Index of content:
Volume 39, Issue 7, July 2012
Radiation-induced lung injury (RILI) is the primary dose-limiting toxicity for radiation therapy of the lung, and although the effects of radiation dose on RILI development have been well characterized, the influence of chronic obstructive pulmonary disease (COPD) on the development of RILI and other outcomes is not well understood. The purpose of this small pilot study was to evaluate the relationship between hyperpolarized3He magnetic resonance imaging(MRI) measurements of COPD with RILI and 12-month survival in lungcancer patients undergoing radical radiotherapy and to evaluate the feasibility of pulmonary functional MRI as an image guidance/planning tool for radiation therapy.Methods:
Fifteen non-small cell and small cell lungcancer patients underwent pulmonary function tests, x-ray computed tomography(CT), and hyperpolarized3He MRI prior to radical radiation therapy (≥60 Gy). Conventional thoracic 1H and hyperpolarized 3He MRI were acquired to generate ventilation defect percent and the apparent diffusion coefficient for the ipsilateral and contralateral lungs independently. CT was acquired postradiation therapy and qualitatively evaluated for radiological evidence of RILI and 12-month survival was reported.Results:
Hyperpolarized3He MRI measurements of COPD classified 10/15 subjects with contralateral lung COPD (CLC), and five subjects without COPD [contralateral lung normal (CLN)]. Of the 10 subjects with CLC, only four had a previous clinical diagnosis of COPD. CTimages were acquired postradiation therapy for 13 subjects, and for eight (62%) of these there was qualitative evidence of RILI, including 5/9 CLC and 3/4 CLN subjects. The one-year survival was 2/10 for CLC and 3/5 for CLN subjects.Conclusions:
In this small pilot study, we report the use of3He MRI to stratify lungcancer patients based on MRI evidence of COPD and showed that comorbid COPD was present in the majority of lungcancer subjects stratified for radiation therapy.Lungcancer patients with imaging evidence of COPD did not have an increased incidence of RILI compared to patients without COPD. However, preliminary data presented here indicated that one-year survival in COPD subjects was lower than expected based on previously published survival rates, which may have implications for radiation therapy in lungcancer patients with comorbid COPD.
39(2012); http://dx.doi.org/10.1118/1.3694115View Description Hide Description
- MEDICAL PHYSICS LETTERS
39(2012); http://dx.doi.org/10.1118/1.4729838View Description Hide DescriptionPurpose:
The high breast density is one of the biggest risk factors for breast cancer. Identifying patient having persistent high breast density is important for breast cancer screening and prevention. In this work the authors propose for the first time an x-ray phase-shifts-based method of breast density measurement.Methods:
When x ray traverses the breast, x ray gets not only its intensity attenuated but also its phase shifted. Studying the x-ray phase-shifts generated by the breast tissues, we derived a general formula for determining the volumetric breast density from the breast phase map. The volumetric breast density is reconstructed by retrieving the breast phase map from just a single phase-sensitive projection of the breast, through the use of an innovative phase retrieval method based on the phase-attenuation duality. In order to numerically validate this phase-shifts-based method for measuring the volumetric breast density, the authors performed computer simulations with a digitally simulated anthropomorphic breast phantom.Results:
Using the proposed phase-shifts-based method, we reconstructed the breast phantom's volumetric breast density, which differs from the phantom's intrinsic breast density by only 0.06%. In the presence of noises in the projection image, the reconstructed volumetric breast density differs from the phantom's intrinsic breast density by only 1.79% for a projection signal-to-noise-ratio (SNR) of 34. The error in reconstructed breast density is further reduced to 1.61% and 1.55% for SNR = 68 and SNR = 134, respectively, achieving good accuracies in the breast density determination.Conclusions
: The authors proposed an x-ray phase-shifts-based method of measuring the volumetric breast density. The simulation results numerically validated the proposed method as a novel method of breast density measurement with good accuracies.
- RADIATION THERAPY PHYSICS
Monte Carlo computed machine-specific correction factors for reference dosimetry of TomoTherapy static beam for several ion chambers39(2012); http://dx.doi.org/10.1118/1.4722752View Description Hide DescriptionPurpose:
To determine correction factors for machine-specific reference (msr) conditions by Monte Carlo(MC) simulations for reference dosimetry of TomoTherapy static beams for ion chambers Exradin A1SL, A12; PTW 30006, 31010 Semiflex, 31014 PinPoint, 31018 microLion; NE 2571.Methods:
For the calibration of TomoTherapy units, reference conditions specified in current codes of practice like IAEA/TRS-398 and AAPM/TG-51 cannot be realized. To cope with this issue,Alfonso et al. [Med. Phys.35, 5179–5186 (2008)] described a new formalism introducing msr factors for reference dosimetry, applicable to static TomoTherapy beams. In this study, those factors were computed directly using MC simulations for Q 0 corresponding to a simplified 60Co beam in TRS-398 reference conditions (at 10 cm depth). The msr conditions were a 10 × 5 cm2 TomoTherapy beam, source-surface distance of 85 cm and 10 cm depth. The chambers were modeled according to technical drawings using the egs++ package and the MC simulations were run with the egs_chamber user code. Phase-space files used as the source input were produced using PENELOPE after simulation of a simplified 60Co beam and the TomoTherapy treatment head modeled according to technical drawings. Correlated sampling, intermediate phase-space storage, and photon cross-section enhancement variance reduction techniques were used. The simulations were stopped when the combined standard uncertainty was below 0.2%.Results:
Computed values were all close to one, in a range from 0.991 for the PinPoint chamber to 1.000 for the Exradin A12 with a statistical uncertainty below 0.2%. Considering a beam quality Q defined as the TPR 20,10 for a 6 MV Elekta photon beam (0.661), the additional correction to defined in Alfonso et al. [Med. Phys.35, 5179–5186 (2008)] formalism was in a range from 0.997 to 1.004.Conclusion:
The MC computed factors in this study are in agreement with measured factors for chamber types already studied in literature. This work provides msr correction factors for additional chambers used in reference dosimetry. All of them were close to one (within 1%).
Technical note: Patient-specific quality assurance methods for TomoDirectTM whole breast treatment delivery39(2012); http://dx.doi.org/10.1118/1.4722967View Description Hide DescriptionPurpose:
To investigate the feasibility of implementing a novel approach for patient-specific QA of TomoDirectTM whole breast treatment.Methods:
The most currently used TomoTherapy DQA method, consisting in the verification of the 2D dose distribution in a coronal or sagittal plane of the Cheese Phantom by means of gafchromic films, was compared with an alternative approach based on the use of two commercially available diode arrays, MapCHECK2TM and ArcCHECKTM. The TomoDirectTM plans of twenty patients with a primary unilateral breast cancer were applied to a CT scan of the Cheese Phantom and a MVCT dataset of the diode arrays. Then measurements of 2D dose distribution were performed and compared with the calculated ones using the gamma analysis method with different sets of DTA and DD criteria (3%-3 mm, 3%-2 mm). The sensitivity of the diode arrays to detect delivery and setup errors was also investigated.Results:
The measured dose distributions showed excellent agreement with the TPS calculations for each detector, with averaged fractions of passed Γ values greater than 95%. The percentage of points satisfying the constraint Γ < 1 was significantly higher for MapCHECK2TM than for ArcCHECKTM and gafchromic films using both the 3%-3 mm and 3%-2 mm gamma criteria. Both the diode arrays show a good sensitivity to delivery and setup errors using a 3%-2 mm gamma criteria.Conclusions:
MapCHECK2™ and ArcCHECKTM may fulfill the demands of an adequate system for TomoDirectTM patient-specific QA.
Experimental evaluation of a spatial resampling technique to improve the accuracy of pencil-beam dose calculation in proton therapy39(2012); http://dx.doi.org/10.1118/1.4722984View Description Hide DescriptionPurpose:
In proton therapy, pencil-beam algorithms (PBAs) are the most widely used dose calculation methods. However, the PB calculations that employ one-dimensional density scaling neglect the effects of lateral density heterogeneity on the dose distributions, whereas some particles included in such pencil beams could overextend beyond the interface of the density heterogeneity. We have simplified a pencil-beam redefinition algorithm (PBRA), which was proposed for electron therapy, by a spatial resampling technique toward an application for proton therapy. The purpose of this study is to evaluate the calculation results of the spatial resampling technique in terms of lateral density heterogeneity by comparison with the dose distributions that were measured in heterogeneous slab phantoms.Methods:
The pencil beams are characterized for multiple residual-range (i.e., proton energy) bins. To simplify the PBRA, the given pencil beams are resampled on one or two transport planes, in which smaller sub-beams that are parallel to each other are generated. We addressed the problem of lateral density heterogeneity comparing the calculation results to the dose distributions measured at different depths in heterogeneous slab phantoms using a two-dimensional detector. Two heterogeneity slab phantoms, namely, phantoms A and B, were designed for the measurements and calculations. In phantom A, the heterogeneity slab was placed close to the surface. On the other hand, in phantom B, it was placed close to the Bragg peak in the mono-energetic protonbeam.Results:
In measurements, lateral dose profiles showed a dose reduction and increment in the vicinity ofx = 0 mm in both phantoms at depths z = 142 and 161 mm due to lateral particle disequilibrium. In phantom B, these dose reduction/increment effects were higher/lower, respectively, than those in phantom A. This is because a longer distance from the surface to the heterogeneous slab increases the strength of protonscattering. Sub-beams, which were generated from the resampling plane, formed a detouring/overextending path that was different from that of elemental pencil beams. Therefore, when the spatial resampling was implemented at the surface and immediately upstream of the lateral heterogeneity, the calculation could predict these dose reduction/increment effects. Without the resampling procedure, these dose reduction/increment effects could not be predicted in both phantoms owing to the blurring of the pencil beam. We found that the PBA with the spatial resampling technique predicted the dose reduction/increment at the dose profiles in both phantoms when the sampling plane was defined immediately upstream of the heterogeneous slab.Conclusions:
We have demonstrated the implementation of a spatial resampling technique for pencil-beam calculation to address the problem of lateral density heterogeneity. While further validation is required for clinical use, this study suggests that the spatial resampling technique can make a significant contribution to proton therapy.
Quality indicators and technique for analyzing permanent I-125 prostate seed implants: Seven years postimplant dosimetry evaluation39(2012); http://dx.doi.org/10.1118/1.4725173View Description Hide DescriptionPurpose:
The roles of postimplant dosimetry (PID) after permanent I-125 implant are to identify and rectify inadequate implants, assess the dosimetric quality indicators, and evaluate dose to the organs at risk. The aim of the current work was to assess the progress of prostate implant quality via postimplant dosimetry over seven years.Methods
: The following factors were investigated to assess the PID results obtained over seven years: the improvement in implant technique, the computed tomography(CT) delineation-based PID versus ultrasound-CT (US-CT) fusion-based PID, and the evolution of parameters such as D90 and NDR (natural dose ratio). The correlation between dosimetric parameters and clinical outcomes were also evaluated.Results
: The seven years PID learning curve shows clear changes in dosimetric trend for the 265 patients studied. Manual target contouring on CT was shown to overestimate the prostate volume when compared to ultrasound data, translating to CT-based D90 values being lower than US-CT D90. It was found that NDR does not contribute with additional dosimetric information to postimplant dosimetry evaluation. Patient follow-up data show that 4.7% patients have relapsed, and urinary retention was reported in 2.7% of the patients.Conclusions
: CT-based PID was found less reliable than US-CT fusion-based PID due to target volume overestimation. This result shows the biased interpretation of low D90 values based on CT-based targeting, providing unreliable correlations between D90 and relapse probability. The low urinary retention statistics are in accordance with the PID data for the organ, as only 5.2% of patients had their PID D10 > 218 Gy, i.e., above the recommended GEC-ESTRO guidelines. Besides the “learning” component, the PID D90 curve is influenced by the PID technique.
A combined dose calculation and verification method for a small animal precision irradiator based on onboard imaging39(2012); http://dx.doi.org/10.1118/1.4725710View Description Hide DescriptionPurpose:
Novel small animal precision microirradiators (micro-IR) are becoming available for preclinical use and are often equipped with onboard imaging (OBI) devices. We investigated the use of OBI as a means to infer the accuracy of the delivered treatment plan.Methods:
Monte Carlo modeling of the micro-IR including an elliptical Gaussian electron beam incident on the x-ray tube was used to score dose and to continue photon transport to the plane of the OBI device. A model of the OBI detector response was used to generate simulated onboard images. Experimental OBI was performed at 225 kVp, gain/offset and scatter-glare were corrected. Simulated and experimentally obtained onboard images of phantoms and a mouse specimen were compared for a range of photon beam sizes from 2.5 cm down to 0.1 cm.Results:
Simulated OBI can be used in small animal radiotherapy to determine if a treatment plan was delivered according to the prescription within an uncertainty of 5% for beams as small as 4 mm in diameter. For collimated beams smaller than 4 mm, beam profile differences remain primarily in the penumbra region of the smallest beams, which may be tolerable for specific preclinical micro-IR investigations.Conclusions:
Comparing simulated to acquired OBI during small animal treatment radiotherapy represents a useful treatment delivery tool.
Development of a novel ArcCHECK™ insert for routine quality assurance of VMAT delivery including dose calculation with inhomogeneities39(2012); http://dx.doi.org/10.1118/1.4728222View Description Hide DescriptionPurpose:
To design a versatile, nonhomogeneous insert for the dose verification phantom ArcCHECK™ (Sun Nuclear Corp., FL) and to demonstrate its usefulness for the verification of dose distributions in inhomogeneous media. As an example, we demonstrate it can be used clinically for routine quality assurance of two volumetric modulated arc therapy (VMAT) systems for lung stereotactic body radiation therapy(SBRT): SmartArc® (Pinnacle3, Philips Radiation Oncology Systems, Fitchburg, WI) and RapidArc® (Eclipse™, Varian Medical Systems, Palo Alto, CA).Methods:
The cylindrical detector array ArcCHECK™ has a retractable homogeneous acrylic insert. In this work, we designed and manufactured a customized heterogeneous insert with densities that simulate soft tissue,lung, bone, and air. The insert offers several possible heterogeneity configurations and multiple locations for point dose measurements. SmartArc® and RapidArc® plans for lungSBRT were generated and copied to ArcCHECK™ for each inhomogeneity configuration. Dose delivery was done on a Varian 2100 ix linac. The evaluation of dose distributions was based on gamma analysis of the diode measurements and point doses measurements at different positions near the inhomogeneities.Results:
The insert was successfully manufactured and tested with different measurements of VMAT plans. Dose distributions measured with the homogeneous insert showed gamma passing rates similar to our clinical results (∼99%) for both treatment-planning systems. Using nonhomogeneous inserts decreased the passing rates by up to 3.6% in the examples studied. Overall, SmartArc® plans showed better gamma passing rates for nonhomogeneous measurements. The discrepancy between calculated and measured point doses was increased up to 6.5% for the nonhomogeneous insert depending on the inhomogeneity configuration and measurement location. SmartArc® and RapidArc® plans had similar plan quality but RapidArc® plans had significantly higher monitor units (up to 70%).Conclusions:
A versatile, nonhomogeneous insert was developed for ArcCHECK™ for an easy and quick evaluation of dose calculations with nonhomogeneous media and for comparison of different treatment planning systems. The device was tested for SmartArc® and RapidArc® plans for lungSBRT, showing the uncertainties of dose calculations with inhomogeneities. The new insert combines the convenience of the ArcCHECK™ and the possibility of assessing dose distributions in inhomogeneous media.
39(2012); http://dx.doi.org/10.1118/1.4728977View Description Hide DescriptionPurpose:
This is a proof-of-concept study addressing volume of interest (VOI) cone beam CT(CBCT)imaging using an x-ray beam produced by 2.35 MeV electrons incident on a carbon linear accelerator target. Methodology is presented relevant to VOI CBCTimage acquisition and reconstruction. Sample image data are given to demonstrate and compare two approaches to minimizing artifacts arising from reconstruction with truncated projections. Dosimetric measurements quantify the potential dose reduction of VOI acquisition relative to full-field CBCT. The dependence of contrast-to-noise ratio(CNR) on VOI dimension is investigated.Methods:
A paradigm is presented linking the treatment planning process with the imaging technique, allowing definition of an imaging VOI to be tailored to the geometry of the patient. Missing data in truncated projection images are completed usinga priori information in the form of digitally reconstructedradiographs(DRRs) generated from the planning CT set. This method is compared to a simpler technique of extrapolating truncated projection data prior to reconstruction. The utility of these approaches is shown through imaging of a geometric phantom and the head-and-neck section of a lamb. The total scatter factor of the 2.35 MV/carbon beam on field size is measured and compared to a standard therapeutic beam to estimate the comparative dose reduction inside the VOI. Thermoluminescent dosimeters and Gafchromic film measurements are used to compare the imaging dose distributions for the 2.35 MV/carbon beam between VOI and full-field techniques. The dependence of CNR on VOI dimension is measured for VOIs ranging from 4 to 15 cm diameter.Results:
Without compensating for missing data outside of truncated projections prior to reconstruction, pronounced boundary artifacts are present, in three dimensions, within 2–3 cm of the edges of the VOI. These artifacts, as well as cupping inside the VOI, can be reduced substantially using either the DRR filling or extrapolation techniques presented. Compared to 6 MV, the 2.35 MV/carbon beam shows a substantially greater dependence of total scatter factor on field size, indicating a comparative advantage of the VOI approach when combined with the low-Z target beam. Dosimetric measurements in the anthropomorphic head phantom demonstrate a dose reduction by up to 15% and 75% inside and outside of the VOI, respectively, compared to full-field imaging. For the 2.35 MV/carbon beam, CNR was shown to be approximately invariant with VOI dimension for bone and lung objects.Conclusions:
The low-Z target, VOI CBCT technique appears to be feasible and combines the desirable characteristics of the low-Z target beam with regard to CNR, with the capacity to localize the imaging dose to the anatomy relevant to the image guidance task.
Development of a phantom to evaluate the positioning accuracy of patient immobilization systems using thermoplastic mask and polyurethane cradle39(2012); http://dx.doi.org/10.1118/1.4728978View Description Hide DescriptionPurpose:
The purpose of this study was to develop a new phantom to evaluate the positioning accuracy of patient immobilization systems.Methods:
The phantom was made of papers formed into a human shape, paper clay, and filling rigid polyester. Acrylonitrile butadiene styrene (ABS) pipes were inserted at anterior-posterior (A-P) and right-left (R-L) directions in the phantom to give static load by pulling ropes through the pipes. First, the positioning precision of the phantom utilizing a target locating system (TLS) was evaluated by moving the phantom on a couch along inferior-superior (I-S), A-P, and R-L directions in a range from −5 mm to +5 mm. The phantom's positions detected with the TLS were compared with values measured by a vernier caliper. Second, the phantom movements in a tensile test were chosen from patient movements determined from 15 patients treated for intracranial lesions and immobilized with a thermoplastic mask and polyurethane cradle. The phantom movement was given by minimum or maximum values of patient movements in each direction. Finally, the relationship between phantom movements and the static load in the tensile test was characterized from measurements using the new phantom and the TLS.Results:
The differences in all positions between the vernier caliper measurement and the TLS detected values were within 0.2 mm with frequencies of 100%, 95%, and 90% in I-S, A-P, and R-L directions, respectively. The phantom movements according to patient movements in clinical application in I-S, A-P, and R-L directions were within 0.58 mm, 0.94 mm, and 0.93 mm from the mean value plus standard deviation, respectively. The regression lines between the phantom movements and static load were given by y = 0.359x, y = 0.241x, and y = 0.451x in I-S, A-P, and R-L directions, respectively, where x is the phantom movement (mm) and y is the static load (kgf). The relationship between the phantom movements and static load may represent the performance of inhibiting patient movements, so the accuracy of the immobilization system in the intracranial lesion will be estimated in advance by basic tensile test on the new phantom.Conclusions:
The newly developed phantom was useful to evaluate the accuracy of immobilization systems for a Cyberknife system for intracranial lesions.
39(2012); http://dx.doi.org/10.1118/1.4729709View Description Hide DescriptionPurpose:
To develop and validate a volume-modulated arc therapy (VMAT) quality assurance (QA) tool that takes as input a time-resolved, low-density (∼10 mm) cylindrical surface dose map from a commercial helical diode array, and outputs a high density, volumetric, time-resolved dose matrix on an arbitrary patient dataset. This first validation study is limited to a homogeneous “patient.”Methods:
A VMAT treatment is delivered to a diode array phantom (ARCCHECK, Sun Nuclear Corp., Melbourne, FL). 3DVH software (Sun Nuclear) derives the high-density volumetric dose using measurement-guided dosereconstruction (MGDR). MGDR cylindrical phantom results are then used to perturb the three-dimensional (3D) treatment planningdose on the patient dataset, producing a semiempirical volumetric dose grid. Four-dimensional (4D) dosereconstruction on the patient is also possible by morphing individual sub-beam doses instead of the composite. For conventional (3D) dose comparison two methods were developed, using the four plans (Multi-Target, C-shape, Mock Prostate, and Head and Neck), including their structures and objectives, from the AAPM TG-119 report. First, 3DVH and treatment planning system (TPS) cumulative point doses were compared to ion chamber in a cube water-equivalent phantom (“patient”). The shape of the phantom is different from the ARCCHECK and furthermore the targets were placed asymmetrically. Second, coronal and sagittal absolute film dose distributions in the cube were compared with 3DVH and TPS. For time-resolved (4D) comparisons, three tests were performed. First, volumetric dose differences were calculated between the 3D MGDR and cumulative time-resolved patient (4D MGDR) dose at the end of delivery, where they ideally should be identical. Second, time-resolved (10 Hz sampling rate) ion chamberdoses were compared to cumulative point dose vs time curves from 4D MGDR. Finally, accelerator output was varied to assess the linearity of the 4D MGDR with global fluence change.Results:
Across four TG-119 plans, the average PTV point dose difference in the cube between 3DVH and ion chamber is 0.1 ± 1.0%. Average film vs TPSγ-analysis passing rates are 83.0%, 91.1%, and 98.4% for 1%/2 mm, 2%/2 mm, and 3%/3 mm threshold combinations, respectively, while average film vs 3DVH γ-analysis passing rates are 88.6%, 96.1%, and 99.5% for the same respective criteria. 4D MGDR was also sufficiently accurate. First, for 99.5% voxels in each case, the doses from 3D and 4D MGDR at the end of delivery agree within 0.5% local dose-error/1 mm distance. Moreover, all failing voxels are confined to the edge of the cylindrical reconstruction volume. Second, dose vs time curves track between the ion chamber and 4D MGDR within 1%. Finally, 4D MGDR dose changes linearly with the accelerator output: the difference between cumulative ion chamber and MGDR dose changed by no more than 1% (randomly) with the output variation range of 10%.Conclusions:
Even for a well-commissioned TPS, comparison metrics show better agreement on average to MGDR than to TPS on the arbitrary-shaped measurable “patient.” The method requires no more accelerator time than standard QA, while producing more clinically relevant information. Validation in a heterogeneous thoracic phantom is under way, as is the ultimate application of 4D MGDR to virtual motion studies.
39(2012); http://dx.doi.org/10.1118/1.4729708View Description Hide DescriptionPurpose:
A new motion-based gated proton therapy for the treatment of orbital tumors using real-time eye-tracking system was designed and evaluated.Methods:
We developed our system by image-pattern matching, using a normalized cross-correlation technique with LabVIEW 8.6 and Vision Assistant 8.6 (National Instruments, Austin, TX). To measure the pixel spacing of an image consistently, four different calibration modes such as the point-detection, the edge-detection, the line-measurement, and the manual measurement mode were suggested and used. After these methods were applied to proton therapy, gating was performed, and radiationdose distributions were evaluated.Results:
Moving phantom verification measurements resulted in errors of less than 0.1 mm for given ranges of translation. Dosimetric evaluation of the beam-gating system versus nongated treatment delivery with a moving phantom shows that while there was only 0.83 mm growth in lateral penumbra for gated radiotherapy, there was 4.95 mm growth in lateral penumbra in case of nongated exposure. The analysis from clinical results suggests that the average of eye movements depends distinctively on each patient by showing 0.44 mm, 0.45 mm, and 0.86 mm for three patients, respectively.Conclusions:
The developed automatic eye-tracking based beam-gating system enabled us to perform high-precision protonradiotherapy of orbital tumors.
Dose optimization with first-order total-variation minimization for dense angularly sampled and sparse intensity modulated radiation therapy (DASSIM-RT)39(2012); http://dx.doi.org/10.1118/1.4729717View Description Hide DescriptionPurpose:
A new treatment scheme coined as dense angularly sampled and sparse intensity modulated radiation therapy (DASSIM-RT) has recently been proposed to bridge the gap between IMRT and VMAT. By increasing the angular sampling of radiation beams while eliminating dispensable segments of the incident fields, DASSIM-RT is capable of providing improved conformity in dose distributions while maintaining high delivery efficiency. The fact that DASSIM-RT utilizes a large number of incident beams represents a major computational challenge for the clinical applications of this powerful treatment scheme. The purpose of this work is to provide a practical solution to the DASSIM-RT inverse planning problem.Methods:
The inverse planning problem is formulated as a fluence-map optimization problem with total-variation (TV) minimization. A newly released L1-solver, template for first-order conic solver (TFOCS), was adopted in this work. TFOCS achieves faster convergence with less memory usage as compared with conventional quadratic programming (QP) for the TV form through the effective use of conic forms, dual-variable updates, and optimal first-order approaches. As such, it is tailored to specifically address the computational challenges of large-scale optimization in DASSIM-RT inverse planning. Two clinical cases (a prostate and a head and neck case) are used to evaluate the effectiveness and efficiency of the proposed planning technique. DASSIM-RT plans with 15 and 30 beams are compared with conventional IMRT plans with 7 beams in terms of plan quality and delivery efficiency, which are quantified by conformation number (CN), the total number of segments and modulation index, respectively. For optimization efficiency, the QP-based approach was compared with the proposed algorithm for the DASSIM-RT plans with 15 beams for both cases.Results:
Plan quality improves with an increasing number of incident beams, while the total number of segments is maintained to be about the same in both cases. For the prostate patient, the conformation number to the target was 0.7509, 0.7565, and 0.7611 with 80 segments for IMRT with 7 beams, and DASSIM-RT with 15 and 30 beams, respectively. For the head and neck (HN) patient with a complicated target shape, conformation numbers of the three treatment plans were 0.7554, 0.7758, and 0.7819 with 75 segments for all beam configurations. With respect to the dose sparing to the critical structures, the organs such as the femoral heads in the prostate case and the brainstem and spinal cord in the HN case were better protected with DASSIM-RT. For both cases, the delivery efficiency has been greatly improved as the beam angular sampling increases with the similar or better conformal dose distribution. Compared with conventional quadratic programming approaches, first-order TFOCS-based optimization achieves far faster convergence and smaller memory requirements in DASSIM-RT.Conclusions:
The new optimization algorithm TFOCS provides a practical and timely solution to the DASSIM-RT or other inverse planning problem requiring large memory space. The new treatment scheme is shown to outperform conventional IMRT in terms of dose conformity to both the targetand the critical structures, while maintaining high delivery efficiency.
39(2012); http://dx.doi.org/10.1118/1.4728226View Description Hide DescriptionPurpose:
In this study, the authors introduce skew line needle configurations for high dose rate (HDR) brachytherapy and needle planning by integer program (NPIP), a computational method for generating these configurations. NPIP generates needle configurations that are specific to the anatomy of the patient, avoid critical structures near the penile bulb and other healthy structures, and avoid needle collisions inside the body.Methods:
NPIP consisted of three major components: a method for generating a set of candidate needles, a needle selection component that chose a candidate needle subset to be inserted, and a dose planner for verifying that the final needle configuration could meet dose objectives. NPIP was used to compute needle configurations for prostate cancer data sets from patients previously treated at our clinic. NPIP took two user-parameters: a number of candidate needles, and needle coverage radius, δ. The candidate needle set consisted of 5000 needles, and a range of δ values was used to compute different needle configurations for each patient. Dose plans were computed for each needle configuration. The number of needles generated and dosimetry were analyzed and compared to the physician implant.Results:
NPIP computed at least one needle configuration for every patient that met dose objectives, avoided healthy structures and needle collisions, and used as many or fewer needles than standard practice. These needle configurations corresponded to a narrow range of δ values, which could be used as default values if this system is used in practice. The average end-to-end runtime for this implementation of NPIP was 286 s, but there was a wide variation from case to case.Conclusions:
The authors have shown that NPIP can automatically generate skew line needle configurations with the aforementioned properties, and that given the correct input parameters, NPIP can generate needle configurations which meet dose objectives and use as many or fewer needles than the current HDR brachytherapy workflow. Combined with robot assisted brachytherapy, this system has the potential to reduce side effects associated with treatment. A physical trial should be done to test the implant feasibility of NPIP needle configurations.
39(2012); http://dx.doi.org/10.1118/1.4729737View Description Hide DescriptionPurpose:
Model-baseddose calculations (MBDCs) are performed using patient computed tomography(CT) data for patients treated with intraoperative125I lungbrachytherapy at the Mayo Clinic Rochester. Various metallic artifact correction and tissue assignment schemes are considered and their effects on dose distributions are studied. Dose distributions are compared to those calculated under TG-43 assumptions.Methods:
Dose distributions for six patients are calculated using phantoms derived from patient CT data and the EGSnrc user-code BrachyDose.125I (GE Healthcare/Oncura model 6711) seeds are fully modeled. Four metallic artifact correction schemes are applied to the CT data phantoms: (1) no correction, (2) a filtered back-projection on a modified virtual sinogram, (3) the reassignment of CT numbers above a threshold in the vicinity of the seeds, and (4) a combination of (2) and (3). Tissue assignment is based on voxel CT number and mass density is assigned using a CT number to mass density calibration. Three tissue assignment schemes with varying levels of detail (20, 11, and 5 tissues) are applied to metallic artifact corrected phantoms. Simulations are also performed under TG-43 assumptions, i.e., seeds in homogeneous water with no interseed attenuation.Results:
Significant dose differences (up to 40% for D90) are observed between uncorrected and metallic artifact corrected phantoms. For phantoms created with metallic artifact correction schemes (3) and (4), dose volume metrics are generally in good agreement (less than 2% differences for all patients) although there are significant local dose differences. The application of the three tissue assignment schemes results in differences of up to 8% for D90; these differences vary between patients. Significant dose differences are seen between fully modeled and TG-43 calculations with TG-43 underestimating the dose (up to 36% in D90) for larger volumes containing higher proportions of healthy lungtissue.Conclusions:
Metallic artifact correction is necessary for accurate application of MBDCs for lungbrachytherapy; simpler threshold replacement methods may be sufficient for early adopters concerned with clinical dose metrics. Rigorous determination of voxel tissue parameters and tissue assignment is required for accurate dose calculations as different tissue assignment schemes can result in significantly different dose distributions. Significant differences are seen between MBDCs and TG-43 dose distributions with TG-43 underestimating dose in volumes containing healthy lungtissue.
Comparison of action levels for patient-specific quality assurance of intensity modulated radiation therapy and volumetric modulated arc therapy treatments39(2012); http://dx.doi.org/10.1118/1.4729738View Description Hide DescriptionPurpose:
To perform a comprehensive and systematic comparison of fixed-beam IMRT and volumetric modulated arc therapy (VMAT) patient-specific QA measurements for a common set of geometries using typical measurement methods.Methods:
Fixed-beam IMRT and VMAT plans were constructed for structure set geometries provided by AAPM Task Group 119. The plans were repeatedly delivered across multiple measurement sessions, and the resulting dose distributions were measured with (1) radiochromic film and ionization chamber and (2) a commercial two-dimensional diode array. The resulting QA measurements from each delivery technique were then analyzed, compared, and tested for statistically significant differences.Results:
Although differences were noted between QA results for some plans, neither modality showed consistently better agreement of measured and planned doses: of the 22 comparisons, IMRT showed better QA results in 11 cases, and VMAT showed better QA results in 11 cases. No statistically significant differences (p < 0.05) between IMRT and VMAT QA results were found for point dosesmeasured with an ionization chamber, planar dosesmeasured with radiochromic film, or planar dosesmeasured with a two-dimensional diode array.Conclusions:
These results suggest that it is appropriate to apply patient-specific QA action levels derived from fixed-beam IMRT to VMAT.
An artificial neural network (ANN)-based lung-tumor motion predictor for intrafractional MR tumor tracking39(2012); http://dx.doi.org/10.1118/1.4730294View Description Hide DescriptionPurpose:
To address practical issues of implementing artificial neural networks (ANN) for lung-tumor motion prediction in MRI-based intrafractional lung-tumor tracking.Methods:
A feedforward four-layered ANN structure is used to predict future tumor positions. A back-propagation algorithm is used for ANN learning. Adaptive learning is incorporated by continuously updating weights and learning rate during prediction. An ANN training scheme specific for MRI-based tracking is developed. A multiple-ANN structure is developed to reduce tracking failures caused by the lower imaging rates of MRI. We used particle swarm optimization to optimize the ANN structure and initial weights (IW) for each patient and treatment fraction. Prediction accuracy is evaluated using the 1D superior–inferior lung-tumor motions of 29 lungcancer patients for system delays of 120–520 ms, in increments of 80 ms. The result is compared with four different scenarios: (1), (2) ANN structure optimization + with/without IW optimization, and (3), (4) no ANN structure optimization + with/without IW optimization, respectively. An additional simulation is performed to assess the value of optimizing the ANN structure for each treatment fraction.Results:
For 120–520 ms system delays, mean RMSE values (ranges 0.0–2.8 mm from 29 patients) of 0.5–0.9 mm are observed, respectively. Using patient specific ANN structures, a 30%–60% decrease in mean RMSE values is observed as a result of IW optimization, alone. No significant advantages in prediction performance are observed, however, by optimizing for each fraction.Conclusions:
A new ANN-based lung-tumor motion predictor is developed for MRI-based intrafractional tumor tracking. The prediction accuracy of our predictor is evaluated using a realistic simulated MR imaging rate and system delays. For 120–520 ms system delays, mean RMSE values of 0.5–0.9 mm (ranges 0.0–2.8 mm from 29 patients) are achieved. Further, the advantage of patient specific ANN structure and IW in lung-tumor motion prediction is demonstrated by a 30%–60% decrease in mean RMSE values.
Dosimetric characterization of a synthetic single crystal diamond detector in clinical radiation therapy small photon beams39(2012); http://dx.doi.org/10.1118/1.4729739View Description Hide DescriptionPurpose:
To determine the potentialities of synthetic single crystaldiamond Schottky diodes for accurate dosemeasurements in radiation therapy small photon beams.Methods:
The dosimetric properties of a diamond-based detector were assessed by comparison with a reference microionization chamber. The diamond device was operated at zero bias voltage under irradiation with high-energy radiotherapic photon beams. The stability of the detector response and its dose and dose rate dependence were measured. Different square field sizes ranging from 1 × 1 cm2 to 10 × 10 cm2 were used during comparative dose distribution measurements by means of percentage depth dose curves (PDDs), lateral beam profiles, and output factors. The angular and temperature dependence of the diamonddetector response were also studied.Results:
The detector response shows a deviation from linearity of less than ±0.5% in the 0.01–7 Gy range and dose rate dependence below ±0.5% in the 1–6 Gy/min range. PDDs and output factors are in good agreement with those measured by the reference ionization chamber within 1%. No angular dependence is observed by rotating the detector along its axis, while ∼3.5% maximum difference is measured by varying the radiation incidence angle in the polar direction. The temperature dependence was investigated as well and a ±0.2% variation of the detector response is found in the 18–40 °C range.Conclusions:
The obtained results indicate the investigated synthetic diamond-based detector as a candidate for small field clinical radiation dosimetry in advanced radiation therapy techniques.
Commissioning a CT-compatible LDR tandem and ovoid applicator using Monte Carlo calculation and 3D dosimetry39(2012); http://dx.doi.org/10.1118/1.4730501View Description Hide DescriptionPurpose:
To determine the geometric and dose attenuation characteristics of a new commercially available CT-compatible LDR tandem and ovoid (T&O) applicator using Monte Carlo calculation and 3D dosimetry.Methods:
For geometric characterization, we quantified physical dimensions and investigated a systematic difference found to exist between nominal ovoid angle and the angle at which the afterloading buckets fall within the ovoid. For dosimetric characterization, we determined source attenuation through asymmetric gold shielding in the buckets using Monte Carlo simulations and 3D dosimetry.Monte Carlo code MCNP5 was used to simulate 1.5 × 109photon histories from a 137Cs source placed in the bucket to achieve statistical uncertainty of 1% at a 6 cm distance. For 3D dosimetry, the distribution about an unshielded source was first measured to evaluate the system for 137Cs, after which the distribution was measured about sources placed in each bucket. Cylindrical PRESAGE®dosimeters (9.5 cm diameter, 9.2 cm height) with a central channel bored for source placement were supplied by Heuris Inc. The dosimeters were scanned with the Duke Large field of view Optical CT-Scanner before and after delivering a nominal dose at 1 cm of 5–8 Gy. During irradiation the dosimeter was placed in a water phantom to provide backscatter. Optical CT scan time lasted 15 min during which 720 projections were acquired at 0.5° increments, and a 3D distribution was reconstructed with a (0.05 cm)3 isotropic voxel size. The distributions about the buckets were used to calculate a 3D distribution of transmission rate through the bucket, which was applied to a clinical CT-based T&O implant plan.Results:
The systematic difference in bucket angle relative to the nominal ovoid angle (105°) was 3.1°–4.7°. A systematic difference in bucket angle of 1°, 5°, and 10° caused a 1% ± 0.1%, 1.7% ± 0.4%, and 2.6% ± 0.7% increase in rectal dose, respectively, with smaller effect to dose to Point A, bladder, sigmoid, and bowel. For 3D dosimetry, 90.6% of voxels had a 3Dγ-index (criteria = 0.1 cm, 3% local signal) below 1.0 when comparing measured and expected dose about the unshielded source. Dose transmission through the gold shielding at a radial distance of 1 cm was 85.9% ± 0.2%, 83.4% ± 0.7%, and 82.5% ± 2.2% for Monte Carlo, and measurement for left and right buckets, respectively. Dose transmission was lowest at oblique angles from the bucket with a minimum of 56.7% ± 0.8%, 65.6% ± 1.7%, and 57.5% ± 1.6%, respectively. For a clinical T&O plan, attenuation from the buckets leads to a decrease in average Point A dose of ∼3.2% and decrease in D2cc to bladder, rectum, bowel, and sigmoid of 5%, 18%, 6%, and 12%, respectively.Conclusions:
Differences between dummy and afterloading bucket position in the ovoids is minor compared to effects from asymmetric ovoid shielding, for which rectal dose is most affected. 3D dosimetry can fulfill a novel role in verifying Monte Carlo calculations of complex dose distributions as are common about brachytherapy sources and applicators.
39(2012); http://dx.doi.org/10.1118/1.4728979View Description Hide DescriptionPurpose:
Contouring a normal anatomical structure during radiation treatment planning requires significant time and effort. The authors present a fast and accurate semiautomatic contour delineation method to reduce the time and effort required of expert users.Methods:
Following an initial segmentation on one CT slice, the user marks the target organ and nontarget pixels with a few simple brush strokes. The algorithm calculates statistics from this information that, in turn, determines the parameters of an energy function containing both boundary and regional components. The method uses a conditional random field graphical model to define the energy function to be minimized for obtaining an estimated optimal segmentation, and a graph partition algorithm to efficiently solve the energy function minimization. Organ boundary statistics are estimated from the segmentation and propagated to subsequent images; regional statistics are estimated from the simple brush strokes that are either propagated or redrawn as needed on subsequent images. This greatly reduces the user input needed and speeds up segmentations. The proposed method can be further accelerated with graph-based interpolation of alternating slices in place of user-guided segmentation. CTimages from phantom and patients were used to evaluate this method. The authors determined the sensitivity and specificity of organ segmentations using physician-drawn contours as ground truth, as well as the predicted-to-ground truth surface distances. Finally, three physicians evaluated the contours for subjective acceptability. Interobserver and intraobserver analysis was also performed and Bland–Altman plots were used to evaluate agreement.Results:
Liver and kidney segmentations in patient volumetric CTimages show that boundary samples provided on a single CT slice can be reused through the entire 3D stack of images to obtain accurate segmentation. In liver, our method has better sensitivity and specificity (0.925 and 0.995) than region growing (0.897 and 0.995) and level set methods (0.912 and 0.985) as well as shorter mean predicted-to-ground truth distance (2.13 mm) compared to regional growing (4.58 mm) and level set methods (8.55 mm and 4.74 mm). Similar results are observed in kidney segmentation. Physician evaluation of ten liver cases showed that 83% of contours did not need any modification, while 6% of contours needed modifications as assessed by two or more evaluators. In interobserver and intraobserver analysis, Bland–Altman plots showed our method to have better repeatability than the manual method while the delineation time was 15% faster on average.Conclusions:
Our method achieves high accuracy in liver and kidney segmentation and considerably reduces the time and labor required for contour delineation. Since it extracts purely statistical information from the samples interactively specified by expert users, the method avoids heuristic assumptions commonly used by other methods. In addition, the method can be expanded to 3D directly without modification because the underlying graphical framework and graph partition optimization method fit naturally with the image grid structure.