Index of content:
Volume 39, Issue 10, October 2012
Mapping of the distribution of local susceptibility strength could be very important in understanding the mechanisms of progression in neurodegenerative diseases, due to depositions of iron and iron-containing plaques. The goal of this study is to map the voxelwise distribution of local cross-termb-value (b c ) effect caused by interactions between the external and internal gradients, in subjects with Alzheimer's disease (AD), mild cognitive impairment (MCI), and cognitively normal (CN) elderly, using a diffusiontensor (DT) MRI.Methods:
Two DT-MRI experiments were conducted with opposite polarities of the external diffusion-sensitizing gradients, and the authors modeled the 3 × 3 tensor matrix ofb c maps and drove a rotationally independent mean b c (i.e., MBc) maps. To test whether AD has higher cross-term gradients than MCI and/or CN, 15 AD patients, 18 MCI patients, and 16 CN controls were acquired from DT-MRI data, with six diffusion encoding directions, five b-values (0, 160, 360, 640, and 1000 s/mm2), and positively and negatively alternating polarities of the external diffusion-sensitizing gradients. The b c and MBc maps were calculated and were spatially normalized into a study specific template for all subjects. The differences of MBc maps across the three subject groups were investigated with voxelwise one-way ANOVA tests for each b-value. The differences of MBc maps, among the four b-values, were also investigated with a voxelwise one-way within-subject ANOVA test for each group.Results:
The authors successfully mapped the local cross-term strength, using a DT-MRI data in the three groups. The MBc differences between the groups were increased with increasing b-values. Compared with the CN group and the MCI group, MBc values in the AD group were significantly increased. However, compared with the CN group, MBc values in the MCI group were not significantly different for all the b-values.Conclusions:
In order to map theb-matrix cross-term effect, the authors developed a rotationally invariant index of MBc, and the index was applied in AD, MCI, and CN subjects. In the AD group, compared with the MCI and CN groups, MBc values were increased. AD patients may have much more local intrinsic gradients in the brain than those MCI or CN subjects. MBc maps may be used to detect the intrinsically susceptible materials in the human brain, such as iron-containing plaques in the brain with AD.
39(2012); http://dx.doi.org/10.1118/1.3694666View Description Hide Description
- RADIATION THERAPY PHYSICS
Second cancer incidence risk estimates using BEIR VII models for standard and complex external beam radiotherapy for early breast cancer39(2012); http://dx.doi.org/10.1118/1.4748332View Description Hide DescriptionPurpose:
To compare organ specific cancer incidence risks for standard and complex external beam radiotherapy (including cone beam CT verification) following breast conservation surgery for early breast cancer.Method:
Doses from breast radiotherapy and kilovoltage cone beam CT(CBCT) exposures were obtained from thermoluminescent dosimeter measurements in an anthropomorphic phantom in which the positions of radiosensitive organs were delineated. Five treatment deliveries were investigated: (i) conventional tangential field whole breast radiotherapy (WBRT), (ii) noncoplanar conformal delivery applicable to accelerated partial beast irradiation (APBI), (iii) two-volume simultaneous integrated boost (SIB) treatment, (iv) forward planned three-volume SIB, and (v) inverse-planned three volume SIB. Conformal and intensity modulated radiotherapy methods were used to plan the complex treatments. Techniques spanned the range from simple methods appropriate for patient cohorts with a low cancer recurrence risk to complex plans relevant to cohorts with high recurrence risk. Delineated organs at risk included brain, salivary glands, thyroid, contralateral breast, left and right lung, esophagus, stomach, liver, colon, and bladder. Biological Effects of Ionizing Radiation (BEIR) VII cancer incidence models were applied to the measured mean organdoses to determine lifetime attributable risk (LAR) for ages at exposure from 35 to 80 yr according to radiotherapy techniques, and included dose from the CBCTimaging.Results:
All LAR decreased with age at exposure and were lowest for brain, thyroid, liver, and bladder (<0.1%). There was little dependence of LAR on radiotherapy technique for these organs and for colon and stomach. LAR values for the lungs for the three SIB techniques were two to three times those from WBRT and APBI. Uncertainties in the LAR models outweigh any differences in lung LAR between the SIB methods. Constraints in the planning of the SIB methods ensured that contralateral breast doses and LAR were comparable to WBRT, despite their added complexity. The smaller irradiated volume of the ABPI plan contributed to a halving of LAR for contralateral breast compared with the other plan types. Daily image guided radiotherapy(IGRT) for a left breast protocol using kilovoltage CBCT contributed <10% to LAR for the majority of organs, and did not exceed 22% of total organdose.Conclusions:
Phantom measurements and calculations of LAR from the BEIR VII models predict that complex breast radiotherapy techniques do not increase the theoretical risk of second cancer incidence for organs distant from the treated breast, or the contralateral breast where appropriate plan constraints are applied. Complex SIB treatments are predicted to increase the risk of second cancer incidence in the lungs compared to standard whole breast radiotherapy; this is outweighed by the threefold reduction in 5 yr local recurrence risk for patients of high risk of recurrence, and young age, from the use of radiotherapy. APBI may have a favorable impact on risk of second cancer in the contralateral breast and lung for older patients at low risk of recurrence. Intensive use of IGRTincreased the estimated values of LAR but these are dominated by the effect of the dose from the radiotherapy, and any increase in LAR from IGRT is much lower than the models’ uncertainties.
39(2012); http://dx.doi.org/10.1118/1.4749932View Description Hide DescriptionPurpose:
To analyze prostate intrafraction motion using both non-gas-release (NGR) and gas-release (GR) rectal balloons and to evaluate the ability of GR rectal balloons to reduce prostate intrafraction motion.Methods:
Twenty-nine patients with NGR rectal balloons and 29 patients with GR balloons were randomly selected from prostate patients treated with proton therapy at the University of Florida Proton Therapy Institute (Jacksonville, FL). Their pretreatment and post-treatment orthogonal radiographs were analyzed, and both pretreatment setup residual error and intrafraction-motion data were obtained. Population histograms of intrafraction motion were plotted for both types of balloons. Population planning target-volume (PTV) margins were calculated with the van Herk formula of 2.5Σ + 0.7σ to account for setup residual errors and intrafraction motion errors.Results:
Pretreatment and post-treatment radiographs indicated that the use of gas-release rectal balloons reduced prostate intrafraction motion along superior–inferior (SI) and anterior–posterior (AP) directions. Similar patient setup residual errors were exhibited for both types of balloons. Gas-release rectal balloons resulted in PTV margin reductions from 3.9 to 2.8 mm in the SI direction, 3.1 to 1.8 mm in the AP direction, and an increase from 1.9 to 2.1 mm in the left–right direction.Conclusions:
Prostate intrafraction motion is an important uncertainty source in radiotherapy after image-guided patient setup with online corrections. Compared to non-gas-release rectal balloons, gas-release balloons can reduce prostate intrafraction motion in the SI and AP directions caused by gas buildup.
39(2012); http://dx.doi.org/10.1118/1.4749965View Description Hide DescriptionPurpose:
Rotational IMRT has been adopted by many clinics for its promise to deliver treatments in a shorter amount of time than other conventional IMRT techniques. In this paper, the authors investigate whether RapidArc is more susceptible to delivery uncertainties than dynamic IMRT using fixed fields.Methods:
Dosimetric effects of delivery uncertainties in dose rate, gantry angle, and MLC leaf positions were evaluated by incorporating these uncertainties into RapidArc and sliding window IMRT (SW IMRT)treatment plans for five head-and-neck and five prostate cases. Dose distributions and dose-volume histograms of original and modified plans were recalculated and compared using Gamma analysis and dose indices of planned treatment volumes (PTV) and organs at risk (OAR). Results of Gamma analyses using passing criteria ranging from 1%–1 mm up to 5%–3 mm were reported.Results:
Systematic shifts in MLC leaf bank positions of SW-IMRT cases resulted in 2–4 times higher average percent differences than RapidArc cases. Uniformly distributed random variations of 2 mm for active MLC leaves had a negligible effect on all dose distributions. Sliding window cases were much more sensitive to systematic shifts in gantry angle. Dose rate variations during RapidArc must be much larger than typical machine tolerances to affect dose distributions significantly; dynamic IMRT is inherently not susceptible to such variations.Conclusions:
RapidArc deliveries were found to be more tolerant to variations in gantry position and MLC leaf position than SW IMRT. This may be attributed to the fact that the average segmental field size or MLC leaf opening is much larger for RapidArc. Clinically acceptable treatments may be delivered successfully using RapidArc despite large fluctuations in dose rate and gantry position.
4D patient dose reconstruction using online measured EPID cine images for lung SBRT treatment validation39(2012); http://dx.doi.org/10.1118/1.4748505View Description Hide DescriptionPurpose:
This study aims to develop an EPID-guided 4D patient dosereconstruction framework and to investigate its feasibility for lungSBRT treatment validation.Methods:
Both the beam apertures and tumor movements were detected based on the continuously acquired EPIDimages during the treatment. Instead of directly using the transit photon fluence measured by the EPID, this method reconstructed the entrance fluence with the measured beam apertures and the delivered MUs. The entrance fluence distributions were sorted into their corresponding phases based on the detected tumor motion pattern and then accumulated for each phase. Together with the in-room 4DCT taken before every treatment to consider the interfractional-motion, the entrance fluence was then used for the patient dose calculation. Deformable registration was performed to sum up the phase doses for final treatment assessment. The feasibility of using the transit EPIDimages for entrance fluence reconstruction was evaluated against EPID in-air measurements. The accuracy of 3D- and 4D-dose reconstruction was validated by experiments with a motor-driven cylindrical diode array for six clinical-SBRT plans.Results:
The average difference between the measured and reconstructed fluence maps was within 0.16%. The reconstructed 3D-dose showed a less than 1.4% difference for the CAX-dose and at least a 98.3% gamma-passing-rate (2%/2 mm) for the peripheral dose. Distorted dose distributions were observed in the measurement with the moving phantom. The comparison between the measured and the reconstructed 4D-dose without considering temporal information failed the gamma-evaluation for most cases. In contrast, when temporal information was considered, the dose distortion phenomena were successfully represented in the reconstructeddose (97.6%–99.7% gamma-passing rate).Conclusions:
The proposed method considered uncertainties of the beam delivery system, the interfractional- and intrafractional-motion, and the interplay effect. The experimental validation demonstrates that this method is practical and accurate for online or offline SBRT patient dose verification.
Detailed high-accuracy megavoltage transmission measurements: A sensitive experimental benchmark of EGSnrc39(2012); http://dx.doi.org/10.1118/1.4745561View Description Hide DescriptionPurpose:
There are three goals for this study: (a) to perform detailed megavoltage transmission measurements in order to identify the factors that affect the measurement accuracy, (b) to use the measured data as a benchmark for theEGSnrc system in order to identify the computational limiting factors, and (c) to provide data for others to benchmark Monte Carlo codes.Methods:
Transmission measurements are performed at the National Research Council Canada on a research linac whose incident electron parameters are independently known. Automated transmission measurements are made on-axis, down to a transmission value of ∼1.7%, for eight beams between 10 MV (the lowest stable MV beam on the linac) and 30 MV, using fully stopping Be, Al, and Pb bremsstrahlung targets and no fattening filters. To diversify energy differentiation, data are acquired for each beam using low-Z and high-Z attenuators (C and Pb) and Farmer chambers with low-Z and high-Z buildup caps. Experimental corrections are applied for beam drifts (2%), polarity (2.5% typical maximum, 6% extreme), ion recombination (0.2%), leakage (0.3%), and room scatter (0.8%)—the values in parentheses are the largest corrections applied. The experimental setup and the detectors are modeled using EGSnrc, with the newly added photonuclear attenuation included (up to a 5.6% effect). A detailed sensitivity analysis is carried out for the measured and calculated transmission data.Results:
The developed experimental protocol allows for transmission measurements with 0.4% uncertainty on the smallest signals. Suggestions for accurate transmission measurements are provided. Measurements andEGSnrc calculations agree typically within 0.2% for the sensitivity of the transmission values to the detector details, to the bremsstrahlung target material, and to the incident electron energy. Direct comparison of the measured and calculated transmission data shows agreement better than 2% for C (3.4% for the 10 MV beam) and typically better than 1% for Pb. The differences can be explained by acceptable photon cross section changes of ⩽0.4%.Conclusions:
Accurate transmission measurements require accounting for a number of influence quantities which, if ignored, can collectively introduce errors larger than 10%. Accurate transmission calculations require the use of the most accurate data and physics options available inEGSnrc, particularly the more accurate bremsstrahlung angular sampling option and the newly added modeling of photonuclear attenuation. Comparison between measurements and calculations implies that EGSnrc is accurate within 0.2% for relative ion chamber response calculations. Photon cross section uncertainties are the ultimate limiting factor for the accuracy of the calculated transmission data (Monte Carlo or analytical).
Motion mitigation in intensity modulated particle therapy by internal target volumes covering range changes39(2012); http://dx.doi.org/10.1118/1.4749964View Description Hide DescriptionPurpose:
Particle therapy offers benefits over conventional photon therapy but also introduces sensitivity to changes in the water-equivalent path length (WEPL) in case of target motion, e.g., breathing. Target motion can be addressed by the internal target volume (ITV) approach, defined as the CTV plus target movement. In photon therapy, the ITV can be constructed as the geometric union of CTVs in all motion states (GEO-ITV) of a 4D-CT, but this does not account for WEPL-changes. An ITV including WEPL-changes can be defined as the union of all CTVs transformed to a WEPL-equivalent axis along beam's eye view. The resulting WEPL-ITV is field-specific and thus unsuitable for intensity modulated particle therapy (IMPT). The purpose of this study was an IMPT-compatible ITV by splitting geometrical motion and field-specific WEPL changes, following ICRU 78 recommendations.Methods:
For all fields, the GEO-ITV was used as a common target. This identical geometry for all fields was mapped to an enlarged WEPL extent with a field-specific transformation. As the dose distribution is determined by the WEPL, this is sufficient to achieve equivalent dose coverage as for a geometrically enlarged target volume. The WEPL enlargement is only visible to the specific field and therefore does not increase the target volume of other fields. This avoids unnecessary lateral field extensions, reducing the dose to normal tissue. Homogeneous dose coverage in IMPT is achieved only if the inhomogeneous doses from the individual fields match up during delivery. As the course of the WEPL within each motion phase differs, this cannot be guaranteed by optimizing the fields only in the reference phase. The WEPL-ITV for the reference phase can be amended by CTVs from a subset of motion phases (4D-WEPL-ITV). Here, end-exhale as the reference phase was combined with end-inhale to cover the whole motion range. The GEO-ITV, WEPL-ITV, and 4D-WEPL-ITV were applied in an IMPT simulation of a lungcancer patient case using a four-field geometry and the heart as an OAR. A static plan of the CTV in end-exhale was computed for reference. The CTV was moving approximately 20 mm in SI and was partly overlapping the heart. For a single fraction a target dose of 17.7 GyE was prescribed, with a 50% maximum dose for the heart.Results:
With 21 rescans to counter interplay, the homogeneity (D5-D95) was 17.0%, 9.0%, 6.0%, and 3.5% for the GEO-ITV, WEPL-ITV, 4D-WEPL-ITV, and a 3D CTV plan computed for reference, respectively. Due to the overlap, the 50% maximum dose was violated by all plans, with V50 of 3.8%, 3.5%, 3.7%, and 2.0% for the four plans.Conclusions:
A 4D-WEPL-ITV method was developed that is suitable for IMPT, covers range changes, and drastically improves dose homogeneity in the target without increasing the OAR dose.
39(2012); http://dx.doi.org/10.1118/1.4747528View Description Hide DescriptionPurpose:
This project proposes using a real tissue phantom for 4D tissue deformation reconstruction (4DTDR) and 4D deformable image registration (DIR) validation, which allows for the complete verification of the motion path rather than limited end-point to end-point of motion.Methods:
Three electro-magnetic-tracking (EMT) fiducials were implanted into fresh porcine liver that was subsequently animated in a clinically realistic phantom. The animation was previously shown to be similar to organ motion, including hysteresis, when driven using a real patient's breathing pattern. For this experiment, 4DCTs and EMT traces were acquired when the phantom was animated using both sinusoidal and recorded patient-breathing traces. Fiducial were masked prior to 4DTDR for reconstruction. The original 4DCT data (with fiducials) were sampled into 20 CT phase sets and fiducials’ coordinates were recorded, resulting in time-resolved fiducial motion paths. Measured values of fiducial location were compared to EMT measured traces and the result calculated by 4DTDR.Results:
For the sinusoidal breathing trace, 95% of EMT measured locations were within 1.2 mm of the measured 4DCT motion path, allowing for repeatable accurate motion characterization. The 4DTDR traces matched 95% of the EMT trace within 1.6 mm. Using the more irregular (in amplitude and frequency) patient trace, 95% of the EMT trace points fitted both 4DCT and 4DTDR motion path within 4.5 mm. The average match of the 4DTDR estimation of the tissue hysteresis over all CT phases was 0.9 mm using a sinusoidal signal for animation and 1.0 mm using the patient trace.Conclusions:
The real tissue phantom is a tool which can be used to accurately characterize tissue deformation, helping to validate or evaluate a DIR or 4DTDR algorithm over a complete motion path. The phantom is capable of validating, evaluating, and quantifying tissue hysteresis, thereby allowing for full motion path validation.
39(2012); http://dx.doi.org/10.1118/1.4749968View Description Hide DescriptionPurpose:
The aim of this study was to design and build a prototype beam shaper to be used on a dedicated mobile accelerator that protects organs at risk within the radiation field and conforms the beam to the target geometry during intraoperative electron radiotherapy (IOERT). A dosimetric characterization of the beam shaper device was performed based on Monte Carlo(MC) simulations, as well as experimental data, at different energies, field sizes, and source to skin distances.Methods:
A mobile light intraoperative accelerator (LIAC®, Sordina, Italy) was used. The design of the beam shaper prototype was based on MC simulations (BEAMnrc/OMEGA and DOSXYZnrc code) for a selection of materials and thicknesses, as well as for dosimetric characterization. Percentage depth dose (PDD) and profile measurements were performed using a p-type silicon diode and a commercial water phantom, while output factors were measured using a PinPoint ion chamber in a PMMA phantom. Planar doses in planes of interest were carried out using radiochromic films (GafchromicTM EBT and EBT2) in PMMA and in a Solid Water® phantom. Several experimental set-ups were investigated with the beam shaper device fixed on the top of the phantom, varying both the short side of the rectangular field and the air gap between the device and the phantom surface, simulating the clinical situation. The output factors (OFs) were determined using different geometrical set-ups and energies.Results:
The beam shaper prototype consists of four blades sliding alongside each other and mounted on a special support at the end of the 10 cm diameter PMMA circular applicator. Each blade is made of an upper layer of 2.6 cm of Teflon ® and a lower layer of 8 mm of stainless steel. All rectangles inscribed in a 5 cm diameter can be achieved in addition to any “squircle-shaped” field. When one side of the rectangular field is held constant and the second side is reduced, both R 50 and R max move towards the phantom surface. Comparing the PDDs obtained with the 5 cm circular applicator and with a 4.4 × 4.4 cm2 square field (that is the equivalent square of the 5 cm circular field) obtained with the beam shaper, a different behavior was observed in the region extending from the surface to a depth of 50% of the maximum dose. Isodoses measured for rectangular fields used for clinical cases (i.e., 4 × 9 cm2 8 MeV) are shown, with different air gaps. For each energy investigated, the normalized OFs slowly increase, when the length of the side decreases down to about 4 cm, and then rapidly decreases for smaller field widths. MC simulation showed an excellent agreement with experimental data (<2%).Conclusions:
The beam shaper device is able to provide square/rectangular/squircle fields with adequate dose homogeneity for mobile dedicated accelerators, thus allowing conformal treatment with IOERT. Monte Carlo simulation can be a very useful tool to simulate any clinical set up and can be used to create a data set to calculate MUs, thereby increasing the accuracy of the delivered dose during IOERT procedures.
39(2012); http://dx.doi.org/10.1118/1.4752211View Description Hide DescriptionPurpose:
Radiosurgery uses small fields and high-radiation doses to treat intra- and extracranial lesions in a single session. The lack of a lateral electronic equilibrium and the presence of high-dose gradients in these fields are challenges for adequate measurements. The availability of radiation detectors with the high spatial resolution required is restricted to only a few. Stereotactic diodes and EBT radiochromic films have been demonstrated to be good detectors for small-beam dosimetry. Because the stereotactic diode is the standard measurement for the dosimetry of radiosurgical beams, the goal of this work was to perform measurements with the radiochromic film Gafchromic® EBT2 and compare its results with a stereotactic diode.Methods:
Total scatter factors, tissue maximum, and off-axis ratios from a 6 MV small photon beams were measured using EBT2 radiochromic film in a water phantom. The film-measured data were evaluated by comparing it with the data measured with a stereotactic field diode (IBA-Dosimetry).Results:
The film and diode measurements had excellent agreement. The differences between the detectors were less than or equal to 2.0% for the tissue maximum and the off-axis ratios. However, for the total scatter factors, there were significant differences, up to 4.9% (relative to the reference field), for field sizes less than 1.0 cm.Conclusions:
This work found that the Gafchromic® EBT2 film is adequate for small photon beam measurements, particularly for tissue maximum and off-axis ratios. However, careful attention must be taken when measuring output factors of small beams below 1.0 cm due to the film's energy dependence. The measurement differences may be attributable to the film's active layer composition because EBT2 incorporates higher Z elements (i.e., bromide and potassium), hence revealing a potential energy dependence for the dosimetry of small photon beams.
39(2012); http://dx.doi.org/10.1118/1.4752236View Description Hide DescriptionPurpose:
The dosimetric accuracy of EDR2 radiographic film has been rigorously assessed in regular and intensity modulated beams for various incidence angles, including the parallel and perpendicular orientation. There clearly exists confusion in literature regarding the effect of film orientation. The primary aim is to clarify potential sources of the confusion and to gain physical insight into the film orientation effect with a link to radiochromic film as well.Methods:
An inverse pyramid IMRT field, consisting of six regular and elongated 3 × 20 cm2 field segments, was studied in perpendicular and parallel orientation. Assessment of film self-perturbation and intrinsic directional sensitivity were also included in the experiments. Finally, the authors investigated the orientational effect in composite beams in the two extreme orientations, i.e., perpendicular and parallel.Results:
The study of an inverse pyramid dose profile revealed good agreement between the perpendicular film and the diamond detector within 0.5% in the low-scatter regions for both 6 and 18 MV. The parallel oriented film demonstrated a 3% under-response at 5-cm (6 MV) depth against the perpendicular orientation, but both orientations over responded equally in the central region, which received only scattered dose, at both 5- and 20-cm depths. In a regular 6-MV 5 × 5 cm2 field, a 4.1% lower film response was observed in the parallel orientation compared to perpendicular orientation. The under response gradually increased to 6% when reducing the field size to 0.5 × 5 cm2. On the other hand, the film showed a 1.7% lower response in parallel orientation for the large field size of 20 × 20 cm2 at 5-cm depth but the difference disappeared at 10 cm. At 18 MV, similar but somewhat lower differences were found between the two orientations. The directional sensitivity of the film diminishes with increasing field size and depth. Surprisingly a composite IMRTbeam consisting of 20 adjacent strip segments also produced a significant orientational dependence of film response, notwithstanding the large total field size of 20 × 20 cm2.Conclusions:
This analysis allowed the development of a hypothesis about the physics behind the orientational dependence of film response in general and to formulate precautions when using film dosimetry in the dosimetric verification of multibeam treatments.
39(2012); http://dx.doi.org/10.1118/1.4752088View Description Hide DescriptionPurpose:
To investigate dose perturbations for pacemaker-implanted patients in partial breast irradiation using high dose rate (HDR) balloon brachytherapy.Methods:
Monte Carlo (MC) simulations were performed to calculate dose distributions involving a pacemaker in Ir-192 HDR balloon brachytherapy. Dose perturbations by varying balloon-to-pacemaker distances (BPD = 50 or 100 mm) and concentrations of iodine contrast medium (2.5%, 5.0%, 7.5%, and 10.0% by volume) in the balloon were investigated for separate parts of the pacemaker (i.e., battery and substrate). Relative measurements using an ion-chamber were also performed to confirm MC results.Results:
The MC and measured results in homogeneous media without a pacemaker agreed with published data within 2% from the balloon surface to 100 mm BPD. Further their dose distributions with a pacemaker were in a comparable agreement. The MC results showed that doses over the battery were increased by a factor of 3, compared to doses without a pacemaker. However, there was no significant dose perturbation in the middle of substrate but up to 70% dose increase in the substrate interface with the titanium capsule. The attenuation by iodine contrast medium lessened doses delivered to the pacemaker by up to 9%.Conclusions:
Due to inhomogeneity of pacemaker and contrast medium as well as low-energy photons in Ir-192 HDR balloon brachytherapy, the actual dose received in a pacemaker is different from the homogeneous medium-based dose and the external beam-based dose. Therefore, the dose perturbations should be considered for pacemaker-implanted patients when evaluating a safe clinical distance between the balloon and pacemaker.
39(2012); http://dx.doi.org/10.1118/1.4752207View Description Hide DescriptionPurpose:
In advanced radiotherapy treatments such as intensity modulated radiation therapy(IMRT) and volumetric modulated arc therapy (VMAT), verification of the performance of the multileaf collimator(MLC) is an essential part of the linac QA program. The purpose of this study is to use the existing measurement methods for geometric QA of the MLCs and extend them to more comprehensive evaluation techniques, and to develop dedicated robust algorithms to quantitatively investigate the MLC performance in a fast, accurate, and efficient manner.Methods:
The behavior of leaves was investigated in the step-and-shoot mode by the analysis of integrated electronic portal imaging device(EPID)images acquired during picket fence tests at fixed gantry angles and arc delivery. The MLC was also studied in dynamic mode by the analysis of cine EPIDimages of a sliding gap pattern delivered in a variety of conditions including different leaf speeds, deliveries at fixed gantry angles or in arc mode, and changing the direction of leaf motion. The accuracy of the method was tested by detection of the intentionally inserted errors in the delivery patterns.Results:
The algorithm developed for the picket fence analysis was able to find each individual leaf position, gap width, and leaf bank skewness in addition to the deviations from expected leaf positions with respect to the beam central axis with sub-pixel accuracy. For the three tested linacs over a period of 5 months, the maximum change in the gap width was 0.5 mm, the maximum deviation from the expected leaf positions was 0.1 mm and the MLC skewness was up to 0.2°. The algorithm developed for the sliding gap analysis could determine the velocity and acceleration/deceleration of each individual leaf as well as the gap width. There was a slight decrease in the accuracy of leaf performance with increasing leaf speeds. The analysis results were presented through several graphs. The accuracy of the method was assessed as 0.01 mm for both the gap size and peak position determination.Conclusions:
This study provides fast, easy, and accurate test methods for routine QA of the MLC performance and helps in faster troubleshooting of MLC problems in both IMRT and VMAT treatments.
39(2012); http://dx.doi.org/10.1118/1.4754297View Description Hide DescriptionPurpose:
To develop a method that allows a commercial treatment planning system (TPS) to perform accurate dose reconstruction for rigidly moving targets and to validate the method in phantom measurements for a range of treatments including intensity modulated radiation therapy(IMRT), volumetric arc therapy (VMAT), and dynamic multileaf collimator (DMLC) tracking.Methods:
An in-house computer program was developed to manipulate Dicom treatment plans exported from a TPS (Eclipse, Varian Medical Systems) such that target motion during treatment delivery was incorporated into the plans. For each treatment, a motion including plan was generated by dividing the intratreatment target motion into 1 mm position bins and construct sub-beams that represented the parts of the treatment that were delivered, while the target was located within each position bin. For each sub-beam, the target shift was modeled by a corresponding isocenter shift. The motion incorporating Dicom plans were reimported into the TPS, where dose calculation resulted in motion including target dose distributions. For experimental validation of the dose reconstruction a thorax phantom with a moveable lung equivalent rod with a tumor insert of solid water was first CT scanned. The tumor insert was delineated as a gross tumor volume (GTV), and a planning target volume (PTV) was formed by adding margins. A conformal plan, two IMRT plans (step-and-shoot and sliding windows), and a VMAT plan were generated giving minimum target doses of 95% (GTV) and 67% (PTV) of the prescription dose (3 Gy). Two conformal fields with MLC leaves perpendicular and parallel to the tumor motion, respectively, were generated for DMLC tracking. All treatment plans were delivered to the thorax phantom without tumor motion and with a sinusoidal tumor motion. The two conformal fields were delivered with and without portal image guided DMLC tracking based on an embedded gold marker. The target dose distribution was measured with a radiochromic film in the moving rod and compared with the reconstructed doses using gamma tests.Results:
Considerable interplay effects between machine motion and target motion were observed for the treatments without tracking. For nontracking experiments, the mean 2 mm/2% gamma pass rate over all investigated scenarios was 99.6% between calculated and measured doses. For tracking experiments, the mean gamma pass rate was 99.4%.Conclusions:
A method for accurate dose reconstruction for moving targets with dynamic treatments was developed and experimentally validated in a variety of delivery scenarios. The method is suitable for integration into TPSs, e.g., for reconstruction of the dose delivered to moving tumors or calculation of target doses delivered with DMLC tracking.
Positional accuracy of novel x-ray-image-based dynamic tumor-tracking irradiation using a gimbaled MV x-ray head of a Vero4DRT (MHI-TM2000)39(2012); http://dx.doi.org/10.1118/1.4754592View Description Hide DescriptionPurpose:
To verify the positional accuracy of a novel x-ray-image-based dynamic tumor-tracking (DTT) irradiation technique using the gimbaled MV x-ray head of a Vero4DRT (MHI-TM2000).Methods:
Verification of the x-ray-image-based DTT was performed using three components: a three-dimensional moving phantom with a steel ball target, a laser displacement gauge, and an orthogonal kV x-ray imaging subsystem with a gimbaled MV x-ray head and the system controller of the Vero4DRT. The moving phantom was driven based on seven periodic patterns [peak-to-peak amplitude (A): 20–40 mm, breathing period (T): 2–5 s] and 15 patients’ aperiodic respiratory patterns (A: 6.5–22.9 mm, T: 1.9–5.8 s). The target position was detected in real time with the orthogonal kV x-ray imaging subsystem using the stereo vision technique. Subsequently, the Vero4DRT predicted the next position of the target, and then the gimbaled MV x-ray head tracked the corresponding orientation of the target. The displacements of the target were measured synchronously using the laser displacement gauge. The difference between the target positions predicted by the Vero4DRT and those measured by the laser displacement gauge was computed as the prediction error (EP), and the difference between the target positions tracked by the gimbaled MV x-ray head and predicted target positions was computed as the mechanical error (EM). Total tracking system error (ET) was defined as the difference between the tracked and measured target positions.Results:
The root mean squares (RMSs) of EP, EM, and ET were up to 0.8, 0.3, and 0.7 mm, respectively, for the periodic patterns. Regarding the aperiodic patterns, the median RMSs of EP, EM, and ET were 1.2 (range, 0.9–1.8) mm, 0.1 (range, 0.1–0.5) mm, and 1.2 (range, 0.9–1.8) mm, respectively. From the results of principal component analysis, tracking efficiency, defined as the ratio of twice the RMS of ET to A, was improved for patients with high respiratory function (R = 0.91; p < 0.01).Conclusions:
The present study demonstrated that the Vero4DRT is capable of high-accuracy x-ray-image-based DTT. ET was caused primarily by EP, and EM was negligible. Furthermore, principal component analysis showed that tracking efficiency could be improved with this system, especially for patients with high respiratory function.
39(2012); http://dx.doi.org/10.1118/1.4754647View Description Hide DescriptionPurpose:
To evaluate the performance of a model based image reconstruction method in reducing metal artifacts in the megavoltage computed tomography (MVCT) images of a phantom representing bilateral hip prostheses and to compare with the filtered-backprojection (FBP) technique.Methods:
An iterative maximum likelihood polychromatic algorithm for CT (IMPACT) is used with an additional model for the pair/triplet production process and the energy dependent response of the detectors. The beam spectra for an in-house bench-top and TomoTherapy™ MVCTs are modeled for use in IMPACT. The empirical energy dependent response of detectors is calculated using a constrained optimization technique that predicts the measured attenuation of the beam by various thicknesses (0–24 cm) of solid water slabs. A cylindrical (19.1 cm diameter) plexiglass phantom containing various cylindrical inserts of relative electron densities 0.295–1.695 positioned between two steel rods (2.7 cm diameter) is scanned in the bench-top MVCT that utilizes the bremsstrahlung radiation from a 6 MeV electron beam passed through 4 cm solid water on the Varian Clinac 2300C and in the imaging beam of the TomoTherapy™ MVCT. The FBP technique in bench-top MVCT reconstructsimages from raw signal normalized to air scan and corrected for beam hardening using a uniform plexiglass cylinder (20 cm diameter). The IMPACT starts with a FBP reconstructed seed image and reconstructs the final image in 150 iterations.Results:
In both MVCTs, FBP produces visible dark shading in the image connecting the steel rods. In the IMPACT reconstructed images this shading is nearly removed and the uniform background is restored. The average attenuation coefficients of the inserts and the background are very close to the corresponding values in the absence of the steel inserts. In the FBP images of the bench-top MVCT, the shading causes 4%–9.5% underestimation of electron density at the central inserts with an average of (6.3 ± 1.8)% for the range of electron densities studied. In the uniform plexiglass background, the shadow creates 0.8%–4.7% underestimation of electron density with an average of (2.9 ± 1.2)%. In the corresponding IMPACT images, the underestimation in the shaded plexiglass background is 0.3%–1.8% with an average of (0.9 ± 0.5)% and 1.4%–6.8% with an average of (2.8 ± 2.7)% in the central insert region. In the FBP images of the TomoTherapy™ MVCT, this shading creates 2.6%–6.7% underestimation of electron density with an average of (3.7 ± 1.4)% at the central inserts and 5.9%–7.2% underestimation in the background with an average of (6.4 ± 0.5)%. In the IMPACT images, the uniform background between the steel rods is restored with 0.3%–1.0% underestimation of electron density with an average of (0.7 ± 0.3)%. The corresponding underestimation at the central inserts of the IMPACT images is −0.4%–0.1% with an average of (−0.1 ± 0.2)%.Conclusions:
The shading metal artifact has been nearly removed in MVCT images using the IMPACT algorithm with the accurate geometry of the system, proper modeling of energy dependent response of detectors, and all relevant photon interaction processes. This results less than 1% difference in electron density in the background plexiglass and less than 3% averaged over the range of electron densities investigated.
39(2012); http://dx.doi.org/10.1118/1.4754659View Description Hide DescriptionPurpose:
Interest in adaptive radiation therapy research is constantly growing, but software tools available for researchers are mostly either expensive, closed proprietary applications, or free open-source packages with limited scope, extensibility, reliability, or user support. To address these limitations, we propose SlicerRT, a customizable, free, and open-source radiation therapy research toolkit. SlicerRT aspires to be an open-source toolkit for RT research, providing fast computations, convenient workflows for researchers, and a general image-guided therapy infrastructure to assist clinical translation of experimental therapeutic approaches. It is a medium into which RT researchers can integrate their methods and algorithms, and conduct comparative testing.Methods:
SlicerRT was implemented as an extension for the widely used 3D Slicer medical imagevisualization and analysis application platform. SlicerRT provides functionality specifically designed for radiation therapy research, in addition to the powerful tools that 3D Slicer offers for visualization, registration, segmentation, and data management. The feature set of SlicerRT was defined through consensus discussions with a large pool of RT researchers, including both radiation oncologists and medical physicists. The development processes used were similar to those of 3D Slicer to ensure software quality. Standardized mechanisms of 3D Slicer were applied for documentation, distribution, and user support. The testing and validation environment was configured to automatically launch a regression test upon each software change and to perform comparison with ground truth results provided by other RT applications.Results:
Modules have been created for importing and loading DICOM-RT data, computing and displaying dose volume histograms, creating accumulated dose volumes, comparing dose volumes, and visualizing isodose lines and surfaces. The effectiveness of using 3D Slicer with the proposed SlicerRT extension for radiation therapy research was demonstrated on multiple use cases.Conclusions:
A new open-source software toolkit has been developed for radiation therapy research. SlicerRT can import treatment plans from various sources into 3D Slicer for visualization, analysis, comparison, and processing. The provided algorithms are extensively tested and they are accessible through a convenient graphical user interface as well as a flexible application programming interface.
An efficient protocol for radiochromic film dosimetry combining calibration and measurement in a single scan39(2012); http://dx.doi.org/10.1118/1.4754797View Description Hide DescriptionPurpose:
Radiochromic film provides dose measurement at high spatial resolution, but often is not preferred for routine evaluation of patient-specific intensity modulated radiation therapy(IMRT) plans owing to ease-of-use factors. The authors have established an efficient protocol that combines calibration and measurement in a single scan and enables measurement results to be obtained in less than 30 min. This avoids complications due to postexposure changes in radiochromic film that delay the completion of a measurement, often for up to 24 h, in commonly used methods. In addition, the protocol addresses the accuracy and integrity of the measurement by eliminating environmental and interscan variability issues.Methods:
The authors collected dose–response data from six production lots of Gafchromic EBT3 film and three production lots of EBT2 film at doses up to 480 cGy. In this work, the authors used seven different scanners of two different models—Epson 10000XL and V700; postexposure times before scanning from 30 min to 9 days; ambient temperatures for scanning spanning 11 °C; and two film orientations. Scanning was in 48-bit RGB format at 72 dpi resolution. Dose evaluation was conducted using a triple-channel dosimetry method. To evaluate the measurement protocol, patient specific IMRT and volumetric modulated arc therapy (VMAT) plans were exposed onto EBT3 films on a Varian Trilogy Linac. Film scanning was done following the protocol under a number of different conditions and the dose maps were analyzed to demonstrate the equivalence of results.Results:
The results indicated that the dose–response data could be fit by a set of related rational functions leading to the description of a generic calibration curve. A simplified dosimetry protocol was established where dose–response data for a specific film lot, scanner, and scanning conditions could be derived from two films exposed to known doses. In most cases only one calibrated exposure was required since the dose for one of the films could be zero. Using the Gamma test criterion of 2%/2 mm to evaluate the measurements, similar passing rates ranging between about 95% and 99% for the fields studied were obtained from application films digitized under a variety of conditions all of them different than the conditions under which the calibration films were scanned.Conclusions:
The authors have developed a simplified and efficient protocol to measure doses delivered by an IMRT or VMAT plan using only the patient film, one calibration film, one unexposed film, and applying a single scan to acquire a digital image for calculation and analysis. The simplification and timesaving offer a potential practical solution for using radiochromic film for routine treatment plan quality assurance without sacrificing spatial resolution for convenience.
39(2012); http://dx.doi.org/10.1118/1.4754798View Description Hide DescriptionPurpose:
Accurate delineation of the rectum is of high importance in off-line adaptive radiation therapy since it is a major dose-limitingorgan in prostate cancer radiotherapy. The intensity-based deformable image registration (DIR) methods cannot create a correct spatial transformation if there is no correspondence between the template and the target images. The variation of rectal filling, gas, or feces, creates a noncorrespondence in image intensities that becomes a great obstacle for intensity-based DIR.Methods:
In this study the authors have designed and implemented a semiautomatic method to create a rectum mask in pelvic computed tomography(CT)images. The method, that includes a DIR based on the demons algorithm, has been tested in 13 prostate cancer cases, each comprising of two CT scans, for a total of 26 CT scans.Results:
The use of the manual segmentation in the planning image and the proposed rectum mask method (RMM) method in the daily image leads to an improvement in the DIR performance in pelvic CTimages, obtaining a mean value of overlap volume index = 0.89, close to the values obtained using the manual segmentations in both images.Conclusions:
The application of the RMM method in the daily image and the manual segmentations in the planning image during prostate cancer treatments increases the performance of the registration in presence of rectal fillings, obtaining very good agreement with a physician's manual contours.
Verification of dosimetric accuracy on the TrueBeam STx: Rounded leaf effect of the high definition MLC39(2012); http://dx.doi.org/10.1118/1.4752444View Description Hide DescriptionPurpose:
The dosimetric leaf gap (DLG) in the Varian Eclipse treatment planning system is determined during commissioning and is used to model the effect of the rounded leaf-end of the multileaf collimator(MLC). This parameter attempts to model the physical difference between the radiation and light field and account for inherent leakage between leaf tips. With the increased use of single fraction high dosetreatments requiring larger monitor units comes an enhanced concern in the accuracy of leakage calculations, as it accounts for much of the patient dose. This study serves to verify the dosimetric accuracy of the algorithm used to model the rounded leaf effect for the TrueBeam STx, and describes a methodology for determining best-practice parameter values, given the novel capabilities of the linear accelerator such as flattening filter free (FFF) treatments and a high definition MLC (HDMLC).Methods:
During commissioning, the nominal MLC position was verified and the DLG parameter was determined using MLC-defined field sizes and moving gap tests, as is common in clinical testing.Treatment plans were created, and the DLG was optimized to achieve less than 1% difference between measured and calculated dose. The DLG value found was tested on treatment plans for all energies (6 MV, 10 MV, 15 MV, 6 MV FFF, 10 MV FFF) and modalities (3D conventional, IMRT, conformal arc, VMAT) available on the TrueBeam STx.Results:
The DLG parameter found during the initial MLCtesting did not match the leaf gap modeling parameter that provided the most accurate dose delivery in clinical treatment plans. Using the physical leaf gap size as the DLG for the HDMLC can lead to 5% differences in measured and calculated doses.Conclusions:
Separate optimization of the DLG parameter using end-to-end tests must be performed to ensure dosimetric accuracy in the modeling of the rounded leaf ends for the Eclipse treatment planning system. The difference in leaf gap modeling versus physical leaf gap dimensions is more pronounced in the more recent versions of Eclipse for both the HDMLC and the Millennium MLC. Once properly commissioned and tested using a methodology based on treatment plan verification, Eclipse is able to accurately model radiationdose delivered for SBRTtreatments using the TrueBeam STx.