V Dumane

SU‐E‐T‐595: Comparison of Volumetric Modulated Arc Therapy (VMAT) and Static Intensity Modulated Radiotherapy (IMRT) for Malignant Pleural Mesothelioma in Patients with Intact Lungs/Post Pleurectomy

PURPOSE This planning study compares VMAT and static gantry, sliding window IMRT for malignant pleural mesothelioma for post pleurectomy. METHODS We compared plans for a left sided (L) and a right sided case (R). Plans used clinically approved planning target volumes (PTVs) and critical organ contours. IMRT plans employed 7-8 6 MV photon beam directions over a 215° range centered on the ipsilateral lung. VMAT plans used 4 partial arcs within the same range and energy. Prescription dose per fraction was 1.8 Gy; case L went to 50.4 Gy, case R to 46.8 Gy. Planning objectives were: Lyman model NTCP for both lungs < 25%; contralateral lung, mean dose < 8 Gy; heart, V30 Gy < 50%, mean < 30 Gy; Each Kidney, V18 Gy < 33%; liver_not_GTV, mean < 30 Gy, V30 Gy < 50%; stomach not PTV, mean < 30 Gy; cord maximum < 45 Gy; bowel maximum < 55 Gy, D05 < 45 Gy; PTV D95 = 94%, V95 = 94%, D05 = 115%. Dose calculation was done with the AAA algorithm. RESULTS VMAT and IMRT both met the dosimetric constraints. The VMAT MU were 887 (L)_and 896 (R) and for IMRT were 1691 (L) and 2409 (R). IMRT required 14-16 fields (wide-field splitting). The delivery times were 8 minutes (VMAT) and 20 minutes (IMRT). For coverage and plan homogeneity parameters within 1.5% - 2%, VMAT better spared organs at risk. CONCLUSIONS Both VMAT and IMRT are feasible techniques for the treatment of malignant pleural mesothelioma with intact lungs, with less MU and a shorter delivery time for VMAT. Additional cases must be planned to test generality of our preliminary results.

SU-E-T-491: Influence of Applicator Dimensions On Doses to Bladder, Rectum and Sigmoid in HDR Brachytherapy for Cervical Cancer

Purpose: To study the influence of dimensions of the tandem and ring applicator on bladder D2cc, rectum D2cc and sigmoid D2cc in HDR treatment planning for cervical cancer. Methods: 53 plans from 13 patients treated at our institution with the tandem and ring applicator were retrospectively reviewed. Prescription doses were one of the following: 8 Gy x 3, 7 Gy x 4 and 5.5 Gy x 5. Doses to the D2ccs of the bladder, rectum and the sigmoid were recorded. These doses were normalized to their relative prescriptions doses. Correlations between the normalized bladder D2cc, rectum D2cc and sigmoid D2cc were investigated and linear regression models were developed to study the dependence of these doses on the ring diameter and the applicator angle. Results: Normalized doses to the D2cc of the bladder, rectum and sigmoid showed statistically significant correlation (P < 0.05) to the applicator angle. Significant correlation was also noted for the normalized D2cc of the rectum and the sigmoid with the ring diameter. The normalized bladder D2cc was found to decrease with applicator angle on an average by 22.65% ± 4.43% while the same for the rectum and sigmoid were found to increase on an average by 14.43% ± 1.65% and 14.01% ± 1.42% respectively. Both the rectum and sigmoid D2cc reduced with increasing ring diameter by 12.93% ± 1.95% and 11.27% ± 1.79%. No correlation was observed between the normalized bladder D2cc and the ring diameter. Conclusion: Preliminary regression models developed in this study can potentially aid in the choice of the appropriate applicator angle and ring diameter for tandem and ring implant so as to optimize doses to the bladder, rectum and sigmoid.

SU-E-T-342: Use of Patient Geometry Measurements to Predict Dosimetric Gain with VMAT Over 3D for Chestwall and Regional Nodal Radiation

PURPOSE To predict the dosimetric gain of VMAT over 3D for the treatment ofchestwall/IMN/supraclavicular nodes using geometric parameters acquired during simulation METHODS: CT scans for 20 left and 20 right sided patients were retrospectively analyzed toobtain percent ipsilateral lung volume included in the PWT and supraclavicular fields, central lung depth (CLD), maximum lung depth (MLD), separation, chestwall concavity (defined here as the product of CLD and separation) and the maximum heart depth (MHD). VMAT, PWT and P/E plans were done for each case. The ipsilateral lung V20 Gy and mean, total lung V20 Gy and mean, heart V25 Gy and mean were noted for each plan. Correlation coefficients were obtained and linear regression models were built using data from the above training set of patients and then tested on 4 new patients. RESULTS The decrease in ipsilateral lung V20 Gy, total lung V20 Gy, ipsilateral lung mean and total lung mean with VMAT over PWT significantly (p<0.05) correlated with the percent volume of ipsilateral lung included in the PWT and supraclavicular fields with correlation coefficient values of r = 0.83, r = 0.77, r = 0.78 and r = 0.75 respectively. Significant correlations were also found between MHD and the decrease in heart V25 Gy and mean of r = 0.77 and r = 0.67 respectively. Dosimetric improvement with VMAT over P/E plans showed no correlation to any of the geometric parameters investigated in this study. The dosimetric gain predicted for the 4 test cases by the linear regression models given their respective percent ipsilateral lung volumes fell within the 95% confidence intervals around the best regression fit. CONCLUSION The percent ipsilateral lung volume appears to be a strong predictor of the dosimetric gain on using VMAT over PWT apriori.

SU-E-T-341: Use of Patient Geometry and Multiple Linear Regression to Predict Prescription Dose for Pleurectomy/decortication Cases in Malignant Pleural Mesothelioma

PURPOSE To identify patient geometric parameters that correlate with the prescription dose in the treatment of malignant pleural mesothelioma (MPM) so as to build a model that could assist in the prediction of the same apriori and this help in planning. METHODS Planning CT scans for ten right sided and ten left sided patients were examined to extract five parameters for each case, which involved ratio of the ipsilateral lung volume to the total lung volume (X1), volume of PTV overlap with ipsilateral lung to ipsilateral lung volume (X2), volume of the PTV overlap with ipsilateral lung to total lung volume (X3), volume of the PTV overlap with ipsilateral lung to PTV volume (X4) and ratio of contralateral lung volume to total lung volume (X5). Each patient for this study was planned with VMAT using 6 MV on Eclipse V11 to a prescription dose that kept the mean total lung dose just under 20 Gy. Correlation coefficients were obtained between the parameters and the dose prescribed. Those that significantly correlated were combined using multiple linear regression. The model built was tested on 9 new cases. RESULTS All geometric parameters investigated except X2 showed significant correlation with the prescription dose. The correlation coefficients were r = -0.5421 for X1 (p = 0.0135), r = -0.6891 for X3 (p = 0.0008), r = -0.6797 for X4 (p = 0.001) and r = 0.5421 for X5 (p = 0.0135). The predictive model D = 4192.2×1 - 3577.1×3 - 471.6×4 + 6253.4×5 had a higher correlation (r = 0.86, p = 1.0657e-06) with dose. For 8 out of the 9 test cases, the predicted dose was either equal to or upto 1 fraction of the prescription dose. CONCLUSION The multiple regression model developed here could serve as a guide for dose prescribed in treatment of MPM.

SU‐E‐T‐315: Planning and Verification of CT‐Based HDR Intraluminal Brachytherapy Treatment for Malignant Obstructive Jaundice

PURPOSE To present our experience using CT to plan and verify intraluminal HDR treatment for a patient with obstructive jaundice. Due to the obstructions proximity to the small bowel, along with small bowel adhesions from past surgical history, it was imperative to verify source position relative to the bowel before each treatment. METHODS Treatment was administered to a total dose of 2000cGy in 5 fractions via a 6F intraluminal catheter inserted into the patients 14F percutaneous drainage catheter. Graduations on the intraluminal catheter were used to measure the exact length of catheter inserted in to the patients drainage tube allowing reproducibility. Dummy seeds inserted during CT were identified by iteratively aligning the planning systems 3D reconstruction axis to the catheter at multiple points as it snaked through the liver. Taking in to account the known offset between actual dwell positions and dummy source positions, we determined what dwell positions to activate for planning. CT verification was performed prior to each treatment to insure that the drainage catheter had not moved and that the distance from treatment site to small bowel was adequate. Dummy seeds and anatomical landmarks were identified on the scout image and correlated to the CT. RESULTS Verification CTs showed remarkable consistency in the day-to-day drainage catheter position. The physician was able to easily identify the small bowel of concern on the CT and determine if a safe distance existed for treatment. CONCLUSIONS The method outlined in this work provides a safe means by which to treat bile duct obstructions using HDR when critical structures are nearby. We were prepared to make real-time adjustments to our treatment plan to account for significant variation, but found it unnecessary to do so in this particular case.

SU‐E‐T‐834: Comparison of Rapidarc, IMRT and 3D Conformal Planning for Treatment of Chestwall, and Regional Nodes

Purpose: To investigate the application of Rapidarc as a technique to improve the dosimetric plan quality for the treatment of the chestwall and regional nodes when 3D conformal planning using the partially wide tangents (PWT) technique requires the irradiation of more than 3 cm of the ipsilateral lung and/or the heart in order to achieve coverage to the IMNs, and to compare it with IMRT. Methods: The PWT plan was done using the field in field technique. IMRT planning consisted of 6 beams at five gantry angles, chosen to minimize the entrance or exit through the contralateral structures. The arc range for the Rapidarc plan was similar to IMRT. Dose calculation was performed with the AAA algorithm in Eclipse V 8.9. Dose prescribed was 5040 cGy. Results: PTV D95 and V95 for both the Rapidarc and IMRT plans were 98% and 97% respectively. The same for 3D was lower by 4%–5% respectively. Rapidarc reduced the left lung mean dose compared to 3D and IMRT by as much as 19 Gy. The V20 Gy and V10 Gy were lower with Rapidarc by as much as 45% and 50%. The total lung mean dose was lower with Rapidarc by 8 Gy while the V20 Gy and V10 Gy reduced by 22% and 25%. Heart doses more than 25 Gy were identical with 3D and Rapidarc while IMRT gave a mean heart dose that was higher by 11 Gy. The mean dose to the contralateral organs was higher with Rapidarc by 3 Gy compared to 3D. Conclusions: For the same coverage, Rapidarc was able to better spare critical organs compared to IMRT. Compared to 3D, Rapidarc had better coverage and lower ipsilateral and total lung dose and identical volumes of heart irradiated above 25 Gy.

SU‐E‐T‐203: Sensitivity Study on the Use of Computed Radiography for Linac Routine Quality Assurance ‐ Light/radiation Field Coincidence

Purpose: To investigate the accuracy of using computed radiography(CR) system for the light/radiation field coincidence test and to verify that the accuracy level can be achieved by using this technique as a linac routine quality assurance check. Methods: Kodak 2000RT CR system together with a Kodak Flexible Phosphor plate was used in this study. The phosphor plate was exposed with background radiation (cover the entire plate), radiation field and then field light. All measurements were carried out by using 6 MV X‐rays (Varian 21iX) and 100 cm SSD setup. The room lights were off during the entire measure procedure to control the amount of light field discharge on the CR plate. Results: Delivering 1 and 2 MUs to background and radiation fields, respectively and shining the field light for 35 seconds can provide a good quality of contrast to notify discrepancy (darker or lighter stripe along field edges) between light and radiation fields. Users can visually indicate the discrepancy approximately 1 mm without using any tool. For discrepancy less than 1 mm in the penumbra region between light and radiation fields, by analyzing the light/radiation field edge profiles of CR plate response, accuracy of less than 1 mm can be achieved. Conclusions: The CR system can be used as a replacement of the film to perform the linac routine QA procedures. For light/radiation coincidence test, the result can be directly determined by eyes if using TG‐40 tolerance (i.e., < 2mm). Compared to conventional marking film technique, our method does not involve any intervention between light/radiation fields, which makes it possible to reach higher accuracy level (<1 mm).

SU‐E‐T‐407: Investigation of the Need for Bolus for Superficial Head and Neck Tumors Treated with 3D, RapidArc, and IMRT

Purpose: To determine whether or not the multiple oblique and tangential beam angles of RapidArc and IMRT treatment plans eliminate the need for bolus when treating superficial tumors. This study uses film measurements to evaluate the efficacy of IMRT and RapidArc planning techniques employed to achieve adequate surface dose without bolus. Methods: Gafchromic EBT2 QD+ film was used to measure skin dose for plans created using Eclipse 8.9 and delivered via a Varian 21iX linac. Dose was delivered to a cylindrical Catphan (CP504) and the film measurements were evaluated using in‐house software. Measurements were compared to results from the treatment planning system calculated using Varians Anisotropic Analytical Algorithm (8.9.08). Results: The Gafchromic film measurements showed that the 7‐field IMRT plan with strategically placed beams tangential to the phantom surface did increase skin dose 63% compared with a non‐bolused 3D plan. RapidArc delivery increased the skin dose by an additional 13% beyond IMRT. However, even with RapidArc, the skin dose obtained was only 44% of the prescribed dose (2Gy) showing that bolus is still required. Attempts to modify planning technique by adding skin flash to lessen the dose falloff and expanding the external contour to match had no effect on skin dose. Incidentally, we found that the 5mm bolus often used for head and neck plans only brought the surface dose to 78% of the prescription dose (in the case of 3D) which may still be clinically inadequate. Conclusions: IMRT and RapidArc treatment plans delivered without bolus do increase skin dose compared with their 3D counterparts, but the effect is not great enough avoid using bolus when prescription surface dose is required. While planning techniques to lessen dose fall off at the skin may be useful for managing setup uncertainty, they do not increase surface dose in any way.