ESTRO 2020 Abstract book

S868 ESTRO 2020

Results The distribution of PTV volumes between the two workflows were vastly different, with CT contoured PTVs averaging 58.9±31.2cm 3 while US/MRI contoured PTVs resulted in much smaller volumes at 37.1±18.6cm 3 . Analyzing the dose coverage of the plans, despite large differences between PTVs, the D90 were very similar, with a median of 105.8±6.3 %PD for CT-based and 100.7±8.3 %PD for US-based procedures. In terms of conformity one can analyze the volume at 100%PD and compare that with the volume of the PTV. Our analysis showed that volumes irradiated at 100%PD exceeded already larger PTV volumes in CT-based procedures by 23.9±13.7 cm 3 compared to much smaller additional volumes 13.9±4.9cm 3 for US- based, pointing to increased conformity. Subvolumes of PTV receiving 150% are, as expected, relatively correlated with target size representing about 22-26% of V100. Not surprising, V200 shows a much lesser correlation with target size, but instead stronger with the number of needles, larger for CT compared to US. Conclusion This comparative study analyzed two different workflows for HDR prostate brachytherapy, based on two imaging methods and two physicians with different styles. Despite largely similar D90 distributions and very good dosimetric quality of all treatments (D90 ranging from 95% to 115%), one can see in Figure 1 that the CT-based V100 are much larger, at least for a subgroup of patients, compared to US- based ones and employing a larger number of needles. In a separate analysis, we concluded that both these factors contribute to more acute toxicity.

Conclusion Non-coplanar trajectory VMAT achieved similar plans compared to the clinical plans. The results indicate a potential patient-specific benefit of using dynamic couch rotations. In the future, the gantry/couch trajectories will be optimized automatically, which is expected to increase the performance of non-coplanar trajectory-based treatment planning. Moreover, treatment delivery efficiency will be evaluated in future studies. PO-1515 A tale of two image-based treatment workflows for prostate HDR: can dosimetry alone tell the story? L. Francis 1 , A. Urdaneta 1 , D. Todor 1 1 VCU Health System, Radiation Oncology, Richmond, USA Purpose or Objective This is a single institution comparative study between two different workflows for HDR prostate brachytherapy. Workflow A, CT-based, and Workflow B, US-based, were each associated with one physician and a different implantation and planning style. While one can imagine multiple perspectives in evaluating differences, this study is aiming to determine if dosimetry parameters alone are capable of completely and accurately describing two vastly different procedures. Material and Methods A total of consecutive 221 delivered patient plans, 74 from Workflow A and 147 from Workflow B, were analyzed. DICOM files containing plans, structures and dose matrices were exported from the treatment planning system (BrachyVision-Varian Medical Sytems, Inc) and analyzed by in-house developed software. In a CT based setting implants had a mean of 18±2.7 needles, while in US-based setting the mean was lower at 14±3.5 needles. In the CT based workflow, patients were implanted using free handed US, then transported under anesthesia to CT and back to the HDR treatment room. In the US based setting, both implantation and planning were done with stepper based acquired images routinely incorporating MRI-US fusions. Patient never moved. All treatments were created using inverse optimization with similar constraints, with physician in workflow A routinely using the graphical tool to ‘adjust’ the optimized plan.

PO-1516 Doses to heart structures in breast cancer: comparing 3D-CRT to IMRT technique M.M. Soraya 1 , M. Vázquez 1 , A. Giraldo 1 , M. Altabas 1 , R. Granado 1 , D. Sanchez 2 , J. Giralt 1 , V. Reyes 1 1 Hopsital Universitari Vall d´Hebron, Radiation Oncology, Barcelona, Spain ; 2 Hospital Universitari Vall d´Hebron, Physics, Barcelona, Spain Purpose or Objective There is strong evidence between radiotherapy in breast cancer and the risk of late cardiac toxicity. Some studies have shown the correlation within Mean Heart Dose (MHD) and the mean dose of its substructures: Left Ventricle (LV) and Left Anterior Descending Artery (LAD). This work wants to demonstrate differences in cardiac dosimetric parameters between 3DCRT and IMRT. Material and Methods We analyzed two groups of breast cancer patients divided for technique, treated at our institution: one treated in 2010 with 3D-CRT technique and another one in 2016 with IMRT technique. The heart, LAD, and LV were contoured, according to RTOG guidelines. Dose-distribution cardiac parameters were extracted from our treatment planning system. Differences were analyzed with two-tailed two-