Volume 38, Issue 4, April 2011
Index of content:
The terminal M.S. degree is no longer appropriate for students interested in a career in clinical medical physics in the United States38(2011); http://dx.doi.org/10.1118/1.3533901View Description Hide Description
- RADIATION THERAPY PHYSICS
Internal margin assessment using cine MRI analysis of deglutition in head and neck cancer radiotherapya)38(2011); http://dx.doi.org/10.1118/1.3560418View Description Hide DescriptionPurpose:
Intensity-modulated radiation therapy(IMRT) is a promising treatment modality for patients with head and neck cancer (HNC). The dose distributions from IMRT are static and, thus, are unable to account for variations and/or uncertainties in the relationship between the patient (region being treated) and the beam. Organ motion comprises one such source of this uncertainty, introduced by physiological variation in the position, size, and shape of organs during treatment. In the head and neck, the predominant source of this variation arises from deglutition (swallowing). The purpose of this study was to investigate whether cinematographic MRI (cine MRI) could be used to determine asymmetric (nonuniform) internal margin (IM) components of tumor planning target volumes based on the actual deglutition-induced tumor displacement.Methods:
Five head and neck cancer patients were set up in treatment position on a 3 T MRI scanner. Two time series of single-slice, sagittal, cine images were acquired using a 2D FLASH sequence. The first time series was a 12.8 min scan designed to capture the frequency and duration of deglutition in the treatment position. The second time series was a short, 15 s scan designed to capture the displacement of deglutition in the treatment position. Deglutition frequency and mean swallow duration were estimated from the long time series acquisition. Swallowing and resting (nonswallowing) events were identified on the short time series acquisition and displacement was estimated based on contours of gross tumor volume (GTV) generated at each time point of a particular event. A simple linear relationship was derived to estimate 1D asymmetric IMs in the presence of resting- and deglutition-induced displacement.Results:
Deglutition was nonperiodic, with frequency and duration ranging from 2.89–24.18 mHz and from 3.86 to 6.10 s, respectively. The deglutition frequency and mean duration were found to vary among patients. Deglutition-induced maximal GTV displacements ranged from 0.00 to 28.36 mm with mean and standard deviation of, , , and in the A, P, I, and S directions, respectively. Resting-induced maximal GTV displacement ranged from 0.00 to 5.59 mm with mean and standard deviation of , , , and in the A, P, I, and S directions, respectively. For both resting and swallowing states, displacement along the S-I direction dominated displacement along the A-P direction. The calculated IMs were dependent on deglutition frequency, ranging from 3.28–4.37 mm for the lowest deglutition frequency patient to 3.76–6.43 mm for the highest deglutition frequency patient. A statistically significant difference was detected between IMs calculated for P and S directions .Conclusions:
Cine MRI is able to capture tumor motion during deglutition. Swallowing events can be demarcated by MR signal intensity changes caused by anatomy containing fully relaxed spins that move medially into the imaging plane during deglutition. Deglutition is nonperiodic and results in dynamic changes in the tumor position. Deglutition-induced displacements are larger and more variable than resting displacements. The nonzero mean maximum resting displacement indicates that some tumor motion occurs even when the patient is not swallowing. Asymmetric IMs, derived from deglutition frequency, duration, and directional displacement, should be employed to account for tumor motion in HNC RT.
A spatially encoded dose difference maximal intensity projection map for patient dose evaluation: A new first line patient quality assurance tool38(2011); http://dx.doi.org/10.1118/1.3560424View Description Hide DescriptionPurpose:
To develop a spatially encoded dose difference maximal intensity projection (DD-MIP) as an online patient dose evaluation tool for visualizing the dose differences between the planning dose and dose on the treatment day.Methods:
Megavoltage cone-beam CT (MVCBCT) images acquired on the treatment day are used for generating the dose difference index. Each index is represented by different colors for underdose, acceptable, and overdose regions. A maximal intensity projection (MIP) algorithm is developed to compress all the information of an arbitrary 3D dose difference index into a 2D DD-MIP image. In such an algorithm, a distance transformation is generated based on the planning CT. Then, two new volumes representing the overdose and underdose regions of the dose difference index are encoded with the distance transformation map. The distance-encoded indices of each volume are normalized using the skin distance obtained on the planning CT. After that, two MIPs are generated based on the underdose and overdose volumes with green-to-blue and green-to-red lookup tables, respectively. Finally, the two MIPs are merged with an appropriate transparency level and rendered in planning CTimages.Results:
The spatially encoded DD-MIP was implemented in a dose-guidedradiotherapy prototype and tested on 33 MVCBCT images from six patients. The user can easily establish the threshold for the overdose and underdose. A 3% difference between the treatment and planning dose was used as the threshold in the study; hence, the DD-MIP shows red or blue color for the dose difference or , respectively. With such a method, the overdose and underdose regions can be visualized and distinguished without being overshadowed by superficial dose differences.Conclusions:
A DD-MIP algorithm was developed that compresses information from 3D into a single or two orthogonal projections while hinting the user whether the dose difference is on the skin surface or deeper.
38(2011); http://dx.doi.org/10.1118/1.3560876View Description Hide DescriptionPurpose:
The report issued by AAPM Task Group No. 119 outlined a procedure for evaluating the effectiveness of IMRT commissioning. The procedure involves measuring gamma pass-rate indices for IMRT plans of standard phantoms and determining if the results fall within a confidence limit set by assuming normally distributed data. As stated in the TG report, the assumption of normally distributed gamma pass rates is a convenient approximation for commissioning purposes, but may not accurately describe the data. Here the authors attempt to better describe gamma pass-rate data by fitting it to different distributions. The authors then calculate updated confidence limits using those distributions and compare them to those derived using TG No. 119 method.Methods:
Gamma pass-rate data from 111 head and neck patients are fitted using the TG No. 119 normal distribution, a truncated normal distribution, and a Weibull distribution. Confidence limits to 95% are calculated for each and compared. A more general analysis of the expected differences between the TG No. 119 method of determining confidence limits and a more time-consuming curve fitting method is performed.Results:
The TG No. 119 standard normal distribution does not fit the measured data. However, due to the small range of measured data points, the inaccuracy of the fit has only a small effect on the final value of the confidence limits. The confidence limits for the 111 patient plans are within 0.1% of each other for all distributions. The maximum expected difference in confidence limits, calculated using TG No. 119’s approximation and a truncated distribution, is 1.2%.Conclusions:
A three-parameter Weibull probability distribution more accurately fits the clinical gamma index pass-rate data than the normal distribution adopted by TG No. 119. However, the sensitivity of the confidence limit on distribution fit is low outside of exceptional circumstances.
38(2011); http://dx.doi.org/10.1118/1.3560417View Description Hide DescriptionPurpose:
To determine conditions under which hypofractionation could be favorable for a normal tissue—even if tumor exceeds the normal tissue’s .Methods:
The hypofractionation sufficiency condition (HSC) for an organ is defined as a dose conformality constraint such that, if satisfied, a family of tumor control probability isoeffective fractionation schemes will showdecreasing normal tissue complication probability with decreasing number of fractions.Results:
In the extended equivalent uniform dose (EUD) model [obtained by replacing dose with linear quadratic (LQ) 2 Gy equivalent dose], the HSC for a normal organ is proven to be satisfied if a suitably weighted average of the relative dose [hypofractionation sufficiency index (HSI)] is less than the ratio of normal tissue to tumor. The HSI is determined solely by dose distribution and the normal tissue volume factor, “.” If the HSC is satisfied for every normal tissue of concern, then there is a therapeutic gain with hypofractionation. The corresponding multifractionation sufficiency condition (therapeutic gain with increasing number of fractions) and multifractionation sufficiency index (MSI) are also derived. A sample clinical case is presented.Conclusions:
Within the context of the LQ/EUD models, conformality measures (HSI and MSI) can be used to inform fractionation decisions.
38(2011); http://dx.doi.org/10.1118/1.3560426View Description Hide DescriptionPurpose:
With the introduction of flattening filter free (FFF) linear accelerators to radiation oncology, new analytical source models for a FFF beam applicable to current treatment planning systems is needed. In this work, a multisource model for the FFF beam and the optimization of involved model parameters were designed.Methods:
The model is based on a previous three source model proposed byYang et al. [“A three-source model for the calculation of head scatter factors,” Med. Phys.29, 2024–2033 (2002)]. An off axis ratio (OAR) of photon fluence was introduced to the primary source term to generate cone shaped profiles. The parameters of the source model were determined from measured head scatter factors using a line search optimization technique. The OAR of the photon fluence was determined from a measured dose profile of a field size with the same optimization technique, but a new method to acquire gradient terms for OARs was developed to enhance the speed of the optimization process. The improved model was validated with measured dose profiles from to field sizes at 6 and 10 MV from a TrueBeam™ STx linear accelerator. Furthermore, planar dose distributions for clinically used radiation fields were also calculated and compared to measurements using a 2D array detector using the gamma index method.Results:
All dose values for the calculated profiles agreed with the measured dose profiles within 0.5% at 6 and 10 MV beams, except for some low dose regions for larger field sizes. A slight overestimation was seen in the lower penumbra region near the field edge for the large field sizes by 1%–4%. The planar dose calculations showed comparable passing rates when the criterion of the gamma index method was selected to be 3%/3 mm.Conclusions:
The developed source model showed good agreements between measured and calculated dose distributions. The model is easily applicable to any other linear accelerator using FFF beams as the required data include only the measured PDD, dose profiles, and output factors for various field sizes, which are easily acquired during conventional beam commissioning process.
Dosimetric accuracy of a deterministic radiation transport based brachytherapy treatment planning system. Part II: Monte Carlo and experimental verification of a multiple source dwell position plan employing a shielded applicator38(2011); http://dx.doi.org/10.1118/1.3567507View Description Hide DescriptionPurpose:
The aim of this work is the dosimetric validation of a deterministic radiation transport based treatment planning system (BRACHYVISION™ v. 8.8, referred to as TPS in the following) for multiple source dwell position brachytherapy applications employing a shielded applicator in homogeneous water geometries.Methods:
TPS calculations for an irradiation plan employing seven VS2000 high dose rate (HDR) source dwell positions and a partially shielded applicator (GM11004380) were compared to corresponding Monte Carlo(MC) simulation results, as well as experimental results obtained using the VIP polymer gel–magnetic resonance imaging three-dimensional dosimetry method with a custom made phantom.Results:
TPS and MCdose distributions were found in agreement which is mainly within ±2%. Considerable differences between TPS and MC results (greater than 2%) were observed at points in the penumbra of the shields (i.e., close to the edges of the “shielded” segment of the geometries). These differences were experimentally verified and therefore attributed to the TPS. Apart from these regions, experimental and TPS dose distributions were found in agreement within 2 mm distance to agreement and 5% dose difference criteria. As shown in this work, these results mark a significant improvement relative to dosimetry algorithms that disregard the presence of the shielded applicator since the use of the latter leads to dosimetry errors on the order of 20%–30% at the edge of the “unshielded” segment of the geometry and even 2%–6% at points corresponding to the potential location of the target volume in clinical applications using the applicator (points in the unshielded segment at short distances from the applicator).Conclusions:
Results of this work attest the capability of the TPS to accurately account for the scatter conditions and the increased attenuation involved in HDR brachytherapy applications employing multiple source dwell positions and partially shielded applicators.
Assessing software upgrades, plan properties and patient geometry using intensity modulated radiation therapy (IMRT) complexity metrics38(2011); http://dx.doi.org/10.1118/1.3562897View Description Hide DescriptionPurpose:
The aim of this study is to compare the sensitivity of different metrics to detect differences in complexity of intensity modulated radiation therapy(IMRT) plans following upgrades, changes to planning parameters, and patient geometry. Correlations between complexity metrics are also assessed.Method:
A program was developed to calculate a series of metrics used to describe the complexity of IMRT fields using monitor units (MUs) and multileaf collimator files: Modulation index (MI), modulation complexity score (MCS), and plan intensity map variation (PIMV). Each metric, including the MUs, was used to assess changes in beam complexity for six prostate patients, following upgrades in the inverse planning optimization software designed to incorporate direct aperture optimization (DAO). All beams were delivered to a 2D ionization chamber array and compared to those calculated using gamma analysis. Each complexity metric was then calculated for all beams, on a different set of six prostate IMRT patients, to assess differences between plans calculated using different minimum field sizes and different maximum segment numbers. Different geometries, including CShape, prostate, and head and neck phantoms, were also assessed using the metrics. Correlations between complexity metrics were calculated for 20 prostate IMRT patients.Results:
MU, MCS, MI, and PIMV could all detect reduced complexity following an upgrade to the optimization leaf sequencer, although only MI and MCS could detect a reduction in complexity when one-step optimization (DAO) was employed rather than two-step optimization. All metrics detected a reduction in complexity when the minimum field size was increased from 1 to 4 cm and all apart from PIMV detected reduced complexity when the number of segments was significantly reduced. All metrics apart from MI showed differences in complexity depending on the treatment site. Significant correlations exist between all metrics apart from MI and PIMV for prostate IMRT patients. Treatment deliverability appeared to be more correlated with MI and MCS than MU or PIMV.Conclusions:
The application of complexity metrics in the IMRTtreatment planning process has been demonstrated. Complexity of treatment plans can vary for different inverse planning software versions and can depend on planning parameters and the treatment site. MCS is most suitable for inclusion within the cost function to limit complexity in IMRT optimization due to its sensitivity to complexity changes and correlation to treatment deliverability.
Objective method to report planner-independent skin/rib maximal dose in balloon-based high dose rate (HDR) brachytherapy for breast cancer38(2011); http://dx.doi.org/10.1118/1.3568927View Description Hide DescriptionPurpose:
An objective method was proposed and compared with a manual selection method to determine planner-independent skin and rib maximal dose in balloon-based high dose rate (HDR) brachytherapy planning.Methods:
The maximal dose to skin and rib was objectively extracted from a dose volume histogram (DVH) of skin and rib volumes. A virtual skin volume was produced by expanding the skinsurface in three dimensions (3D) external to the breast with a certain thickness in the planning computed tomography(CT)images. Therefore, the maximal dose to this volume occurs on the skinsurface the same with a conventional manual selection method. The rib was also delineated in the planning CTimages and its maximal dose was extracted from its DVH. The absolute and relative maximal skin and rib dose differences between the manual selection method and the objective method were measured for 50 balloon-based HDR (25 MammoSite® and 25 Contura®) patients.Results:
The deviation of maximal dose difference was of the prescribed dose (PD). No statistical difference was observed between MammoSite® and Contura® patients for both Abdiff and Rediff[%] values. However, a statistically significant difference ( value ) was observed in maximal rib dose difference compared with maximal skindose difference for both Abdiff ( vs ) and Rediff[%] ( vs ). In general, rib has a more irregular contour and it is more proximally located to the balloon for 50 HDR patients. Due to the inverse square law factor, more dose difference was observed in higher dose range compared with lower dose range : vs with value of 0.0049. However, the Rediff[%] analysis eliminated the inverse square factor and there was no statistically significant difference between high and low dose ranges.Conclusions:
The objective method using volumetric information of skin and rib can determine the planner-independent maximal dose compared with the manual selection method. However, the difference was of PD, on average, if appropriate attention is paid to selecting a manual dose point in 3D planning CTimages.
38(2011); http://dx.doi.org/10.1118/1.3554647View Description Hide DescriptionPurpose:
The purpose of this work was to implement and validate a deformable CT to cone-beam computed tomography(CBCT)image registration method in head-and-neck cancer to eventually facilitate automatic target delineation on CBCT.Methods:
Twelve head-and-neck cancer patients underwent a planning CT and weekly CBCT during the 5–7 week treatment period. The 12 planning CTimages (moving images) of these patients were registered to their weekly CBCTimages (fixed images) via the symmetric force Demons algorithm and using a multiresolution scheme. Histogram matching was used to compensate for the intensity difference between the two types of images. Using nine known anatomic points as registration targets, the accuracy of the registration was evaluated using the target registration error (TRE). In addition, region-of-interest (ROI) contours drawn on the planning CT were morphed to the CBCTimages and the volume overlap index (VOI) between registered contours and manually delineated contours was evaluated.Results:
The mean TRE value of the nine target points was less than 3.0 mm, the slice thickness of the planning CT. Of the 369 target points evaluated for registration accuracy, the average TRE value was. The mean TRE for bony tissue targets was , while the mean TRE for soft tissue targets was . The average VOI between the registered and manually delineated ROI contours was , which is consistent with that reported in previous studies.Conclusions:
The authors have implemented and validated a deformable image registration method to register planning CTimages to weekly CBCTimages in head-and-neck cancer cases. The accuracy of the TRE values suggests that they can be used as a promising tool for automatic target delineation on CBCT.
A method for robust segmentation of arbitrarily shaped radiopaque structures in cone-beam CT projectionsa)38(2011); http://dx.doi.org/10.1118/1.3555295View Description Hide DescriptionPurpose:
Implanted markers are commonly used in radiotherapy for x-ray based target localization. The projected marker position in a series of cone-beam CT(CBCT) projections can be used to estimate the three dimensional (3D) target trajectory during the CBCT acquisition. This has important applications in tumor motion management such as motion inclusive, gating, and tumor tracking strategies. However, for irregularly shaped markers, reliable segmentation is challenged by large variations in the marker shape with projection angle. The purpose of this study was to develop a semiautomated method for robust and reliable segmentation of arbitrarily shaped radiopaque markers in CBCT projections.Methods:
The segmentation method involved the following three steps: (1) Threshold based segmentation of the marker in three to six selected projections with large angular separation, good marker contrast, and uniform background; (2) construction of a 3D marker model by coalignment and backprojection of the threshold-based segmentations; and (3) construction of marker templates at all imaging angles by projection of the 3D model and use of these templates for template-based segmentation. The versatility of the segmentation method was demonstrated by segmentation of the following structures in the projections from two clinical CBCT scans: (1) Three linear fiducial markers (Visicoil) implanted in or near a lung tumor and (2) an artificial cardiac valve in a lung cancer patient.Results:
Automatic marker segmentation was obtained in more than 99.9% of the cases. The segmentation failed in a few cases where the marker was either close to a structure of similar appearance or hidden behind a dense structure (data cable).Conclusions:
A robust template-based method for segmentation of arbitrarily shaped radiopaque markers in CBCT projections was developed.
38(2011); http://dx.doi.org/10.1118/1.3560425View Description Hide DescriptionPurpose:
To design and optimize a minibeam collimator for minibeam radiation therapy studies using a 250 kVp x-ray machine as a simulated synchrotron source.Methods:
A Philips RT250 orthovoltage x-ray machine was modeled using the EGSnrc/BEAMnrc Monte Carlo software. The resulting machine model was coupled to a model of a minibeam collimator with a beam aperture of 1 mm. Interaperture spacing and collimator thickness were varied to produce a minibeam with the desired peak-to-valley ratio.Results:
Proper design of a minibeam collimator with Monte Carlo methods requires detailed knowledge of the x-ray source setup. For a cathode-ray tube source, the beam spot size, target angle, and source shielding all determine the final valley-to-peak dose ratio.Conclusions:
A minibeam collimator setup was created, which can deliver a 30 Gy peak dose minibeam radiation therapytreatment at depths less than 1 cm with a valley-to-peak dose ratio on the order of 23%.
38(2011); http://dx.doi.org/10.1118/1.3567146View Description Hide DescriptionPurpose:
The dosimetric accuracy of the recently released Acuros® XB advanced dose calculation algorithm (Varian Medical Systems, Palo Alto, CA) is investigated for single radiation fields incident on homogeneous and heterogeneous geometries, and a comparison is made to the analytical anisotropic algorithm (AAA).Methods:
Ion chamber measurements for the 6 and 18 MV beams within a range of field sizes (from to ) are used to validate Acuros® XB dose calculations within a unit density phantom. The dosimetric accuracy of Acuros® XB in the presence of lung, low-density lung, air, and bone is determined using BEAMnrc/DOSXYZnrc calculations as a benchmark. Calculations using the AAA are included for reference to a current superposition/convolution standard.Results:
Basic open field tests in a homogeneous phantom reveal an Acuros® XB agreement with measurement to within ±1.9% in the inner field region for all field sizes and energies. Calculations on a heterogeneous interface phantom were found to agree with Monte Carlo calculations to within ±2.0% in lung and within ±2.9% in low-density lung. In comparison, differences of up to 10.2% and 17.5% in lung and low-density lung were observed in the equivalent AAA calculations. Acuros® XB dose calculations performed on a phantom containing an air cavity were found to be within the range of ±1.5% to ±4.5% of the BEAMnrc/DOSXYZnrc calculated benchmark in the tissue above and below the air cavity. A comparison of Acuros® XB dose calculations performed on a lungCT dataset with a BEAMnrc/DOSXYZnrc benchmark shows agreement within ±2%/2mm and indicates that the remaining differences are primarily a result of differences in physical material assignments within a CT dataset.Conclusions:
By considering the fundamental particle interactions in matter based on theoretical interaction cross sections, the Acuros® XB algorithm is capable of modeling radiotherapy dose deposition with accuracy only previously achievable with Monte Carlo techniques.
On the use of the MLC dosimetric leaf gap as a quality control tool for accurate dynamic IMRT delivery38(2011); http://dx.doi.org/10.1118/1.3567148View Description Hide DescriptionPurpose:
MLC leaf gap consistency is critical for the accurate delivery of dynamic IMRT plans. It is estimated that a systematic MLC leaf gap change of 0.6 mm will result in a 2% change to the equivalent uniform dose to a clinical target volume for a typical head and neck sliding window (SW) IMRT plan. The aim of this work is to use the measured dosimetric leaf gap (DLG) to verify the dosimetric reproducibility of dynamically delivered SW IMRT plans. This study focuses on Varian linacs equipped with the 120 Millennium MLC and the Eclipse™ treatment planning system (TPS), but can be extended to other linac/MLC/TPS combination.Methods:
An ionization chamber, a diode array, and an electronic portal imaging device(EPID) were used to assess the DLG in zero (central axis), one, and two dimensions, respectively. The DLG for zero and two dimensions was derived from measurements of SW fields of decreasing width (2, 1.5, 1, and 0.5 cm). The DLG in one dimension was measured directly from a single SW sweeping across a linear diode array. This one-dimensional DLG measurement was based on the full width at half maximum (FWHM) of the dose rate versus time spectrum.Results:
The DLG derived from ion chamber measurements at central axis agrees to within 0.1 mm, with the DLG measured directly from the FWHM of dose rate versus time spectrum. The measured DLG depends on the control points used for the MLC SW fields. When two control points were used, the DLG measured at central axis showed an increase of 0.6 mm with respect to the same measurements performed using three or more control points. The two-dimensional distribution of DLG obtained using the EPID identified leaf gap errors as small as ±0.2 mm in isolated areas away from central axis.Conclusions:
Comprehensive measurements of the DLG in 0D, 1D, and 2D provide an accurate assessment of DLG value required during TPS commissioning. These DLG measurements can also be used as a quality control tool to quantify changes of the MLCcalibration and leaf gap consistency, which is critical for the accurate delivery of dynamically delivered SW IMRT plans.
38(2011); http://dx.doi.org/10.1118/1.3566067View Description Hide DescriptionPurpose:
Helical tomotherapy is a complex delivery technique, integrating CT image guidance and intensity modulated radiotherapy in a single system. The integration of the CTdetector ring on the gantry not only allows patient position verification but is also often used to perform various QA procedures. This convenience lacks the rigor of a machine-independent QA process.Methods:
In this article, a Si strip detector, known as the Dose Magnifying Glass (DMG), was used to perform machine-independent QA measurements of the multileaf collimator alignment, leaf open time threshold, and leaf fluence output factor (LFOF).Results:
The DMG measurements showed good agreements with EDR2 film for the MLC alignment test while the CTdetector agrees well with DMG measurements for leaf open time threshold and LFOF measurements. The leaf open time threshold was found to be approximately 20 ms. The LFOF measured with the DMG agreed within error with the CTdetectormeasured LFOF.Conclusions:
The DMG with its 0.2 mm spatial resolution coupled to TERA ASIC allowed real-time high temporal resolution measurements of the tomotherapy leaf movement. In conclusion, DMG was shown to be a suitable tool for machine-independent QA of a tomotherapy unit.
38(2011); http://dx.doi.org/10.1118/1.3566016View Description Hide DescriptionPurpose:
To present a conceptually new method for metal artifact reduction (MAR) that can be used on patients with multiple objects within the scan plane that are also of small sized along the longitudinal (scanning) direction, such as dental fillings.Methods:
The proposed algorithm, named opposite view replacement, achieves MAR by first detecting the projection data affected by metal objects and then replacing the affected projections by the corresponding opposite view projections, which are not affected by metal objects. The authors also applied a fading process to avoid producing any discontinuities in the boundary of the affected projection areas in the sinogram. A skull phantom with and without a variety of dental metal inserts was made to extract the performance metric of the algorithm. A head and neck case, typical of IMRT planning, was also tested.Results:
The reconstructedCTimages based on this new replacement scheme show a significant improvement in image quality for patients with metallic dental objects compared to the MAR algorithms based on the interpolation scheme. For the phantom, the authors showed that the artifact reduction algorithm can efficiently recover the CT numbers in the area next to the metallic objects.Conclusions:
The authors presented a new and efficient method for artifact reduction due to multiple small metallic objects. The obtained results from phantoms and clinical cases fully validate the proposed approach.
38(2011); http://dx.doi.org/10.1118/1.3567498View Description Hide DescriptionPurpose:
To verify water equivalent path length (WEPL) before treatment in protonradiotherapy using time resolvedin vivo diode dosimetry.Methods:
Using a passively scattered range modulated proton beam, the output of a diode driving a fast current-to-voltage amplifier is recorded at a number of depths in a water tank. At each depth, a burst of overlapping single proton pulses is observed. The rms duration of the burst is computed and the resulting data set is fitted with a cubic polynomial.Results:
When the diode is subsequently set to an arbitrary depth and the polynomial is used as a calibration curve, the “unknown” depth is determined within 0.3 mm rms.Conclusions:
A diode or a diode array, placed (for instance) in the rectum in conjunction with a rectal balloon, can potentially determine the WEPL at that point, just prior to treatment, with submillimeter accuracy, allowing the beam energy to be adjusted. The associated unwanted dose is about 0.2% of a typical single fraction treatment dose.
Dose calculation for permanent prostate implants incorporating spatially anisotropic linearly time-resolving edema38(2011); http://dx.doi.org/10.1118/1.3568926View Description Hide DescriptionPurpose:
The objectives of this study were (i) to develop a dose calculation method for permanent prostate implants that incorporates a clinically motivated model for edema and (ii) to illustrate the use of the method by calculating the preimplant dosimetry error for a reference configuration of, , and seeds subject to edema-induced motions corresponding to a variety of model parameters.Methods:
A model for spatiallyanisotropic edema that resolves linearly with time was developed based on serial magnetic resonance imaging measurements made previously at our center to characterize the edema for a group of prostate implant patients [R. S. Sloboda et al., “Time course of prostatic edema post permanent seed implant determined by magnetic resonanceimaging,”Brachytherapy9, 354–361 (2010)]. Model parameters consisted of edema magnitude, , and period, . The TG-43 dose calculation formalism for a point source was extended to incorporate the edema model, thus enabling calculation via numerical integration of the cumulative dose around an individual seed in the presence of edema. Using an even power piecewise-continuous polynomial representation for the radial dose function, the cumulative dose was also expressed in closed analytical form. Application of the method was illustrated by calculating the preimplant dosimetry error, , in a volume for (Oncura 6711), (Theragenics 200), and (IsoRay CS-1) seeds arranged in the Radiological Physics Center test case 2 configuration for a range of edema relative magnitudes and periods . Results were compared to preimplant dosimetry errors calculated using a variation of the isotropic edema model developed by Chen et al. [“Dosimetric effects of edema in permanent prostate seed implants: A rigorous solution,” Int. J. Radiat. Oncol., Biol., Phys.47, 1405–1419 (2000)].Results:
As expected, for our edema model indicated underdosage in the calculation volume with a clear dependence on seed and calculation point positions, and increased with increasing values of and . Values of were generally larger near the ends of the virtual prostate in the RPC phantom compared with more central locations. For edema characteristics similar to the population average values previously measured at our center, i.e., and , mean values of in an axial plane located 1.5 cm from the center of the seed distribution were 8.3% for seeds, 7.5% for seeds, and 2.2% for seeds. Maximum values of in the same plane were about 1.5 times greater. Note that detailed results strictly apply only for loose seed implants where the seeds are fixed in tissue and move in synchrony with that tissue.Conclusions:
A dose calculation method for permanent prostate implants incorporating spatiallyanisotropic linearly time-resolving edema was developed for which cumulative dose can be written in closed form. The method yields values for that differ from those for spatially isotropic edema. The method is suitable for calculating pre- and postimplant dosimetry correction factors for clinical seed configurations when edema characteristics can be measured or estimated.
38(2011); http://dx.doi.org/10.1118/1.3569581View Description Hide DescriptionPurpose:
Current commercial treatment planning systems are not able to accurately predict output factors and calculate monitor units for proton fields. Patient-specific field output factors are thus determined by either measurements or empirical modeling based on commissioning data. The objective of this study is to commission output factors for uniform scanning beams utilized at the ProCure proton therapy centers.Methods:
Using water phantoms and a plane parallel ionization chamber, the authors first measured output factors with a fixed 10 cm diameter aperture as a function of proton range and modulation width for clinically available proton beams with ranges between 4 and 31.5 cm and modulation widths between 2 and 15 cm. The authors then measured the output factor as a function of collimated field size at various calibration depths for proton beams of various ranges and modulation widths. The authors further examined the dependence of the output factor on the scanning area (i.e., uncollimated proton field), snout position, and phantom material. An empirical model was developed to calculate the output factor for patient-specific fields and the model-predicted output factors were compared to measurements.Results:
The output factor increased with proton range and field size, and decreased with modulation width. The scanning area and snout position have a small but non-negligible effect on the output factors. The predicted output factors based on the empirical modeling agreed within 2% of measurements for all prostate treatment fields and within 3% for 98.5% of all treatment fields.Conclusions:
Comprehensive measurements at a large subset of available beam conditions are needed to commission output factors for proton therapy beams. The empirical modeling agrees well with the measured output factor data. This investigation indicates that it is possible to accurately predict output factors and thus eliminate or reduce time-consuming patient-specific output measurements for protontreatments.
- RADIATION IMAGING PHYSICS
Analysis of Fourier-domain task-based detectability index in tomosynthesis and cone-beam CT in relation to human observer performance38(2011); http://dx.doi.org/10.1118/1.3560428View Description Hide DescriptionPurpose:
Design and optimization of medical imagingsystems benefit from accurate theoretical modeling that identifies the physical factors governing image quality, particularly in the early stages of system development. This work extends Fourier metrics of imaging performance and detectability index to tomosynthesis and cone-beam CT(CBCT) and investigates the extent to which is a valid descriptor of task-based imaging performance as assessed by human observers.Methods:
The detectability index for tasks presented in 2D slices was derived from 3D cascaded systems analysis of tomosynthesis and CBCT. Anatomical background noisemeasured in a physical phantom presenting power-law spectral density was incorporated in the “generalized” noise-equivalent quanta. Theoretical calculations of were performed as a function of total angular extent of source-detector orbit ranging 10°–360° under two acquisition schemes: (i) Constant angular separation between projections (constant-), giving variable number of projections and dose vs and (ii) constant number of projections (constant-), giving constant dose (but variable angular sampling) with . Five simple observer models were investigated: Prewhitening (PW), prewhitening with eye filter and internal noise (PWEi), nonprewhitening (NPW), nonprewhitening with eye filter (NPWE), and nonprewhitening with eye filter and internal noise (NPWEi). Human observer performance was measured in 9AFC tests for five simple imaging tasks presented within uniform and power-law clutter backgrounds. Measurements (from 9AFC tests) and theoretical calculations (from cascaded systems analysis of ) were compared in terms of area under the ROC curveResults:
Reasonable correspondence between theoretical calculations and human observer performance was achieved for all imaging tasks over the broad range of experimental conditions and acquisition schemes. The PW and PWEi observer models tended to overestimate detectability, while the various NPW models predicted observer performance fairly well, with NPWEi giving the best overall agreement. Detectability was shown to increase with due to the reduction of out-of-plane clutter, reaching a plateau after a particular that depended on the imaging task. Depending on the acquisition scheme, however (i.e., constant- or ), detectability was seen in some cases to decline at higher due to tradeoffs among quantum noise, background clutter, and view sampling.Conclusions:
Generalized detectability index derived from a 3D cascaded systems model shows reasonable correspondence with human observer performance over a fairly broad range of imaging tasks and conditions, although discrepancies were observed in cases relating to orbits intermediate to 180° and 360°. The basic correspondence of theoretical and measured performance supports the application of such a theoretical framework for system design and optimization of tomosynthesis and CBCT.