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DOE OSTI2015

SU-E-T-135: Assessing the Clinical Impact of Approximations in Analytical Dose Calculations for Proton Therapy

Purpose: To estimate the clinical relevance of approximations made in analytical dose calculation methods (ADCs) used for treatment planning on tumor coverage and tumor control probability (TCP) in proton therapy. Methods: We compared dose distributions planned with ADC to delivered dose distributions (as determined by TOPAS Monte Carlo (MC) simulations). We investigated 10 patients per site for 5 treatment sites (head-and-neck, lung, breast, prostate, liver). We evaluated differences between the two dose distributions analyzing dosimetric indices based on the dose-volume-histograms, the γ-index and the TCP. The normal tissue complication probability (NTCP) was estimated for the bladder and anterior rectum for the prostate patients. Results: We find that the target doses are overestimated by the ADC by 1–2% on average for all patients considered. All dosimetric indices (the mean dose, D95, D50 and D02, the dose values covering 95%, 50% and 2% of the target volume, respectively) are predicted within 5% of the delivered dose. A γ-index with a 3%/3mm criteria had a passing rate for target volumes above 96% for all patients. The TCP predicted by the two algorithms was up to 2%, 2.5%, 6%, 6.5%, and 11% for liver and breast, prostate, head-and-neck and lung patients, respectively. Differences in NTCP for anterior-rectum and bladder for prostate patients were less than 3%. Conclusion: We show that ADC provide adequate dose distributions for most patients, however, they can Result in underdosage of the target by as much as 5%. The TCP was found to be up to 11% lower than predicted. Advanced dose-calculation methods like MC simulations may be necessary in proton therapy to ensure target coverage for heterogeneous patient geometries, in clinical trials comparing proton therapy to conventional radiotherapy to avoid biases due to systematic discrepancies in calculated dose distributions, and, if tighter range margins are considered. Fully funded by NIH grants.

60 APPLIED LIFE SCIENCES↗

DOE OSTI2015

SU-E-T-170: Characterization of the Location, Extent, and Proximity to Critical Structures of Target Volumes Provides Detail for Improved Outcome Predictions Among Pancreatic Cancer Patients

Purpose: In radiotherapy, size, location and proximity of the target to critical structures influence treatment decisions. It has been shown that proximity of the target predicts dosimetric sparing of critical structures. In addition to dosimetry, precise location of disease has further implications such as tumor invasion, or proximity to major arteries that inhibit surgery. Knowledge of which patients can be converted to surgical candidates by radiation may have high impact on future treat/no-treat decisions. We propose a method to improve our characterization of the location of pancreatic cancer and treatment volume extent with respect to nearby arteries with the goal of developing features to improve clinical predictions and decisions. Methods: Oncospace is a local learning health system that systematically captures clinical outcomes and all aspects of radiotherapy treatment plans, including overlap volume histograms (OVH) – a measure of spatial relationships between two structures. Minimum and maximum distances of PTV and OARs based on OVH, PTV volume, anatomic location by ICD-9 code, and surgical outcome were queried. Normalized distance to center from the left and right kidney was calculated to indicate tumor location and laterality. Distance to critical arteries (celiac, superior mesenteric, common hepatic) is validated by surgical status (borderline resectable, locally advanced converted to resectable). Results: There were 205 pancreas stereotactic body radiotherapy patients treated from 2009–2015 queried. Location/laterality of tumor based on kidney OVH show strong trends between location by OVH and by ICD-9. Compared to the locally advanced group, the borderline resectable group showed larger geometrical distance from critical arteries (p=0.03). Conclusion: Our platform enabled analysis of shape/size-location relationships. These data suggest that PTV volume and attention to distance between PTVs and surrounding OARs and major arteries may be promising for improving characterization of treatment anatomy that can refine our ability for outcome predictions and decision making. Elekta, Toshiba.

60 APPLIED LIFE SCIENCES↗

DOE OSTI2015

SU-E-T-178: Clinical Feasibility of Multi-Leaf Collimator Based Dynamic Wedge

Purpose: A multi-leaf collimator (MLC) based dynamic wedge (MDW), which provide similar dose profile of physical wedge (PW) along x-jaw direction while significant monitor unit (MU) reduction, was developed and investigated for clinical use. Methods: A novel technique was used to create the wedge profile using MLC. A modification was applied to the DICOM-RT format file of the plan made with the PW to replace PW with MDW. The Varian enhanced dynamic wedge profile was used to produce MLC sequence, while the MU of the wedged field was recalculated using PW factor and fluence map. The profiles for all possible MDWs to substitute PWs were verified in 6/15 MV x-ray irradiations. New plans with MDWs were compared with the original plans in 5 rectal, 5 RT breast and 5 liver cases. Results: The wedge profile of the MDW fields were well matched with those of PWs inside the fields while less scatter than PW out of the fields. For plan comparisons of the clinical cases no significant dose discrepancy was observed between MDW plan and PW’s with the dose volume histograms. The maximum and mean doses in PTVs are agreed within 1.0%. The Result of OARs of MDW plans are slightly improved in the maximum doses (3.22 ∼ 150.4 cGy) and the mean doses (17.18 ∼ 85.52 cGy) on average for all cases while the prescribed doses are 45 Gy for rectal cases, 40 or 45 Gy for liver cases and 50 Gy for breast cases. The MUs of the fields which replace PW with MDW are reduced to 68% of those of PW. Conclusion: We developed a novel dynamic wedge technique with MLC that shows clinical advantage compared to PW.

60 APPLIED LIFE SCIENCES↗

DOE OSTI2015

SU-E-T-210: Comparison of Proton with Electron Boost in Breast Cancer Treatment

Purpose: Electron beams are commonly used for boost radiation following whole breast irradiation (WBI) to improve the in-breast local control. Proton beams have a finite range and a sharper distal dose falloff compared to electron beams, thus potentially sparing more heart and lung in breast treatment. The purpose of the study is to compare protons with electrons for boost breast treatment in terms of target coverage and normal tissue sparing. Methods: Six breast cancer patients were included in this study. All women received WBI to 45–50 Gy, followed by a 10–16.2 Gy boost with standard fractionation. If proton beams were used for the boost treatment, an electron plan was retrospectively generated for comparison using the same CT set and structures, and vice versa if electron beams were used for treatment. Proton plans were generated using the treatment planning system (TPS) with two to three uniform scanning proton beams. Electron plans were generated using the Pinnacle TPS with one single en face beam. Dose-volume histograms (DVH) were calculated and compared between proton and electron boost plans. Results: Proton plans show a similar boost target coverage, similar skin dose, and much better heart and lung sparing. For an example patient, V95% for PTV was 99.98% and skin (5 mm shell) received a max dose close to the prescription dose for both protons and electrons; however, V2 and V5 for the ipsilateral lung and heart were 37.5%, 17.9% and 19.9%, 4.9% respectively for electrons, but were essentially 0 for protons. Conclusions: This dosimetric comparison demonstrates that while both proton therapy and electron therapy provided similar coverage and skin dose, proton therapy could largely reduce the dose to lung and heart, thus leading to potential less side effects.

60 APPLIED LIFE SCIENCES↗

DOE OSTI2015

SU-E-T-215: Comparison of VMAT-SABR Treatment Plans with Flattened Filter (FF) Beam and Flattening Filter-Free (FFF) Beam for Localized Prostate Cancer

Purpose: The purpose of this study is to access VMAT-SABR plan using flattening filter (FF) and flattening filter-free (FFF) beam, and compare the verification results for all pretreatment plans. Methods: SABR plans for 20 prostate patients were optimized in the Eclipse treatment planning system. A prescription dose was 42.7 Gy/7 fractions. Four SABR plans for each patient were calculated using Acuros XB algorithm with both FF and FFF beams of 6- and 10-MV. The dose-volume histograms (DVH) and technical parameters were recorded and compared. A pretreatment verification was performed and the gamma analysis was used to quantify the agreement between calculations and measurements. Results: For each patient, the DVHs are closely similar for plans of four different beams. There are small differences showed in dose distributions and corresponding DVHs when comparing the each plan related to the same patient. Sparing on bladder and rectum was slightly better on plans with 10-MV FF and FFF than with 6-MV FF and FFF, but this difference was negligible. However, there was no significance in the other OARs. The mean agreement of 3%/3mm criteria was higher than 97% in all plans. The mean MUs and deliver time employed was 1701±101 and 3.02±0.17 min for 6-MV FF, 1870±116 and 1.69±0.08 min for 6-MV FFF, 1471±86 and 2.68±0.14 min for 10-MV FF, and 1619±101 and 0.98±0.04 min for 10-MV FFF, respectively. Conclusion: Dose distributions on prostate SABR plans using FFF beams were similar to those generated by FF beams. However, the use of FFF beam offers a clear benefit in delivery time when compared to FF beam. Verification of pretreatment also represented the acceptable and comparable results in all plans using FF beam as well as FFF beam. Therefore, this study suggests that the use of FFF beam is feasible and efficient technique for prostate SABR.

60 APPLIED LIFE SCIENCES↗

DOE OSTI2015

SU-E-T-233: Cyberknife Versus Linac IMRT for Dose Comparision in Hypofractionated Hemi Larynx Irradiation of Early Stage True Vocal Cord Cancer: A Dosimetric Study

Purpose: To compare dosimetric data of patients treated for early-stage larynx cancer on Cyberknife and Linac IMRT. Methods: Nine patients were treated with Cyberknife to a dose of 45 Gy in 10 fractions of the involved hemilarynx. The prescription dose provided at least 95% of PTV coverage. After Cyberknife treatment, the CT images and contours were sent to Pinnacle treatment planning system for IMRT planning on a regular SBRT linac with same dose prescription and constrains. Dose to target and normal tissue, including the arytenoids, cord, carotid arteries, thyroid, and skin, were analyzed using dose volume histograms. Results: For Cyberknife plan, the conformity indices are within 1.11–1.33. The average dose to the contralateral arytenoids for Cyberknife plans was 28.9±6.5Gy), which is lower than the same mean dose for IMRT plans (34.0±5.2 Gy). The average maximum dose to the ipsilateral and contralateral carotid artery were 20.6 ±9.1 Gy and 10.2±6.0 Gy respectively for Cybeknife comparing with 22.1±8.0 Gy and 12.0±5.1 Gy for IMRT. The mean dose to the thyroid was 3.6±2.2 Gy for Cyberknife and 3.4±2.4 Gy for IMRT. As shown in DVH, the Cyberknife can deliver less dose to the normal tissue which is close to target area comparing with IMRT Plans. However, IMRT plan’s can give more sparing for the critical organs which is far away from the target area. Conclusion: We have compared the dosimetric parameters of Cyberknife and linac IMRT plans for patients with early-stage larynx cancer. Both Cyberknife and IMRT plans can achieve conformal dose distribution to the target area. Cyberknife was able to reduce normal tissue dose in high doses region while IMRT plans can reduce the dose of the normal tissue at the low dose region. These dosimetric parameters can be used to guide future prospective protocols using SBRT for larynx cancer.

60 APPLIED LIFE SCIENCES↗

DOE OSTI2015

SU-E-T-277: Dose Calculation Comparisons Between Monaco, Pinnacle and Eclipse Treatment Planning Systems

Purpose: Monaco treatment planning system (TPS) version 5.0 uses a Monte-Carlo based dose calculation engine. The aim of this study is to verify and compare the Monaco based dose calculations with both Pinnacle{sup 3} collapsed cone convolution superposition (CCC) and Eclipse analytical anisotropic algorithm (AAA) calculations. Methods: For this study, previously treated SBRT lung, head and neck and abdomen patients were chosen to compare dose calculations between Pinnacle, Monaco and Eclipse. Plans were chosen from those that had been treated using the Elekta VersaHD or a NovalisTX linac. The plans included 3D conventional and IMRT beams using 6MV and 6MV Flattening filter free (FFF) photon beams. The original plans calculated with CCCS or AAA along with the recalculated ones using MC from the three TPS were exported into Velocity software for inter-comparison. Results: To compare the dose calculations, Mean Lung Dose (MLD), lung V5 and V20 values, and PTV Heterogeneity indexes (HI) and Conformity indexes (CI) were all calculated and recorded from the dose volume histograms (DVH). For each patient, the CI values were identical but there were differences in all other parameters. The HI was computed higher by 5 and 4% for calculated plans AAA and CCCS respectively, compared to the MC ones. The DVH graphs showed large differences between the CCCS and AAA and Monaco for 3D FFF, VMAT and IMRT plans. Better DVH agreement between was observed for 3D conventional plans. Conclusion: Better agreement was observed between CCCS and MC calculations than AAA and MC calculations. Those differences were more profound as the field size was decreasing and in the presence of inhomogeneities.

60 APPLIED LIFE SCIENCES↗

DOE OSTI2015

SU-E-T-292: Dosimetric Advantage of Prone Breast Radiotherapy for Korean Left-Sided Breast Cancer Patients

Purpose: To evaluate the dosimetric benefit of prone breast radiotherapy for Korean left-sided early-stage breast cancer patients who have relatively small breast Methods: From April to June, 2014, 10 left-sided breast cancer patients received the whole breast irradiation in prone position after partial mastectomy with sentinel lymph node biopsy or axillary lymph node dissection. All patients were pTmi-2N0-1mi. Each patient underwent two computed tomoradiography (CT) simulations in supine and prone positions. The whole breast, ipsilateral lung, heart, and left anterior descending coronary artery (LAD) were contoured on each simulation CT images, and then tangential-fields treatment plan in each position was designed for the whole breast irradiation with the total dose of 50 Gy in 2 Gy fractions. Dose-volume histograms of two setups were compared for target coverage and radiation dose to normal organs with Wilcoxon signed rank tests. Results: The median age of patients was 47 years (range, 37 to 53). The median chest size was 82.5 cm (range, 75 to 90) and bra cup size was A in 4, B in 4, and C in 2 patients. The radiation dose to the whole breast was similar when comparing mean dose (Dmean) and dose covering 95% of the breast volume, but maximum dose (Dmax) of breast was higher in supine (median 52.3 vs. 52.7 Gy, p=0.013). Prone position reduced significantly the radiation dose in ipsilateral lung, heart, and LAD by median 5.7, 1.1, and 6.9 Gy of Dmean (p=0.005, 0.007, and 0.005) and 28.2, 18.8, and 35.0 Gy of Dmax (p=0.005, 0.005, and 0.007), respectively. Conclusion: Prone breast radiotherapy could be beneficial for Korean breast cancer patients since it substantially spared normal organs while achieving adequate coverage of the breast tissue. Further prospective study is required to validate the potential benefit of prone breast radiotherapy.

60 APPLIED LIFE SCIENCES↗

DOE OSTI2015

SU-E-T-300: Dosimetric Comparision of 4D Radiation Therapy and 3D Radiation Therapy for the Liver Tumor Based On 4D Medical Image

Purpose: The purpose of this work was to determine the dosimetric benefit to normal tissues by tracking liver tumor dose in four dimensional radiation therapy (4DRT) on ten phases of four dimensional computer tomagraphy(4DCT) images. Methods: Target tracking each phase with the beam aperture for ten liver cancer patients were converted to cumulative plan and compared to the 3D plan with a merged target volume based on 4DCT image in radiation treatment planning system (TPS). The change in normal tissue dose was evaluated in the plan by using the parameters V5, V10, V15, V20,V25, V30, V35 and V40 (volumes receiving 5, 10, 15, 20, 25, 30, 35 and 40Gy, respectively) in the dose-volume histogram for the liver; mean dose for the following structures: liver, left kidney and right kidney; and maximum dose for the following structures: bowel, duodenum, esophagus, stomach and heart. Results: There was significant difference between 4D PTV(average 115.71cm3 )and ITV(169.86 cm3). When the planning objective is 95% volume of PTV covered by the prescription dose, the mean dose for the liver, left kidney and right kidney have an average decrease 23.13%, 49.51%, and 54.38%, respectively. The maximum dose for bowel, duodenum,esophagus, stomach and heart have an average decrease 16.77%, 28.07%, 24.28%, 4.89%, and 4.45%, respectively. Compared to 3D RT, radiation volume for the liver V5, V10, V15, V20, V25, V30, V35 and V40 by using the 4D plans have a significant decrease(P≤0.05). Conclusion: The 4D plan method creates plans that permit better sparing of the normal structures than the commonly used ITV method, which delivers the same dosimetric effects to the target.

60 APPLIED LIFE SCIENCES↗

DOE OSTI2015

SU-E-T-306: Dosimetric Comparison of Leaf with Or Without Interdigitation in Multiple Brain Metastasis VMAT Treatment Planning

Purpose: To evaluate the effects of leaf with or without interdigitation in multiple brain metastasis volumetric modulated arc therapy (VMAT) plans. Methods: Twenty patients with 2 to 6 brain metastases of our hospital were retrospectively studied to be planned with dual arc VMAT using Monaco 3.3 TPS on the Elekta Synergy linear accelerator. The prescription dose of PTV was 60Gy/30 fractions. Two plans with or without leaf interdigitation were designed. The homogeneity index (HI), conformity index (CI), dose volume histograms (DVHs), monitor unit (MU), treatment time (T), the segments, the dose coverage of the target, were all evaluated. Results: The plans with leaf interdigitation could achieve better CI (p<0.05) than without leaf interdigitation, while no significant difference were found in HI (p> 0.05) and the dose coverage of the target (p> 0.05).The MU,T, and the segments of the plan with leaf interdigitation were more than the plan without leaf interdigitation (p<0.05). There was no significant difference found in radiation dose of spinal cord, lenses and parotids, while the maximum dose of brain stem of leaf without interdigitation was higher than leaf with interdigitation (p< 0.05). It was worth noting that the areas of low dose regions with leaf interdigitation plan were much less than the without leaf interdigitation plan in the doublication planes (p< 0.05). Conclusion: This study shows that leaf with interdigitation has some advantages than leaf without interdigitation in multiple brain metastasis VMAT plans although the clinical relevance remains to be proven.

60 APPLIED LIFE SCIENCES↗

DOE OSTI2015

SU-E-T-311: Dosimetric Comparison of Volumetric Modulated Arc Therapy Plans for Preoperative Radiotherapy Rectal Cancer Using Flattening Filter-Free and Flattening Filter Modes

Purpose: To compare the dosimetric difference of volumetric modulated arc therapy(VMAT) for preoperative radiotherapy rectal cancer using 6MV X-ray flattening filter free(FFF) and flattening filter(FF) modes. Methods: FF-VMAT and FFF-VMAT plans were designed to 15 rectal cancer patients with preoperative radiotherapy by planning treatment system(Eclipse 10.0),respectively. Dose prescription was 50 Gy in 25 fractions. All plans were normalized to 50 Gy to 95% of PTV. The Dose Volume Histogram (DVH), target and risk organ doses, conformity indexes (CI), homogeneity indexes (HI), low dose volume of normal tissue(BP), monitor units(MU) and treatment time (TT) were compared between the two kinds of plans. Results: FF-VMAT provided the lower Dmean, V105, HI, and higher CI as compared with FFF-VMAT. The small intestine of D5, Bladder of D5, Dmean, V40, V50, L-femoral head of V40, R-femoral head of Dmean were lower in FF-VMAT than in FFF-VMAT. FF-VMAT had higher BP of V5, but no significantly different of V10, V15, V20, V30 as compared with FFF-VMAT. FF-VMAT reduceed the monitor units(MU) by 21%(P<0.05), as well as the treatment time(TT) was no significantly different(P>0.05), as compared with FFF-VMAT. Conclusion: The plan qualities of FF and FFF VMAT plans were comparable and both clinically acceptable. FF-VMAT as compared with FFF-VMAT, showing better target coverage, some of OARs sparing, the MUs of FFF-VMAT were higher than FF-VMAT, yet were delivered within the same time. This work was supported by the Medical Scientific Research Foundation of Guangdong Procvince (A2014455 to Changchun Ma)

60 APPLIED LIFE SCIENCES↗

DOE OSTI2015

SU-E-T-319: Dosimetric Evaluation of IMRT with Mix-Energy Beam for Deep Seated Targets

Purpose: IMRT is preferred in the range of 6–10MV X-rays. Partially adding high energy (>10MV) treatment fields, may provide advantage of both higher and lower energies. To study IMRT dose distribution obtained from treatment plans with single (6MV) and mixed-energy (6MV and 15MV) for deep seated targets (separation more than 30cm). Methods: Five patients of carcinoma of cervix were studied using eclipse planning system. Two different dynamic IMRT plans were generated for Varian CL2300C/D linear accelerator; one is by using 6MV X-ray with seven equally spaced coplanar beams. In second plan, 2 lateral oblique fields (gantry angle 102°, 255°) beam energy was modified to 15MV by keeping all other parameters and dose volume constraints constant. Dose prescription for the planning target volume (PTV) was (5040cGy/28f). For plan comparison, dose volume histogram (DVH) was used and PTV coverage index (CI=Target volume covered by prescription dose/Target volume), heterogeneity index (D5/D95), mean dose to organ at risk (OAR) and normal tissue integral dose (NTID, liter-Gray) was also noted. Total monitor unit (MU) required to deliver a plan was also noted. Results: Mixed-energy plan showed a better conformity and CI values were 0.942±0.032 and 0.960±0.040 for single-energy and mixed-energy plan, respectively. In addition, HI value of mixed energy beam is comparable to that of single energy and the values were within 1.084±0.034 and 1.082±0.032 for single energy and mixed-energy plan, respectively. Variation in mean dose to bladder, rectum and bowel were within 1.05%, 0.87% and 0.90%. NTID was lesser for mixed-energy beam due to use of two high-energy fields. NTID were 1573.40±214.60 and 1510.20±249.80 litre-Gray for single energy and mixed-energy plan. MU needed to deliver a plan was similar in both plans and MUs were 238±45 and 237±47. Conclusion: Partial use of 15MV treatment fields in IMRT plan for deep seated targets showed dosimetric advantage over 6MV plan.

60 APPLIED LIFE SCIENCES↗

DOE OSTI2015

SU-E-T-357: Electronic Compensation Technique to Deliver Total Body Dose

Purpose: Total body irradiation (TBI) uses large parallel-opposed radiation fields to suppress the patient’s immune system and eradicate the residual cancer cells in preparation of recipient for bone marrow transplant. The manual placement of lead compensators has conventionally been used to compensate for the varying thickness through the entire body in large-field TBI. The goal of this study is to pursue utilizing the modern electronic compensation technique to more accurately and efficiently deliver dose to patients in need of TBI. Methods: Treatment plans utilizing electronic compensation to deliver a total body dose were created retrospectively for patients for whom CT data had been previously acquired. Each treatment plan includes two, specifically weighted, pair of opposed fields. One pair of open, large fields (collimator=45°), to encompass the patient’s entire anatomy, and one pair of smaller fields (collimator=0°) focused only on the thicker midsection of the patient. The optimal fluence for each one of the smaller fields was calculated at a patient specific penetration depth. Irregular surface compensators provide a more uniform dose distribution within the smaller opposed fields. Results: Dose-volume histograms (DVH) were calculated for the evaluating the electronic compensation technique. In one case, the maximum body doses calculated from the DVH were reduced from the non-compensated 195.8% to 165.3% in the electronically compensated plans, indicating a more uniform dose with the region of electronic compensation. The mean body doses calculated from the DVH were also reduced from the non-compensated 120.6% to 112.7% in the electronically compensated plans, indicating a more accurate delivery of the prescription dose. All calculated monitor units were well within clinically acceptable limits. Conclusion: Electronic compensation technique for TBI will not substantially increase the beam on time while it can significantly reduce the compensator setup time and the potential risk of errors in manually placing lead compensators.

60 APPLIED LIFE SCIENCES↗

DOE OSTI2015

SU-E-T-385: Evaluation of DVH Change for PTV Due to Patient Weight Loss in Prostate VMAT Using Gaussian Error Function

Purpose: We proposed a method to predict the change of dose-volume histogram (DVH) for PTV due to patient weight loss in prostate volumetric modulated arc therapy (VMAT). This method is based on a pre-calculated patient dataset and DVH curve fitting using the Gaussian error function (GEF). Methods: Pre-calculated dose-volume data from patients having weight loss in prostate VMAT was employed to predict the change of PTV coverage due to reduced depth in external contour. The effect of patient weight loss in treatment was described by a prostate dose-volume factor (PDVF), which was evaluated by the prostate PTV. Along with the PDVF, the GEF was used to fit into the DVH curve for the PTV. To predict a new DVH due to weight loss, parameters from the GEF describing the shape of DVH curve were determined. Since the parameters were related to the PDVF as per the specific reduced depth, we could first predict the PDVF at a reduced depth based on the prostate size from the pre-calculated dataset. Then parameters of the GEF could be determined from the PDVF to plot the new DVH for the PTV corresponding to the reduced depth. Results: A MATLAB program was built basing on the patient dataset with different prostate sizes. We input data of the prostate size and reduced depth of the patient into the program. The program then calculated the PDVF and DVH for the PTV considering the patient weight loss. The program was verified by different patient cases with various reduced depths. Conclusion: Our method can estimate the change of DVH for the PTV due to patient weight loss quickly without CT rescan and replan. This would help the radiation staff to predict the change of PTV coverage, when patient’s external contour reduced in prostate VMAT.

60 APPLIED LIFE SCIENCES↗

DOE OSTI2015

SU-E-T-435: Flattening Filter Free Beams for Head and Neck IMRT and VMAT Optimization

Purpose: To investigate if there is benefits of using flattening filter free (FFF) versus flattening filter (FF) beams during plan optimization for head and neck targets. Methods: Five head and neck cancer patients previously treated were selected for this study. Each plan was optimized for volumetric modulated arc therapy (VMAT) using 2 full arcs. Target volumes and the organs at risk (OAR) were outlined by the same physician. The prescription doses for each patient and the dose limits for each OAR were specified by the physician. For each patient four plans were created by varying the photon beam energy (6MV vs 10MV) and the use of FF or FFF. For each patient the prescription dose and OAR dose limits remained unchanged. Furthermore, the number of optimization iterations remained the same for each plan without user interference. The plans were compared using dose volume histograms, conformity and homogeneity indices. Results: Our results showed that the plans optimized with FF beams produced more homogeneous dose distributions in the PTV than the ones with FFF beams (0.92 vs. 0.86 and 0.92 vs.85 for 6MVFF, 6MVFFF, 10MVFF and 10MVFFF respectively). The conformity was better for FFF beams than for FF beams (1.26 vs. 1.20 and 1.27 vs. 1.21 for 6MVFF, 6MVFFF, 10MVFF and 10MVFFF respectively). The overall minimum, maximum and means doses were reduced for the FFF plans by approximately 4% to 6%. All FFF plans allowed more sparing of organs at risk by 7% to 9% on average. Only one 10MVFFF plan had a 1% higher dose than the respective 10MVFF for the spinal cord. Conclusion: The 6MVFFF and 10MVFFF beams illustrated the improvement of normal tissue sparing while achieving similar target dose distribution compared to 6MVFF and 10MVFF beams. Further investigation is required to determine the magnitude of improvement. case specific, fluence-based optimization improved both PTV and OAR dosimetry in 80% of cases.

60 APPLIED LIFE SCIENCES↗

DOE OSTI2015

SU-E-T-448: Heightened Apical Positivity of 2-Year Post-Radiotherapy Biopsies Is Not Related to Suboptimal Dosimetry

Purpose: Previous research has demonstrated that following radiation therapy for prostate cancer, there is a relative increase in positive biopsies in the apex versus the rest of the prostate. The increase could be due to: 1) Inter-fraction apex motion or deformation, 2) Intra-fraction apex motion or deformation, 3) Suboptimal dose coverage in the apex, 4) Tissue composition in the apex and/or 5) Prostate size. In this initial study, the potential for suboptimal dose coverage in the apex was assessed by splitting the prostate planning target volume into the apex (inferior third) and remainder. Methods: 69 patients were selected from 303 patients treated on a clinical radiotherapy trial for prostate cancer. These patients were selected as they had both a localized (sextant template) 2-year post-treatment biopsy and 3D dose information. Of these patients, 10 had positive biopsies in the apex, 8 in the remainder and 11 in both locations. For all patients, the following dosimetric data was acquired from the apex dose volume histogram: Dmean, Dmax, Dmin, D95% and V100%. Unpaired, one-tailed t-tests were used to test for statistical significance (p < 0.05) between all dosimetric parameters for patients with positive versus negative apical biopsies. Additionally, D95% for the apex was plotted against D95% of the remainder. Results: There was no statistical difference for the selected apical dosimetric parameters for patients with positive versus negative biopsies (p-values > 0.05). No correlation was found between D95% (normalized to the prescription dose) for the apex and remainder (R{sup 2} = 0.0116). Conclusion: No correlation was found between positive apical biopsy and suboptimal dosimetric coverage. Current research is looking into inter-fraction apex motion and deformation as a potential source of the increased apical failure using daily CBCT images.

60 APPLIED LIFE SCIENCES↗

DOE OSTI2015

SU-E-T-456: Impact of Dose Calculation Algorithms On Biologically Optimized VMAT Plans for Esophageal Cancer

Purpose: To evaluate the impact of dose calculation algorithm on the dose distribution of biologically optimized Volumatric Modulated Arc Therapy (VMAT) plans for Esophgeal cancer. Methods: Eighteen retrospectively treated patients with carcinoma esophagus were studied. VMAT plans were optimized using biological objectives in Monaco (5.0) TPS for 6MV photon beam (Elekta Infinity). These plans were calculated for final dose using Monte Carlo (MC), Collapsed Cone Convolution (CCC) & Pencil Beam Convolution (PBC) algorithms from Monaco and Oncentra Masterplan TPS. A dose grid of 2mm was used for all algorithms and 1% per plan uncertainty maintained for MC calculation. MC based calculations were considered as the reference for CCC & PBC. Dose volume histogram (DVH) indices (D95, D98, D50 etc) of Target (PTV) and critical structures were compared to study the impact of all three algorithms. Results: Beam models were consistent with measured data. The mean difference observed in reference with MC calculation for D98, D95, D50 & D2 of PTV were 0.37%, −0.21%, 1.51% & 1.18% respectively for CCC and 3.28%, 2.75%, 3.61% & 3.08% for PBC. Heart D25 mean difference was 4.94% & 11.21% for CCC and PBC respectively. Lung Dmean mean difference was 1.5% (CCC) and 4.1% (PBC). Spinal cord D2 mean difference was 2.35% (CCC) and 3.98% (PBC). Similar differences were observed for liver and kidneys. The overall mean difference found for target and critical structures was 0.71±1.52%, 2.71±3.10% for CCC and 3.18±1.55%, 6.61±5.1% for PBC respectively. Conclusion: We observed a significant overestimate of dose distribution by CCC and PBC as compared to MC. The dose prediction of CCC is closer (<3%) to MC than that of PBC. This can be attributed to poor performance of CCC and PBC in inhomogeneous regions around esophagus. CCC can be considered as an alternate in the absence of MC algorithm.

60 APPLIED LIFE SCIENCES↗

DOE OSTI2015

SU-E-T-470: Importance of HU-Mass Density Calibration Technique in Proton Pencil Beam Dose Calculation

Purpose: Stoichiometric calibration of Hounsfield Units (HUs) for conversion to proton relative stopping powers (RStPs) is vital for accurate dose calculation in proton therapy. However proton dose distributions are not only dependent on RStP, but also on relative scattering power (RScP) of patient tissues. RScP is approximated from material density but a stoichiometric calibration of HU-density tables is commonly neglected. The purpose of this work was to quantify the difference in calculated dose of a commercial TPS when using HU-density tables based on tissue substitute materials and stoichiometric calibrated ICRU tissues. Methods: Two HU-density calibration tables were generated based on scans of the CIRS electron density phantom. The first table was based directly on measured HU and manufacturer quoted density of tissue substitute materials. The second was based on the same CT scan of the CIRS phantom followed by a stoichiometric calibration of ICRU44 tissue materials. The research version of Pinnacle{sup 3} proton therapy was used to compute dose in a patient CT data set utilizing both HU-density tables. Results: The two HU-density tables showed significant differences for bone tissues; the difference increasing with increasing HU. Differences in density calibration table translated to a difference in calculated RScP of −2.5% for ICRU skeletal muscle and 9.2% for ICRU femur. Dose-volume histogram analysis of a parallel opposed proton therapy prostate plan showed that the difference in calculated dose was negligible when using the two different HU-density calibration tables. Conclusion: The impact of HU-density calibration technique on proton therapy dose calculation was assessed. While differences were found in the calculated RScP of bony tissues, the difference in dose distribution for realistic treatment scenarios was found to be insignificant.

60 APPLIED LIFE SCIENCES↗