ESTRO 2020 Abstract Book

S819 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.

sample unpaired Mann-Whitney U-test(α=0.05).The association between MHD and mean LV and LAD doses was determined using linear regression. Results The study cohort consisted of 117 3DCRT patients (68 left(L)-sided and 49 right(R)-sided BC) and 114 IMRT patients (76L-sided and 38R-sided). IMRT was offered to those patients whose treatments did not meet risk organ restriction doses with 3DCRT. Patient characteristics and dosimetric parameters are shown in Table1. For the L-BC patients we observe that in IMRT group, despite having a higher MHD the rest of the parameters are lower. In the R- BC patients this correlation is similar. As we can see in Figure1, in IMRT groups the correlation between MHD and the rest of the cardiac parameters was not as strong as in 3DCRT ipsilateral group, and the data dispersion was greater, especially in LAD.

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-

Conclusion IMRT in left-BC compared to 3DCRT decreases doses of contoured cardiac substructures based on MHD as a reference. Correlation between MHD and other cardiac dosimetric parameters is weaker in IMRT than in 3DCRT, as is the ability to predict doses in cardiac substructures from MHD, especially LAD.