C Yang

Modeling the output ratio in air for megavoltage photon beams.

The output ratio in air, OR, for a high-energy x-ray beam describes how the incident central axis photon fluence varies with collimator setting. For field sizes larger than 3 x 3 cm2, its variation is caused by the scatter of photons in structures in the accelerator head (primarily the flattening filter and the wedge, if one is used) and by the backscatter of radiation into the monitor ionization chamber. The objective of this study was to evaluate the use of an analytical function to parametrize OR for square collimator setting c: OR = (1 + a1.c).[1 + a2.erf(c/lambda)2].H0. For open beams, these parameters can be attributed to explicit physical meanings within the systematical uncertainty of the model: a1 accounts for backscatter into the monitor, a2 is the maximum scatter-to-primary ratio for head-scattered photons, and lambda represents the effective width of the source of head-scatter photons. H0 is a constant that sets OR = 1 for c = 10 cm. This formula also fits OR for wedge beams and a Co-60 unit, although the fitting parameters lose their physical interpretations. To calculate the output ratio for a rectangular field, cx x cy, an equivalent square can be used: c = (1 + k).cy x cx/(k.cx + cy), where k is a constant. The study included a number of different accelerators and a cobalt-60 unit. The fits for square fields agreed with measurements with a standard deviation (SD) of less than 0.5%. Using k = lx.(f - ly)/ly.(f - lx), where lx and ly are the source-to-collimator distances and f is the source-to-detector distance, measurements and calculations agreed within a SD of 0.7% for rectangular fields. Sufficient data for the three parameters are presented to suggest constraints that can be used for quality assurance of the measured output ratio in air.

SU-E-P-09: Feasibility of Axillary Target Coverage Utilizing High Tangent Prone Breast Radiation

PURPOSEnTo evaluate the ability to treat level I and level II axillary lymph nodes utilizing a high tangent prone breast radiotherapy technique.nnnMETHODSnTwenty-one patients were treated with whole breast radiation utilizing a prone technique. Retrospectively, axillary nodes were contoured as levels I and low lying level II with the intent of elective nodal coverage. Treatment plans were designed as tangential fields with the superior border set to cover the contoured axillary volume (high tangents). Dose-volume histograms (DVH) were evaluated for axillary, breast, lung and heart (for left breast treatments) coverage. The percent volume receiving greater than or equal to 95% of the prescribed dose (V95) was obtained for the breast and axillary regions.nnnRESULTSnThe 95% isodose line encompassed on average 98.0% of breast volumes (range, 93.2 to 100.0%). The mean V95 for the level I and level II axillary regions were 99.5% (range, 98.3 to 100.0%) and 97.4% (range, 92.5 to 100.0%), respectively. The mean percent volume of the ipsilateral lung receiving greater than 20 Gy was 7.7% (range, 4.0 to 13.4%), and the percent volume of the heart receiving greater than 40 Gy was 0.57% (range, 0 to 2.1%).nnnCONCLUSIONnThese results imply that prone breast radiation with high tangents is an effective and safe technique for the treatment of level I and level II axilla in addition to breast volumes. With increasing trends to limit axillary dissections in sentinel node positive patients, radiotherapy techniques to adequately cover nodal volumes has become more important. With historic data that suggested lower coverage in a prone position, we present techniques to adequately cover elective nodal volumes with good success.

SU‐GG‐I‐103: Comparison of Model‐Based Segmentation Systems for Contouring of Male Pelvic Structures

Purpose: Manual delineation of structures for radiation therapytreatment planning often is a laborious and time consuming process, particularly with IMRT, adaptive planning, and 4D CT data sets. In this study, two model‐based segmentation (MBS) systems are compared and evaluated for contouring of structures in the male pelvis. Method and Materials: Two commercially available MBS systems, Oncentra (Nucletron, Veenendaal NL) and Pinnacle (Philips Medical Systems, Madison WI) are used to delineate structures of the male pelvis for radiation therapytreatment planning. Five patients undergoing prostate IMRTtreatment were randomly selected and contoured in Pinnacle using non‐MBS tools; this is considered the benchmark. Contours were also generated using both MBS systems and subsequently corrected using manual non‐MBS tools that are similar to both Oncentra and Pinnacle. Contouring times and volumes for both MBS systems were compared and evaluated against the benchmark non‐MBS contours. Results: The average time to contour male pelvic structures with non‐MBS tools was 14.93 minutes. Using MBS, this was improved by 25.0% in Oncentra and 30.1% in Pinnacle. The mean reduction in time contouring using MBS with Oncentra and Pinnacle, respectively, was 2.8%±24.0 and 10.5%±15.5 for the prostate, 18.5%±24.0 and 25.5%±17.1 for the bladder, 2.6%±12.2 and 7.2% ±8.6 for the rectum, and 42.1%±15.1 and 45.4%±13.4 for both femurs. Using the MBS technique in Oncentra and Pinnacle, 28.9% ±8.0 and 31.6%±5.5 of the respective total contouring time was spent generating the MBS contours and the remainder of editing with non‐MBS tools. Contour volumes were similar for the benchmark and the MBS systems. Conclusion: Both MBS systems require manual editing of auto generated contours to better match the benchmarks. Nevertheless, the MBS systems resulted in similar time savings over utilizing only non‐MBS tools when delineating structures of the male pelvis for radiation therapytreatment planning.

TU‐C‐224A‐05: Dosimetry Comparison of the Newly Implemented Multi‐Criteria Optimization Tool for IMRT Planning

Purpose: To apply a biological model based algorithm for acquiring optimized IMRT planning solutions. This interactive planning tool will help users to select the best available plans in the IMRTsolution space. Method and Materials:IMRT often is a time consuming iterative optimization process between evaluation of the dose distribution and redefinition of the object function. An IMRT planning optimization tool (Multi‐Criteria Optimization, MCO™) has been introduced for non‐clinical evaluation to acquire the best available solutions. Based on a Paretos solution concept, this tool could search the solution space and offer users a limited set of deliverable IMRT plans. With this interactive process, users can set the target and critical structures dose constraints with the biological model (EUD) to obtain the best solution. We used Pinnacle system as the benchmark to compare the dosimetric gain from the MCO algorithm, DVH indicated excellent sparing with better PTV coverage is achievable from the MCO process in KonRad system. Results: Dosimetric findings are summarized as 1) MCO optimizationtesting shows that much better dose distribution can be achieved compared to the current planning results (Fig. 1 and Fig. 2). Due to the confined solution space, the optimal results are easily achievable. 2) MCO with Paretos approach is durable in the solution searching process. It is interactive with the graphical interface which the dose distribution along with the DVH can be compared simultaneously (Fig. 3). 3) IMRT dose optimization and summary based on the MCO methodology are very conceivable. With pre‐calculated IMRTsolutions, final results help users to select the best available plan from the solution domain in real time (Fig. 4). Conclusion: From this interactive MCO planning tool, we can calculate the best IMRT results in a very reasonable time frame. Human factors for determining an acceptable plan can be dramatically reduced.

SU-F-I-51: CT/MR Image Deformation: The Clinical Assessment QA in Target Delineation.

PURPOSEnTo study the deformation effects in CT/MR image registration of head and neck (HN) cancers. We present a clinical indication in guiding and simplifying registration procedures of this process while CT images possessed artifacts.nnnMETHODSnCT/MR image fusion provides better soft tissue contrast in intracranial GTV definition with artifacts. However, whether the fusion process should include the deformation process is questionable and not recommended. We performed CT/MR image registration of a HN patient with tonsil GTV and nodes delineation on Varian Velocity™ system. Both rigid transformation and deformable registration of the same CT/MR imaging data were processed separately. Physicians selection of target delineation was implemented to identify the variations. Transformation matrix was shown with visual identification, as well as the deformation QA numbers and figures were assessed.nnnRESULTSnThe deformable CT/MR images were traced with the calculated matrix, both translation and rotational parameters were summarized. In deformable quality QA, the calculated Jacobian matrix was analyzed, which the min/mean/max of 0.73/0/99/1.37, respectively. Jacobian matrix of right neck node was 0.84/1.13/1.41, which present dis-similarity of the nodal area. If Jacobian = 1, the deformation is at the optimum situation. In this case, the deformation results have shown better target delineation for CT/MR deformation than rigid transformation. Though the root-mean-square vector difference is 1.48 mm, with similar rotational components, the cord and vertebrae position were aligned much better in the deformable MR images than the rigid transformation.nnnCONCLUSIONnCT/MR with/without image deformation presents similar image registration matrix; there were significant differentiate the anatomical structures in the region of interest by deformable process. Though vendor suggested only rigid transformation between CT/MR assuming the geometry remain similar, our findings indicated with patient positional variations, deformation registration is needed to generate proper GTV coverage, which will be irradiated more accurately in the following boost phase.

SU-F-T-603: Safe Treatment of the Maximum Tolerable Brain Metastases with Gamma Knife in Single Patient Clinical Findings.

PURPOSEnSafe treatment of the maximum tolerable brain metastases with GK in single patient-clinical findings METHODS: A patient diagnosed with stage IV NSCLC has received 6 courses of Gamma Knife radiosurgery (GKRS) treatments. The plans were generated with GammaPlan (version 10.1.1), six courses of treatment were administered in 2.5 years. A total of 53 metastases were treated with retreatment foci (n = 5) and planned on newly identified lesions (n = 48). Individual prescription dose varied from 12 Gy to 24 Gy.The homogeneity index (HI) was evaluated as the ratio of maximum dose to prescription dose (HI = (D2%-D98%) /DRx). The gradient index (GI) was evaluated as the ratio of prescription isodose volume (PIV) at half of the prescription isodose line to PIV (GI = PIV1/2 / PIV). Composite dose of brain from all six courses was generated on Velocity AI (version 3.0.1). All V5, V10, and V12 of brain were estimated.nnnRESULTSnMean target volume was 0.39 ± 0.96 cm3 (range from 0.047 to 6.55 cm3 ). The mean volume of half of prescribed dose was 1.08 ± 2.46 cm3 . HI and GI were calculated as 1.37 ± 0.32 and 2.98 ± 0.52, respectively. The patient had no new neurologic symptoms. V5 , V10 , and V12 of brain were 820, 118, and 71.94 cc, respectively.All doses were delivered with focused and sharp dose gradients, with superior indexes compared to LINAC based delivery.nnnCONCLUSIONnWith exceptional steep dose gradients, GKRS is ideal to treat multiple metastases in continuous courses. The hot spots were observed in the overlapped area for the given high prescription dose with no side effects.GKRS has advantages of treating multiple brain metastases from NSCLC with low whole brain dose. Our data presented conclusions of excellent local control of large numbers of metastases being treated on GKRS with superior clinical outcome.

SU-E-T-303: Dosimetric Comparison of a LINAC Fallback Plan Generated From Tomotherapy System

Purpose: Quantitatively evaluate the Multi Criteria Optimization (MCO) based MLC step and shoot (sMLC) fallback plan derived from Tomotherapy of multiple lesions lung SBRT Methods: Inter-comparison of various IMRT planning systems tends to be difficult due to the vendor-specific functionalities. The methodology of defining dose constraints and beam geometries is challenging. Raysearch™ planning system offers an alternative replanning to deliver same intensity map from Tomotherapy without modifying original fluence. This intuitive comparison comes from the final fluence map converted without any embedded system dependent dose optimization. This planner independent approach could be utilized to study the merits of individual machines. The term “fallback” was utilized to (A) transfer plans in among treatment delivery systems; (B) maintain acceptable plan qualities; and (C) minimize the biological dose impact due to machine breakdown. The Tomotherapy specific DICOM RT dose and plan are retrieved into Raystation’s pre-defined sMLC protocol. Based on specific machine characteristics, same fluence maps were converted to generate equivalent deliverable segments (Fig.1). Therefore, the treatment plans were evaluated among two planning tools, Tomotherapy and MCO based sMLC delivery plans. Results: By converting the fluence map with the pre-defined machine characteristics, the 9-fields fallback plan has similar ITV coverage compared to the original Tomotherapy plan. ITV average doses, the D95 consisted of 0.9% variation. The total lung doses of fallback plan drifted from 17.4% to 30.5% which represents the limitations of the static beam delivery. D2 of fallback spinal cord increased from 22.4% to 36.4% but still within tolerances (Fig.2). Ipsilateral lung changed from 11.0% to 22.6%. Low dose region between ITVs presented increased dose to the normal lung tissues (Fig.3). Conclusion: Acceptable fallback plan for Tomotherapy SBRT has similar ITVs coverage, but lack of the normal tissues avoidance due to the limitation of geometrical factors to reduce doses in between ITVs.

SU-E-T-331: To Evaluate Planning Quality of SBRT with Multiple Lung Metastases Generated with Pinnacle and Tomotherapy

PURPOSEnTo evaluate planning quality of SBRT with multiple lung metastases generated with Pinnacle and Tomotherapy METHODS: Nine randomly selected patients diagnosed with non small-cell lung cancer with multiple lesions were planned with Pinnacle (version 9.2) and Tomotherapy (version 4.2). Coplanar and non_coplanar plans were generated on Pinnacle. A total dose of 60 Gy was prescribed to 95% of PTV in 3 fractions. Single isocenter was used. Nine static beams were used for Pinnacle plans. Planning outcomes such as minimum and mean dose, V95 , D95 (95% of target volume receives prescription dose), D5 , and D1 to PTV, maximum dose to heart, esophagus, cord, trachea, brachial plexus, rib, chest wall, and liver, mean dose to liver, total lung, right and left lung, volume of chest wall receives 30 Gy, volume of lungs receives 5 Gy and 20Gy, conformity index (CI = PIV / PTV) and heterogeneity index (HI = D5 / D95 ) were reported for evaluation.nnnRESULTSnThe mean volume of PTV was 37.77 ± 23.4 cm3. D95 of PTV with Tomo, coplanar, non_coplanar was 60.2 ± 0.3 Gy, 58.6 ± 1.2 Gy, and 59.1 ± 0.7 Gy, respectively. Mean dose to PTV was lower for Tomo (p < 0.0001), so were D5 (p < 0.0001) and D1 (p = 0.001). CI was better with Tomo (p < 0.0001), so was HI (p < 0.0001). Maximum dose to other critical organs were also lower exclusively with Tomo plans. Treatment time was recorded only for Tomo plans (73.0 ± 20.6 min).nnnCONCLUSIONnWith 51 beam angles, Tomo plans could generally achieve better tumor coverage while sparing more critical structures for multiple lung lesions study. Non_coplanar also has better tumor coverage with lower dose to critical organs such as lungs, liver, chest wall and cord compare to coplanar plans.

SU-E-T-40: Analysis of Composite MVCT Planning Dosimetry with SBRT of Upper Peripheral Lung Cancer

PURPOSEnQuantitatively evaluate and compare the final adaptive planning doses of upper peripherally located lung SBRT treated with Tomotherapy using 3rd party software tool.nnnMETHODSnWith tumor located in the upper quadrant of lung, a 3rd party software tool was implemented to evaluate the Tomotherapy composite dosimetry created by adaptive fan beam MVCT images described by RTOG 0915 dose criteria (48 Gy / 4 fractions). The composite doses was then summarized with deformable registration in this package with corresponding target and critical structures. The final dosimetry variation, both for target and critical structures, were evaluated in a tabular format and isodose distribution comparisons.nnnRESULTSnComposite SBRT treatment doses were evaluated with adaptive planning. The PTV and several critical structures were mapped/deformed into the package via DICOM from Tomotherapy after the final composite doses were created. Initial plan versus the final composite plan calculated from verification images were compared. The ITV defined by 4D CT and contoured on MVCT images were correlated in patient repositioning. Final composite dose calculated for PTV coverage has shown 0.1-0.17 cGy coverage (0.2-0.4% of prescription dose) variation. Total lung and cord were both less than 0.17 Gy which represented <0.4% difference. All other critical structure were within statistical significance. The adaptive plans justified/included the breathing and motion during the treatment process. Final 95% isotope line coverage from prescription has been met without issues.nnnCONCLUSIONSnWith lung tumor location in the upper peripheral area, breathing control was not necessary required during SBRT treatment using Tomotherapy technique. Slow fan beam CT provides definitive ITV information and the adaptive composite plan for all fractions were suitable for final dose delivery. The final composite dose calculated with Tomotherapy adaptive tool indicated that the composite dosimetry justified the target location with SBRT delivery, safe with minimum margin of errors.

SU‐E‐T‐719: Dosimetric Differences of Post HDR Prostate IMRT Plans Generated From RayStation Multi‐Criteria Optimization (MCO) and Pinnacle Direct Machine Parameter Optimization (DMPO)

Purpose: To evaluate dosimetric differences of post HDR prostate IMRT plans generated from RayStation Multi‐Criteria Optimization (MCO) and Pinnacle3 Direct Machine Parameter Optimization (DMPO) Methods: Ten patients previously treated with DMPO plans on Pinnacle3 v9.0 were selected. CT images with same RT structures from Pinnacle were sent to RayStation v2.4.8.180 for re‐planning using MCO Pareto surface plans with the same prescription and beam angles. The prescription was 50.4 Gy to 95% of PTV for all cases. Planning outcomes such as D95 (95% of volume receiving the prescribed dose), D 5, D2, D1 for PTV, D 53 for bladder, D53 , D30, and D1cc for rectum, D1cc for small and large bowel, conformity index (CI = PIV / PTV), and homogeneity index (HI = D5 / D95) were evaluated according to the clinical protocol. Results: The mean volume of PTV was 222.6 cm3 (range from 178.5 to 276.3 cm3). The D95 of PTV coverage of RayStation vs. Pinnacle was improved and more uniform with 49.8 Gy vs. 49.7 Gy, respectively. D5, D2, and D1 of PTV were lower with RayStation. So were D53 for bladder and rectum, D30 for rectum (p = 0.017), D1cc for rectum and D1cc for small bowel, D1cc for large bowel except one case on RayStation. CI was better with RayStation compared to Pinnacle (1.01 vs. 1.07). HI was better with RayStation compared to Pinnacle (1.06 vs. 1.07). Conclusion: MCO planning automatically generates a set of Pareto optimized solutions for given objectives to allow tradeoffs between targets and critical organs. RayStation can achieve better uniform tumor coverage with fewer hot spots while sparing more critical structures in post HDR prostate plans. MCO Pareto based plan is helpful in determining the best optimized dosimetry solutions.