Dose Difference Criteria Research Articles

3D-printed applicators for high dose rate brachytherapy: Dosimetric assessment at different infill percentage

PurposeDosimetric assessment of high dose rate (HDR) brachytherapy applicators, printed in 3D with acrylonitrile butadiene styrene (ABS) at different infill percentage. Materials and methodsA low-cost, desktop, 3D printer (Hamlet 3DX100, Hamlet, Dublin, IE) was used for manufacturing simple HDR applicators, reproducing typical geometries in brachytherapy: cylindrical (common in vaginal treatment) and flat configurations (generally used to treat superficial lesions). Printer accuracy was investigated through physical measurements. The dosimetric consequences of varying the applicator’s density by tuning the printing infill percentage were analysed experimentally by measuring depth dose profiles and superficial dose distribution with Gafchromic EBT3 films (International Specialty Products, Wayne, NJ). Dose distributions were compared to those obtained with a commercial superficial applicator. ResultsMeasured printing accuracy was within 0.5mm. Dose attenuation was not sensitive to the density of the material. Surface dose distribution comparison of the 3D printed flat applicators with respect to the commercial superficial applicator showed an overall passing rate greater than 94% for gamma analysis with 3% dose difference criteria, 3mm distance-to-agreement criteria and 10% dose threshold. ConclusionLow-cost 3D printers are a promising solution for the customization of the HDR brachytherapy applicators. However, further assessment of 3D printing techniques and regulatory materials approval are required for clinical application.

An effective calibration technique for radiochromic films using a single-shot dose distribution in Gamma Knife®

PurposeA method of calibrating radiochromic films for Gamma Knife® (GK) dosimetry was developed. The applicability and accuracy of the new method were examined. MethodsThe dose distribution for a sixteen millimeter single-shot from a GK was built using a reference film that was calibrated using the conventional multi-film calibration (MFC) method. Another film, the test film, from a different set of films was irradiated under the same conditions as the reference film. The calibration curve for the second set of films was obtained by assigning the dose distribution of the reference film to the optical density of the test film, point by point. To assess the accuracy of this single-film calibration (SFC) method, differences between gamma index pass rates (GIPRs) were calculated. ResultsThe SFC curves were successfully obtained with estimated errors of 1.46%. GIPRs obtained with the SFC method for films irradiated using a single-shot showed differences less than one percentage point when dose difference criterion (ΔD) was 2% and the distance to agreement criterion (Δd) was 1 mm. The GIPRs of the SFC method when the films were irradiated following a virtual target treatment plan were consistent with the GIPRs of the MFC method, with differences of less than 0.2 percentage points for ΔD = 1% and Δd = 1 mm. ConclusionThe accuracy of the SFC method is comparable to that of conventional multi-film calibration method for GK film dosimetry.

Dose distributions verification for High dose rate brachytherapy plans by using ionization chambers 2D array

The purpose of this paper is to perform dosimetric verification (in-phantom) of dose distributions calculated with treatment planning system (TPS) by using 2D chamber array device in HDR brachytherapy. HDR brachytherapy treatment plans’dose distribution verification is performed using the two-dimensional (2D) ionization chamber array MatriXX Evolution developed by IBA Dosimetry (IBA Dosimetry, Germany) whose detector area is covered with the Nucletron Freiburg Flap Applicator Set (Nucletron BV,Veenendaal, the Netherlands) with catheters such that the detector plane was set to 0.86 cm from the catheter plane. Fixed slabs of RW3 (Perspex) were added below the 2D-ARRAY to provide full scattering conditions. The phantom was scanned on computed tomography (CT) for treatment planning with 2.5-mm slice thickness. Based on the CT data of the phantom, three different plans were calculated by the planning system (Oncentrabrachy version 4.3, Nucletron BV) and then are exported to the VERISOFT software for comparison with measured data. For comparison of dose distributions, both dose planes – measured & calculated – were normalized  to the global maximum dose of the reference matrix (measured data set) and compared using the Gamma index method. Gamma indexes were evaluated using a dose-difference criterion of 3% and a distance criterion of 3 mm (γ≤1).The total number of evaluated dose points for the vault test case is 9755, 98.6 % of them (9623 point) passed the criteria of acceptability (3% delta dose and 3-mm distance criteria) and 1.4% of them (132 point) failed it. The total number of evaluated dose points for the full test case is 19964, 93.6 % of them (18683 point) passed  the criteria of acceptability and 6.4% of them (1281 point) failed it. And the total number of evaluated dose points for the cylinder test case is 19871, 96.9 % of them (19258 point) passed the criteria of acceptability and 3.1% of them (613 point) failed it. By thisThe use of the two-dimensional (2D) ionization chamber array MatriXX Evolution for brachytherapy applications has been successfully demonstrated. The array measurements in these planes have shown acceptable agreement with the TPS, generally within 3% delta dose and 3-mm distance agreement criteria within each plane. The comparisons made led to make a relation between the passing percentage values to the number of the evaluated dose points for each test case and it was found that as the number of these pixels increases the possibility of having more failing points increases.

Verification of the pure alanine in PMMA tube dosimeter applicability for dosimetry of radiotherapy photon beams: a feasibility study.

Alanine dosimeters in the form of pure alanine powder in PMMA plastic tubes were investigated for dosimetry in a clinical application. Electron paramagnetic resonance (EPR) spectroscopy was used to measure absorbed radiation doses by detection of signals from radicals generated in irradiated alanine. The measurements were performed for low-dose ranges typical for single-fraction doses often used in external photon beam radiotherapy. First, the dosimeters were irradiated in a solid water phantom to establish calibration curves in the dose range from 0.3 to 3 Gy for 6 and 18 MV X-ray beams from a clinical linear accelerator. Next, the dosimeters were placed at various locations in an anthropomorphic pelvic phantom to measure the dose delivery of a conventional four-field box technique treatment plan to the pelvis. Finally, the doses measured with alanine dosimeters were compared against the doses calculated with a commercial treatment planning system (TPS). The results showed that the alanine dosimeters have a highly sensitive dose response with good linearity and no energy dependence in the dose range and photon beams used in this work. Also, a fairly good agreement was found between the in-phantom dose measurements with alanine dosimeters and the TPS dose calculations. The mean value of the ratios of measured to calculated dose values was found to be near unity. The measured points in the in-field region passed dose-difference acceptance criterion of 3% and those in the penumbral region passed distance-to-agreement acceptance criterion of 3 mm. These findings suggest that the pure alanine powder in PMMA tube dosimeter is a suitable option for dosimetry of radiotherapy photon beams.

Breaking bad IMRT QA practice.

Agreement between planned and delivered dose distributions for patient‐specific quality assurance in routine clinical practice is predominantly assessed utilizing the gamma index method. Several reports, however, fundamentally question current IMRT QA practice due to poor sensitivity and specificity of the standard gamma index implementation. An alternative is to employ dose volume histogram (DVH)‐based metrics. An analysis based on the AAPM TG 53 and ESTRO booklet No.7 recommendations for QA of treatment planning systems reveals deficiencies in the current “state of the art” IMRT QA, no matter which metric is selected. The set of IMRT benchmark plans were planned, delivered, and analyzed by following guidance of the AAPM TG 119 report. The recommended point dose and planar dose measurements were obtained using a PinPoint ionization chamber, EDR2 radiographic film, and a 2D ionization chamber array. Gamma index criteria {3%(global),3 mm} and {3%(global),3 mm} were used to assess the agreement between calculated and delivered planar dose distributions. Next, the AAPM TG 53 and ESTRO booklet No.7 recommendations were followed by dividing dose distributions into four distinct regions: the high‐dose (HD) or umbra region, the high‐gradient (HG) or penumbra region, the medium‐dose (MD) region, and the low‐dose (LD) region. A different gamma passing criteria was defined for each region, i.e., a “divide and conquer” (D&C) gamma method was utilized. The D&C gamma analysis was subsequently tested on 50 datasets of previously treated patients. Measured point dose and planar dose distributions compared favorably with TG 119 benchmark data. For all complex tests, the percentage of points passing the conventional {3%(global),3 mm} gamma criteria was 97.2%±3.2% and 95.7%±1.2% for film and 2D ionization chamber array, respectively. By dividing 2D ionization chamber array dose measurements into regions and applying 3 mm isodose point distance and variable local point dose difference criteria of 7%, 15%, 25%, and 40% for HD, HG, MD, and LD regions, respectively, a 93.4%±2.3% gamma passing rate was obtained. Identical criteria applied using the D&C gamma technique on 50 clinical treatment plans resulted in a 97.9%±2.3% gamma passing score. Based on the TG 119 standard, meeting or exceeding the benchmark results would indicate an exemplary IMRT QA program. In contrast to TG 119 analysis, a different scrutiny on the same set of data, which follows the AAPM TG 53 and ESTRO booklet No.7 guidelines, reveals a much poorer agreement between calculated and measured dose distributions with large local point dose differences within different dose regions. This observation may challenge the conventional wisdom that an IMRT QA program is producing acceptable results.PACS number: 87.55.Qr

SU‐E‐T‐247: Multi‐Leaf Collimator Model Adjustments Improve Small Field Dosimetry in VMAT Plans

Purpose:The Elekta beam modulator linac employs a 4‐mm micro multileaf collimator (MLC) backed by a fixed jaw. Out‐of‐field dose discrepancies between treatment planning system (TPS) calculations and output water phantom measurements are caused by the 1‐mm leaf gap required for all moving MLCs in a VMAT arc. In this study, MLC parameters are optimized to improve TPS out‐of‐field dose approximations.Methods:Static 2.4 cm square fields were created with a 1‐mm leaf gap for MLCs that would normally park behind the jaw. Doses in the open field and leaf gap were measured with an A16 micro ion chamber and EDR2 film for comparison with corresponding point doses in the Pinnacle TPS. The MLC offset table and tip radius were adjusted until TPS point doses agreed with photon measurements. Improvements to the beam models were tested using static arcs consisting of square fields ranging from 1.6 to 14.0 cm, with 45° collimator rotation, and 1‐mm leaf gap to replicate VMAT conditions. Gamma values for the 3‐mm distance, 3% dose difference criteria were evaluated using standard QA procedures with a cylindrical detector array.Results:The best agreement in point doses within the leaf gap and open field was achieved by offsetting the default rounded leaf end table by 0.1 cm and adjusting the leaf tip radius to 13 cm. Improvements in TPS models for 6 and 10 MV photon beams were more significant for smaller field sizes 3.6 cm or less where the initial gamma factors progressively increased as field size decreased, i.e. for a 1.6cm field size, the Gamma increased from 56.1% to 98.8%.Conclusion:The MLC optimization techniques developed will achieve greater dosimetric accuracy in small field VMAT treatment plans for fixed jaw linear accelerators. Accurate predictions of dose to organs at risk may reduce adverse effects of radiotherapy.

The impact of low-Z and high-Z metal implants in IMRT: A Monte Carlo study of dose inaccuracies in commercial dose algorithms

The aim of the study was to evaluate the dosimetric impact of low-Z and high-Z metallic implants on IMRT plans. Computed tomography (CT) scans of three patients were analyzed to study effects due to the presence of Titanium (low-Z), Platinum and Gold (high-Z) inserts. To eliminate artifacts in CT images, a sinogram-based metal artifact reduction algorithm was applied. IMRT dose calculations were performed on both the uncorrected and corrected images using a commercial planning system (convolution/superposition algorithm) and an in-house Monte Carlo platform. Dose differences between uncorrected and corrected datasets were computed and analyzed using gamma index (Pγ<1) and setting 2 mm and 2% as distance to agreement and dose difference criteria, respectively. Beam specific depth dose profiles across the metal were also examined. Dose discrepancies between corrected and uncorrected datasets were not significant for low-Z material. High-Z materials caused under-dosage of 20%-25% in the region surrounding the metal and over dosage of 10%-15% downstream of the hardware. Gamma index test yielded Pγ<1>99% for all low-Z cases; while for high-Z cases it returned 91% < Pγ<1< 99%. Analysis of the depth dose curve of a single beam for low-Z cases revealed that, although the dose attenuation is altered inside the metal, it does not differ downstream of the insert. However, for high-Z metal implants the dose is increased up to 10%-12% around the insert. In addition, Monte Carlo method was more sensitive to the presence of metal inserts than superposition/convolution algorithm. The reduction in terms of dose of metal artifacts in CT images is relevant for high-Z implants. In this case, dose distribution should be calculated using Monte Carlo algorithms, given their superior accuracy in dose modeling in and around the metal. In addition, the knowledge of the composition of metal inserts improves the accuracy of the Monte Carlo dose calculation significantly.

Evaluation of an aSi‐EPID with flattening filter free beams: Applicability to the GLAaS algorithm for portal dosimetry and first experience for pretreatment QA of RapidArc

To demonstrate the feasibility of portal dosimetry with an amorphous silicon mega voltage imager for flattening filter free (FFF) photon beams by means of the GLAaS methodology and to validate it for pretreatment quality assurance of volumetric modulated arc therapy (RapidArc). The GLAaS algorithm, developed for flattened beams, was applied to FFF beams of nominal energy of 6 and 10 MV generated by a Varian TrueBeam (TB). The amorphous silicon electronic portal imager [named mega voltage imager (MVI) on TB] was used to generate integrated images that were converted into matrices of absorbed dose to water. To enable GLAaS use under the increased dose-per-pulse and dose-rate conditions of the FFF beams, new operational source-detector-distance (SDD) was identified to solve detector saturation issues. Empirical corrections were defined to account for the shape of the profiles of the FFF beams to expand the original methodology of beam profile and arm backscattering correction. GLAaS for FFF beams was validated on pretreatment verification of RapidArc plans for three different TB linacs. In addition, the first pretreatment results from clinical experience on 74 arcs were reported in terms of γ analysis. MVI saturates at 100 cm SDD for FFF beams but this can be avoided if images are acquired at 150 cm for all nominal dose rates of FFF beams. Rotational stability of the gantry-imager system was tested and resulted in a minimal apparent imager displacement during rotation of 0.2 ± 0.2 mm at SDD = 150 cm. The accuracy of this approach was tested with three different Varian TrueBeam linacs from different institutes. Data were stratified per energy and machine and showed no dependence with beam quality and MLC model. The results from clinical pretreatment quality assurance, provided a gamma agreement index (GAI) in the field area for six and ten FFF beams of (99.8 ± 0.3)% and (99.5 ± 0.6)% with distance to agreement and dose difference criteria set to 3 mm/3% with 2 mm/2% thresholds, GAI resulted (95.7.0 ± 2.3)% and (97.2 ± 2.1)%. The GLAaS methodology, introduced in clinical practice for conventional flattened photon beams for machine, IMRT, and RapidArc quality assurance, was successfully adapted for FFF beams of Varian TrueBeam Linac. The detector saturation effects could be eliminated if the portal images acquired at 150 cm for all nominal dose rates of FFF beams.

Does the γ dose distribution comparison technique default to the distance to agreement test in clinical dose distributions?

To determine the validity of the assumption that the γ dose distribution comparison tool defaults to the distance to agreement test under conditions of clinically relevant steep dose gradients and γ test criteria. The assumption was tested by computing the angle θ between the dose axis and γ vector for clinical treatment plans. θ was a function of the evaluated dose distribution dose gradient and the ratio (α) of the dose difference to distance to agreement (DTA) criteria. Dose distributions from prostate, head and neck, and lung clinical treatment plans were examined: 50 treatment plans were selected for each of the prostate and head neck sites and 27 treatment plans were selected for lung. Dose-gradient histograms were prepared for each of the treatment plans using α = 1%/mm (e.g., 3%, 3 mm dose difference and DTA criteria, respectively). To determine how frequently different values of α were used in publications, papers that referenced the original γ paper were examined to identify the dose difference and DTA criteria used in those publications. In order to compare θ calculated using α = 1%/mm to θ for other values of α, the relationship between θ and α was determined. For most of the targets and critical structures, the maximum value of θ approached 90°, so the assumption that the γ tool defaulted to the DTA test in steep dose gradients was correct. Most of the published papers using the γ tool employed the 3%, 3 mm dose difference and DTA criteria, respectively. Most of the other evaluations used criteria such that α ≥ 1%/mm, so the conclusions relating to the examined dose distributions applied. There were a few papers employing very small values of α (including one where α = 0.17%/mm), breaking the assumption that the γ dose comparison tool defaulted to the DTA tool in steep dose gradients. Most published cases utilized values of α ≥ 1%/mm, and for those the implicit assumption that the γ dose comparison tool defaults to the DTA test in steep dose gradient regions was true. There were a few cases for which α was small enough to potentially invalidate this assumption. Care should be taken by investigators when selecting the γ test criteria to assure that the γ test will appropriately default to the DTA test in the steepest dose gradients being evaluated.

MO‐D‐108‐04: Validation of a Simple Portal Dose Calculation Model for Plan QA and In‐Vivo Dosimetry

Purpose: To validate a simple portal dose calculator for plan QA and in‐vivo dosimetry. Methods: We model portal dose as a function of the fluence map, patient attenuation, patient scatter and portal response. Fluence maps are reconstructed using control‐point sequence in RTPlan. Patient attenuation is calculated via ray‐tracing through the patient CT. The effect of patient scatter and portal response is modeled by convolution, where the convolution kernel is derived from the commissioning measurements of different beam energies, different field sizes, different phantom thickness, and different source to image distances (SIDs). For various IMRT/3D plan, phantom and patient geometry, both in‐air and in‐transit portals were calculated. The calculations were compared with portal measurements. The Gamma Index of measurements against predicted portals with various dose difference (DD) criteria (1%, 2%, 3%, 4%, 5%, etc) and distance to agreement (DTA) criteria (1 mm, 2 mm, 3 mm, 4 mm, 5 mm, etc) were calculated. The Gamma pass rates of various DD and DTA criteria were evaluated and formed a Gamma table. Results: For various IMRT beams, the head, body and lung phantoms, the in‐air and in‐transit portal calculations matched well with portal measurements. The Gamma pass rates for in‐air portal are above 97% for 2 mm, 2% criteria and above 99% for 3 mm, 3% criteria. The Gamma pass rates for in‐transit portal were above 90% for 2 mm, 2% criteria and above 95% for 3 mm, 3% criteria. Conclusion: The simple portal dose calculation model is validated via phantom measurements. The model could be used in clinic for in‐air and intransit portal prediction.

SU‐E‐T‐20: Validating That the Gamma Dose Distribution Comparison Defaults to the Distance‐To‐Agreement (DTA) Test in Steep Dose Gradients for Clinical Dose Distributions

Purpose: To test the validity of the central assumption that the gamma dose comparison test defaults to a distance‐to‐agreement (DTA) test in steep dose gradient regions of clinical dose distributions for typically employed dose difference and DTA criteria. Methods: The angle θ between the dose axis and the gamma vector was computed usingθ = tan−1(Λ), where Λ was the local dose gradient of the evaluated dose distribution. θ depended only on the ratio α between the two test criteria and its computation required only one dose distribution. While the most common test criteria were 3%/3mm (α = 1%/mm), we also evaluated 2%/3mm (α = 0.67%/mm) and 3%/2mm (α = 1.5%/mm). Clinical treatment plans were used to calculate θ using the three values of α. 50 cases of both prostate and head and neck and 27 cases for lung cancer were examined. The planning target volumes and selected critical structures were examined. The maximum values of θ were determined for these structures to determine if these maximum θ angles were close to 90 degrees for the selected a values. Results: Most examined targets and critical structures exhibited large values of θ. The exception was for some parotid glands where the doses were very low and relatively homogeneous. The fact that we found that most structures had at least some voxels with sufficiently large dose gradients to yield θ values near 90 degrees validated our hypothesis that for steep gradients, the gamma test defaulted to distance‐to‐agreement test for clinical dose gradients. Conclusion: The implicit assumption that the gamma dose comparison tool defaults to the DTA test in steep dose gradient regions was validated for prostate, had‐and‐neck, and lung cancer sites. While dose distributions from other sites have not been examined, it is likely that they too will yield similar results.

SU-E-J-30: Using Shifting Planned Dose Matrix to Evaluate Daily Dose Changes for IMRT Prostate Treatment.

Summation of daily DVH from KV-cone beam CT (KV-CBCT) to obtain a composite dose volume histogram (DVH) is challenging. Directly translating the planned dose matrix according to measured daily prostate displacements provided a common reference frame for a composite DVH from daily DVHs. The purpose of this study is to evaluate the shifting planned dose matrix method compared to the dose recalculation method using daily KV-CBCT. Six patients, who received concurrent IMRT treatment for prostate and pelvic lymph nodes with 124 daily CBCTs, were selected for this study. Contours for CBCT's were transferred from the planning CT after soft tissue registration for prostate and bony registration for pelvic lymph nodes. Using the same planning beam configurations, we re-calculated doses for these CBCTs after shifting to corrected treatment isocenters. The planned dose matrix translation was performed by an in-house program written in MATLAB and incorporated with Computational Environment for Radiotherapy Research (CERR) software. The corresponding daily DVH was obtained by shifting the planned dose matrix according to shifts of treatment iso-centers. To compare these two methods, selected endpoint doses for tumor targets and sensitive structures were extracted from DVHs. For prostate displacement less then 1.5 cm, the dose matrix shifting method resulted in 93% and 98% fractions within 5% differences from the recalculation method for D95 of prostate and pelvic lymph nodes, respectively. These numbers decreased to 58% and 71% when 2% dose difference criterion was used. Allowing 5% daily dose difference, shifting planned dose matrix provides effective means to evaluating daily dose changes for concurrent IMRT treatment for prostate and pelvic lymph nodes. The utility of this tool is to provide a common coordinate frame to obtain composite dose distributions.

A novel technique to enable experimental validation of deformable dose accumulation

To propose a novel technique to experimentally validate deformable dose algorithms by measuring 3D dose distributions under the condition of deformation using deformable gel dosimeters produced by a novel gel fabrication method. Five gel dosimeters, two rigid control gels and three deformable gels, were manufactured and treated with the same conformal plan that prescribed 400 cGy to the isocenter. The control gels were treated statically; the deformable gels were treated while being compressed by an actuation device to simulate breathing motion (amplitude of compression = 1, 1.5, and 2 cm, respectively; frequency = 16 rpm). Comparison between the dose measured by the control gels and the corresponding static dose distribution calculated in the treatment planning system (TPS) has determined the intrinsic dose measurement uncertainty of the gel dosimeters. Doses accumulated using MORFEUS, a biomechanical model-based deformable registration and dose accumulation algorithm, were compared with the doses measured by the deformable gel dosimeters to verify the accuracy of MORFEUS using dose differences at each voxel as well as the gamma index test. Flexible plastic wraps were used to contain and protect the deformable gels from oxygen infiltration, which inhibits the gels' dose sensitizing ability. Since the wraps were imperfect oxygen barrier, dose comparison between MORFEUS and the deformable gels was performed only in the central region with a received dose of 200 cGy or above to exclude the peripheral region where oxygen penetration had likely affected dose measurements. Dose measured with the control gels showed that the intrinsic dose measurement uncertainty of the gel dosimeters was 11.8 cGy or 4.7% compared to the TPS. The absolute mean voxel-by-voxel dose difference between the accumulated dose and the dose measured with the deformable gels was 4.7 cGy (SD = 36.0 cGy) or 1.5% (SD = 13.4%) for the three deformable gels. The absolute mean vector distance between the 250, 300, 350, and 400 cGy isodose surfaces on the accumulated and measured distributions was 1.2 mm (SD < 1.5 mm). The gamma index test that used the dose measurement precision of the control gels as the dose difference criterion and 2 mm as the distance criterion was performed, and the average pass rate of the accumulated dose distributions for all three deformable gels was 92.7%. When the distance criterion was relaxed to 3 mm, the average pass rate increased to 96.9%. This study has proposed a novel technique to manufacture deformable volumetric gel dosimeters. By comparing the doses accumulated in MORFEUS and the doses measured with the dosimeters under the condition of deformation, the study has also demonstrated the potential of using deformable gel dosimetry to experimentally validate algorithms that include deformations into dose computation. Since dose less than 200 cGy was not evaluated in this study, future investigations will focus more on low dose regions by either using bigger gel dosimeters or prescribing a lower dose to provide a more complete experimental validation of MORFEUS across a wider dose range.

Modification of the gamma function for the recognition of over- and under-dose regions in three dimensions

In order to evaluate two-dimensional radiation dose distributions, an algorithm called the Gamma function has recently been modified. The current study concentrates on modification of the gamma function as a three-dimensional dose distribution evaluation tool, and includes the recognition of over-dose/under-dose areas. Using a sign term, the conventional gamma function separates the disagreed areas into two parts: over-dose and under-dose areas. The new gamma function was modified using an extension of the dose difference criterion, ΔD, from two dimensions into three dimensions. In order to provide two-dimensional dose maps for analysis, several images were acquired for a range of regular and irregular radiation fields using a Scanning Liquid Ionization Chamber Electronic Portal Imaging Device. The raw images were then converted into two-dimensional transmitted dose maps using an empirical method. They were utilized as reference dose maps. Translational and rotational manipulations were performed on the reference dose distribution maps to provide evaluated dose maps. The reference and evaluated dose maps were then compared using conventional and modified gamma tools. The results indicated that the modified algorithm is able to enhance the over- and under-dose regions. In addition, a slight increase of the agreement percentage for reference and evaluated dose maps were observed by the extension of ΔD to three dimensions. It is concluded that the modified method is more realistic and applicable for the evaluation of both two-dimensional and three-dimensional dose distributions.

Toward a better understanding of the gamma index: Investigation of parameters with a surface‐based distance methoda)

The purpose of this work was to clarify the interactions between the parameters used in the γ index with the surface-based distance method, which itself can be viewed as a generalized version of the γ index. The examined parameters included the distance to agreement (DTA)/dose difference (DD) criteria, the percentage used as a passing criterion, and the passing percentage for given DTA/DD criteria. The specific aims of our work were (1) to understand the relationships between the parameters used in the γ index, (2) to determine the detection limit, or the minimum detectable error, of the γ index with a given set of parameters, and (3) to establish a procedure to determine parameters that are consistent with the capacity of an IMRT QA system. The surface-based distance technique with dose gradient factor was derived, and then the relationship between surface-based distance and γ index was established. The dose gradient factor for plans and measurements of 10 IMRT patients, 10 spine stereotactic radiosurgery (SRS) patients, and 3 Radiological Physics Center (RPC) head and neck phantom were calculated and evaluated. The detection limits of the surface-based distance and γ index methods were examined by introducing known shifts to the 10 IMRT plans. The means of the dose gradient factors were 0.434 mm/% and 0.956 mm/% for the SRS and IMRT plans, respectively. Key quantities (including the mean and 90th and 99th percentiles of the distance distribution) of the surface-based distance distribution between two dose distributions were linearly proportional to the actual shifts. However, the passing percentage of the γ index for a given set of DTA/DD criteria was not associated with the actual shift. For IMRT, using the standard quality assurance criteria of 3 mm/3% DTA/DD and a 90% passing rate, we found that the detection limit of the γ index in terms of global shift was 4.07 mm/4.07 % without noise. Surface-based distance is a direct measure of the difference between two dose distributions and can be used to evaluate or determine parameters for use in calculating the γ index. The dose gradient factor represents the weighting between spatial and dose shift and should be determined before DTA/DD criteria are set. The authors also present a procedure to determine γ index parameters from measurements.

SU‐E‐J‐73: Preventing Errors in Radiotherapy: A Real‐Time System Using in Vivo EPID Dosimetry

Purpose: In vivo dosimetry using an electronic portal imaging device (EPID) may provide a uniquely effective means for preventing errors and may allow for real‐time beam interlocks. We present graphical processor (GPU)‐based fast EPID dose computation and simulate the sensitivity for detecting errors caused by patient mispositioning. Methods: We implemented an enhanced superposition convolution algorithm to compute the EPID dose. The patient was simulated using a digital cylinder phantom (15cm length and 15cm diameter) with air and bone structure built in. Setup parameters are: SAD 100cm, 12×12‐cm2 open field, 1.5×1.5×1.5‐mm3 dose grid, 6‐MV photon and isocenter at the center of cylinder. 2D dose images were obtained at 1.6‐cm depth of a water slab positioned 60‐cm below the isocenter to simulate the EPID readout. To examine errors caused by mispositioning, we shifted the phantom left‐and‐right and up‐and‐down and compared dose profiles to those with no shifts. Gamma value of each pixel was calculated as an index to identify dose mismatch. Dose failure was determined as gamma &gt; 1 using 3‐mm distance‐to‐agreement and 3% dose difference criteria. Results: Dose failures occur mainly in air‐tissue and bone‐tissue interfaces and in regions with rapid thickness change. Gamma is sensitive to the left‐and‐right shift, with a 3‐mm shift causing dose failure in ∼10% pixels; but is much less sensitive to the up‐and‐down shift, with dose failure detected only after a shift of several centimeters. Analysis with patient CT data sets is underway and will be presented. Conclusions: In vivo EPID dosimetry enabled by GPU‐based dose computation offers the potential for real‐time beam interlocks. Future directions include simulation of other error scenarios, optimization of dose computation, improved EPID modeling and comparison with measured EPID profiles.

SU‐E‐T‐185: Passing Rates of 83 Patient‐Specific IMRT QA Measurements with ArcCHECK Using Various Gamma Criteria

Purpose: Three‐dimensional diode arrays are an important tool in patient‐specific intensity modulated radiotherapy quality assurance (IMRT QA) for arc therapy. We investigated the results of 83 patient‐specific IMRT QA measurements made on TomoTherapy using the Sun Nuclear ArcCHECK. Methods: Our ArcCHECK setup consists of a water‐equivalent plug and ion chamber at the center of the device. Planned dose is recalculated for ArcCHECK using the TomoTherapy planning workstation on the “Normal” setting creating dose voxels sized 4.8mm in each dimension. Patient‐specific IMRT QA measurements are made for all patients on TomoTherapy using megavoltage computed tomography to register the device. Analysis is performed in Sun Nuclearˈs MapCHECK software using various dose difference and distance gamma criteria using absolute dose (AD). The average passing rates, uncertainty, and upper/lower quartiles (UQ and LQ) of the data were computed. Four measurements deemed unacceptable were left in the data to determine the number of false negatives (FN) and false positives (FP) given when using various passing thresholds. Results: The average passing rates using gamma criteria 5%/5mm, 5%/3mm, 3%/5mm, 3%/3mm, 2%/5mm, 2%/3mm, 1%/5mm and 1%/3mm were 99.7 +/−0.5%, 98.4 +/−1.9%, 98.6 +/−1.5%, 94.9 +/−3.5%, 97.0 +/2.5%, 90.4 +/−5.1%, 94.1 +/−4.6% and 83.6 +/−7.1% respectively. The distribution of points passing using 2%/5mm (UQ 98.8 LQ 95.8) was significantly better than the typical criteria of 3%/3mm (UQ 97.8 LQ 92.7). 2%/5mm were the only criteria that gave no FN or FP using a percentage of points passing threshold of 93%. At this threshold 3%/3mm produced 21 FN and 1 FP. Conclusions: The performance of various criteria at or below the 3% dose difference threshold demonstrates improved results using 5mm as the distance parameter. This is likely due to the “Normal” dose voxel size in TomoTherapy and has led us to investigate using the “Fine” dose grid of 2.4mm.

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 applicator

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 192Ir source dwell position brachytherapy applications employing a shielded applicator in homogeneous water geometries. TPS calculations for an irradiation plan employing seven VS2000 192Ir 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. TPS and MC dose 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). 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.

Validation of the Swiss Monte Carlo Plan for a static and dynamic 6 MV photon beam

Monte Carlo (MC) based dose calculations can compute dose distributions with an accuracy surpassing that of conventional algorithms used in radiotherapy, especially in regions of tissue inhomogeneities and surface discontinuities. The Swiss Monte Carlo Plan (SMCP) is a GUI-based framework for photon MC treatment planning (MCTP) interfaced to the Eclipse treatment planning system (TPS). As for any dose calculation algorithm, also the MCTP needs to be commissioned and validated before using the algorithm for clinical cases. Aim of this study is the investigation of a 6 MV beam for clinical situations within the framework of the SMCP. In this respect, all parts i.e. open fields and all the clinically available beam modifiers have to be configured so that the calculated dose distributions match the corresponding measurements. Dose distributions for the 6 MV beam were simulated in a water phantom using a phase space source above the beam modifiers. The VMC++ code was used for the radiation transport through the beam modifiers (jaws, wedges, block and multileaf collimator (MLC)) as well as for the calculation of the dose distributions within the phantom. The voxel size of the dose distributions was 2 mm in all directions. The statistical uncertainty of the calculated dose distributions was below 0.4%. Simulated depth dose curves and dose profiles in terms of [Gy/MU] for static and dynamic fields were compared with the corresponding measurements using dose difference and γ analysis. For the dose difference criterion of ±1% of D max and the distance to agreement criterion of ±1 mm, the γ analysis showed an excellent agreement between measurements and simulations for all static open and MLC fields. The tuning of the density and the thickness for all hard wedges lead to an agreement with the corresponding measurements within 1% or 1 mm. Similar results have been achieved for the block. For the validation of the tuned hard wedges, a very good agreement between calculated and measured dose distributions was achieved using a 1%/1 mm criteria for the γ analysis. The calculated dose distributions of the enhanced dynamic wedges (10°, 15°, 20°, 25°, 30°, 45° and 60°) met the criteria of 1%/1 mm when compared with the measurements for all situations considered. For the IMRT fields all compared measured dose values agreed with the calculated dose values within a 2% dose difference or within 1 mm distance. The SMCP has been successfully validated for a static and dynamic 6 MV photon beam, thus resulting in accurate dose calculations suitable for applications in clinical cases.

SU‐GG‐T‐212: Verification of IMRT Plans for SRT with 2D Detector Array

Purpose: To evaluate the use of 2D detector array for verification of intensity modulated radiotherapy (IMRT) fields for stereotactic radiotherapy (SRT) treatments. Method and Materials: We evaluated ten SRT plans with field sizes ranging from 3 cm to 6 cm with MapCHECK device that is routinely used for IMRT plan verification. We calculated the percent of passing points (PPP) for the distance to agreement (DTA) for absolute dose between measured and planned isodose lines above 10% using the MapCHECK software. We selected a value of 3 mm for DTA and 3% dose difference criteria for comparison. Dose sampling by MapCHECK detector array is limited for small field sizes due to detector geometry with 0.7 cm spatial resolution diagonally. To increase the number of points for more accurate evaluation, we repeated the measurement four times by shifting the array 0.5 cm in orthogonal directions. We then compare the combined measurement for each plan. We also measured the relative dose fluence using Kodak EDR2 films. We performed the γ analysis both for 3%, 3 mm and 2%, 2 mm criteria. The OmniPro‐I'mRT software was used for film comparison with treatment plans. There was no direct comparison between film results and MapCHECK measurements. Results: The average PPP for merged shifted fields is 98.7%, while for single plan delivery is 98.5%. Slightly lower PPP was shown for a 3 cm field size plan with high dose gradient. For each individual plan, the variation between shifted measurements could rise as high as 8% for smaller fields. The average PPPs for film and MapCHECK compared to planned dose are 98.7% vs. 100% for γ analysis with 3%, 3 mm criteria and 92.2% vs. 97.7% for γ with 2%, 2 mm, correspondingly. Conclusion: MapCHECK is an appropriate clinical tool for verification of IMRT plans for SRT.