ESTRO 2022 - Abstract Book

S1676

Abstract book

ESTRO 2022

Materials and Methods One-hundred and twenty-five patients planned for adjuvant breast cancer RT with indication of IMC irradiation were evaluated: 75 patients with left-sided BC and 50 patients with right-sided BC. All patients were treated with hypofractionated 3D conformal RT, by tangential fields +/- supraclavicular field, using a mono-isocentric technique. The prescription dose was 40 Gy delivered in 15 fractions +/- an additional boost of 13.35 Gy. Cardiac subvolumes were contoured based on the Duane’s cardiac contouring atlas. For each patient, DVHs for coronary arteries and cardiac chambers were generated with and without including IMC in the tangential fields. DVHs were statistically compared and the contribution of IMC irradiation to cardiac substructures mean and maximum doses was assessed. Results RT was delivered to the chest wall in 58% and to the remaining breast in 42% of patients. Higher doses to the heart and cardiac subvolumes were reported in the left-sided BC RT when compared to the right-sided BC RT (p<0.01). Regardless of BC laterality, IMC RT was strongly correlated with MHD, Dmean and Dmax to coronary arteries and cardiac chambers with Pearson coefficients ranging between 0.73 and 0.86. The MHD was significantly higher with IMC irradiation with mean values of 3.83 Gy vs 2.49 Gy (p<0.01) in the left-sided BC group, and 1.56 Gy vs 0.83 Gy (p=0.02) in the right-sided BC group. IMC RT contributed to significant increase in Dmean and Dmax to all cardiac chambers and coronary arteries in both left and right-sided BC RT. The greatest contribution of IMC RT to cardiac subvolumes doses was found for the left-anterior descending artery (LAD)/left ventricle (LV) in the left-sided BC RT and for the right coronary artery (RCA) /right atrium (RA) in the right-sided BC RT. IMC RT yielded, on average, 23% increase in Dmax LAD and 29.5% in Dmean LV. For the right- sided BC RT, IMC RT increased on average the Dmax RCA by 17% and Dmean RA by 24.6%. Conclusion IMC RT was related to higher exposure of coronary arteries and cardiac chambers in both right and left-sided BC RT. The exclusive contribution of IMC RT to heart subvolumes doses was not negligible, which may translate into an excessive risk of late cardiovascular morbidity. Heart sparing techniques should be considered when IMC RT is indicated. Purpose or Objective Prostate cancer being the leading cancer disease for men shows high sensitivity to increasing fractional dose. Therefore, ultra-hypofractionated radiotherapy based on high fractional doses is an alternative to the classical radiotherapy scheme. Materials and Methods In this study we performed a retrospective analysis of dose metrics in selected group of patients classified for the HYPOPROST trial comparing ultra-hypofractionated radiotherapy (UF-RT) used as a boost to conventionally fractionated radiotherapy (CF-RT) in high-risk prostate cancer. This dosimetrical analysis includes 179 patients treated by VMAT technique - 86 plans for UF-RT and 93 plans for CF-RT. The physical doses obtained from the second phase of UF-RT were recalculated to biologically equivalent doses using EQD2 formula based on the linear quadratic model. The a/b ratios used in the recalculations were 1,5 for the tumor and 3,0 for the OARs. Before the plan acceptance for delivery, the biologically equivalent doses from the first and second phases were summed and checked for adherence to required dose constraints for OARs as well as to adequate dose distribution in the PTVs. The ICRU-83 plan normalization criteria for the PTVs were followed, with prescription to the median dose. The treatment plans for the CF-RT and UF-RT arms were analyzed for the potential differences between dose metrics using the following parameters: • · the mean EQD2 [Gy], and the EQD2 related to 2% (D2 [Gy]), and 95% (D95 [Gy]) of the PTV (the prostate and the basal volume of seminal vesicles), • · the EQD2 related to 5% (D5 [Gy]), 25% (D25 [Gy]), 30% (D30 [Gy]), and 40% (D40 [Gy]) of the rectum volume, • · the EQD2 related to 10% (D10 [Gy]), 25% (D25 [Gy]), 30% (D30 [Gy]), and 40% (D40 [Gy]) of the bladder volume, • · the EQD2 related to 10% (D10 [Gy]) of the volumes of femoral heads and the bowels. Results Despite the increase in dose delivered to PTV (e.g., mean EQD2 for UF-RT equal to 84,6 Gy vs 76,3 Gy for CF-RT) the doses delivered to OARs were clinically comparable. Due to the clinical constraints, the statistical superiority of UF-RT for the doses D25 (CF-RT 54,6 Gy vs UF-RT 51,2 Gy), D30 (CF-RT 51,9 Gy vs UF-RT 48,0 Gy), D40 (CF-RT 47,4 Gy vs UF-RT 43,5 Gy) of the rectum, and D10 (CF-RT 38,5 Gy vs UF-RT 34,5 Gy) of the femoral heads were clinically not relevant. Conclusion The statistical differences between mean values of EQD2 parameters obtained for PTV are caused by the scheme of treatment. Dosimetric analysis of the boost plans prepared for UF-RT and CF-RT schemes showed that biologically equivalent doses EQD2 for organs at risk were statistically close for both schemes in agreement with clinically observed similar toxicity outcomes for these patient groups. PO-1892 Dosimetric comparison of conventionally and ultra-hypofractionated RT boost in prostate cancer J. Krawczyk 1 , A. Ginter 1 1 Greater Poland Cancer Centre, Radiotherapy Department I, Pozna ń , Poland

PO-1893 could we consider the Pancreas as an organ at risk during gastric cancer radiotherapy?