ESTRO 2021 Abstract Book

S1599

ESTRO 2021

A cohort of 7 left and 7 right sided breast patients, planned on RS v7 or v9 with dual partial arc VMAT have undergone IMC Radiotherapy Treatment Planning. Breast, IMC and SCF PTV coverage and Ipsilateral and Contralateral Lung, Heart and Contralateral Breast are used to assess the plan quality achieved. Robustness of the breast CTV to lateral and anterior motion of 10mm and IMC coverage robustness to posterior motion of 5mm were evaluated using RayStation’s perturbed dose features. DIBH methods were not available for this study and the cohort includes some cases where the relative locations of OARs and Targets are very challenging. Results PTV breast, SCF and IMC coverage at V38Gy > 95%, > 88.5Gy and >96.6G% respectively were achieved, with V36Gy > 95.2% and >99.7% for IMC and SCF respectively. Maximum doses were less than 44.0Gy (110%) in all cases, with a maximum volume of 1.7% receiving 42Gy (105%) in any case (average 0.6%). Average mean heart doses of 5.2Gy (range 3.2Gy to 7.5Gy) for left and 2.5Gy (range 1.7Gy to 3.4Gy) for right sided cases were recorded. Average contralateral breast 3.52Gy (range 2.3-6.54Gy), average contralateral mean lung dose of 2.4Gy (range 1.4Gy to 3.2Gy) and average ipsilateral lung V17Gy = 29.3% (range 19% to 43%). In all plans Breast CTV robust to 10mm anterior and lateral motion and IMC CTV robustness to 5mm posterior motion were assessed by visual assessment and 38Gy coverage, and visual assessment of planned and perturbed dose distributions were acceptable in all cases. Conclusion High quality plans, robust to delivery uncertainties and likely inter and intra-fraction patient shape changes, can be created using partial arc VMAT for both left and right sided breast cohorts. TPS manufacturers should be encouraged to continue to develop optimisation tools to achieve high plan quality in a robust way. PO-1879 plan complexity and delivery accuracy of dose escalation to DIL in high-risk prostate cancer K.S. Lew 1,1 , A.L.K. Ong 1 , H.Q. Tan 1 , W.Y.C. Koh 1 , K.W. Ang 1 , J.C.L. Lee 1 , J.K.L. Tuan 1 , S.Y. Park 1 1 National Cancer Centre Singapore, Department of Radiation Oncology, Singapore, Singapore Purpose or Objective Dose escalation in Prostate cancer has shown to be correlated to reduced risk of biochemical failure. A SIB boost to Dominant Intra-prostatic lesion (DIL) is capable of achieving more than 90 Gy (2 Gy equivalent dose) with limited toxicity and obtain good local tumor control. However, a SIB boost plan tends to require more MLC modulation and result in a more complex plan. Thus, in this study, we compare the patient-specific QA (PSQA) result and quantify the dose delivery error. Materials and Methods Sixteen patients were recruited in this study (n=16). Eight patients were planned with DIL boost and the remaining eight without. All patients were treated with 2-3 VMAT arcs with a prescription dose (PD) of 46 Gy to the pelvic lymph nodes and 60 Gy to the prostate. DILs were identified by multi-parametric MRI and dose were escalated to 107-110% of the PD depending on the location in relative to the proximal organs-at-risk. PSQA was performed using portal dosimetry and plan complexity was calculated using modulation complexity score (MCS). The log file for the QA was used to generate a new plan with the actual gantry and MLC positions. The dose distribution in the new plan was calculated in the TPS and was compared to the planned dose distribution to determine the dosimetric errors. Mann-Whitney U tests were used to test for difference in the MCS and PSQA gamma passing rate. Results There is a statistically significant difference between the MCS ( P =0.036) and gamma passing rate results at 2%/2mm ( P <0.01) and 1%/1mm ( P=0.046 ). The MCS of DIL boost plan is lower indicating higher plan complexity. The gamma passing rate of the boost plan is lower indicating lesser delivery accuracy. The reconstructed plan from log file shows 0.35% to 0.67% and 0.03% to 0.25% reduction in DIL D95% and D98% respectively.