ESTRO 2020 Abstract Book

S1048 ESTRO 2020

structure with a D mean

of 4.78 (1.35-7.8) Gy and a D max of

mediastinal envelope, PTV conformity and OAR compromise indices. The remaining OAR doses although assessed, are not reported in this table as they were easily achieved for this case. All plans were assessed as per the requirements described (table 2) and were considered sub- optimal if not achieving the criteria for at least two of the following: PTV coverage, OAR dose-volume constraints, PTV conformity and OAR compromise indices. All plans were also visually assessed in all slices.

37 (7.6-45.4) Gy. Internal mammary node irradiation resulted in significantly higher doses to the LAD and LV (p=0.002; p=0.003). Pearson correlation coefficients between MHD and D mean for cardiac substructures were all statistically significant. The strongest correlation with MHD was found for D mean to the LAD (r=0.81). For every 1 Gy increase in MHD, D mean LAD rose by 3.4 Gy. Howerver, the proportion of the variance in D mean LAD predictable from MHD was moderate (R² = 0.65). For all other cardiac substructure, the predictive value of MHD was moderate to weak with no R² value above 0.7. Conclusion Our dosimetry study illustrates that the LAD and LV are the most exposed cardiac substructures in left-sided breast cancer radiotherapy. With poor predictive value, MHD was not precise enough to reflect with confidence doses to cardiac substructures, including LAD. For optimal heart sparing radiotherapy, it would be necessary to delineate LV and LAD and to assess doses within these substructures as well as to the whole heart. PO-1881 Dosimetric impact of left anterior descending artery contouring in left-sided breast irradiation Z. Naimi 1 , R. Moujahed 1 , M. Ben Rejeb 1 , S. Ghorbel 1 , J. Yahyaoui 1 , A. Hamdoun 1 , L. Kochbati 1 1 Abderrahmen Mami Hospital, Radiation Oncology, Ariana, Tunisia Purpose or Objective The aim of our study was to assess the impact of contouring the left anterior descending artery on treatment planning in left-sided breast cancer radiotherapy. Material and Methods The data of fifty women treated for left-sided breast cancer were prospectively evaluated. Patients were planned for 3D conformal RT with or without lymph node irradiation. The prescription dose was 40 Gy delivered in 2.67 daily fractions (+/- additional boost of 13.35 Gy). Delineation of the heart substructures and coronary arteries was performed using the cardiac contouring atlas of F.Duane. For each patient, two different treatment plans were generated by two separate dosimetrists. The first plan considered only the heart as organ at risk while the second plan considered the heart as well as the LAD. The following dose metrics were chosen for plan comparisons: The D mean and D max to the heart, LAD and left ventricle, the volume of the heart that received more than 20 Gy (V20 heart ) and the volume of the CTV that received more than 95% of the prescription dose (V95%). Student’s t -test was used to compare these parameters. Results The mean heart dose was significantly lower for the second plan (2.34 vs 3.08 Gy; p<0.001) as well as the D mean (7.1 vs 11.45 Gy ; p<0.001) and D max (18.9 vs 29.5 Gy; p<0.001) to the LAD. Despite no specific dose constraint, optimization on the LAD showed significant reduction in V20 heart (2 vs 3.83%; p<0.001), D mean (3.43 vs 4.78 Gy; p<0.001) and D max (34.5 vs 37 Gy; p<0.001) to the left ventricle. The coverage of the CTV was not substantially affected as there was no significant difference in the V95% between the two plans (96.46 vs 95.06%; p=0.06). Conclusion Despite no specific dose constraint, delineation of the LAD resulted in considerable cardiac dose-sparing with sufficient coverage of the clinical target volume. Since left-sided breast irradiation has been associated with an increased cardiac mortality, the LAD should be routinely contoured and considered as a separate organ at risk. PO-1882 The risk of pelvic insufficiency fractures after radiotherapy using planned dose analysis. L. Devlin 1 , S. Millar 1 , S. Morris 1 , A. Sadozye 1 , R. Harrand 1 , K. Graham 1 , A. Kerr 1 , C. Duncanson 1 , A. Duffton 1

Conclusion 45.5% of the submitted plans were sub-optimal as plans had compromised PTV coverage in favour of meeting OAR dose constraints, overdosed OARs or did not escalated the prescription dose as high is achievable. We attribute the high number of sub-optimal plans to the learning curve in the new individualised planning techniques included in ADSCaN. Assessing plans that use novel dose escalation can be challenging, particularly in determining whether a plan is optimal. This is a novel and objective evaluation that can be applicable to any dose escalated lung RT plan. PO-1880 Cardiac exposure in left-sided breast cancer radiotherapy: which substructure should be contoured? Z. NaimI 1 , R. Moujahed 1 , M. Ben Rejeb 1 , S. Ghorbel 1 , J. Yahyaoui 1 , A. Hamdoun 1 , M. Bohli 1 , L. Kochbati 1 1 Abderrahmen Mami Hospital, Radiation Oncology, Ariana, Tunisia Purpose or Objective The aim of our study was to analyse radiation dose distribution to cardiac chambers and coronary arteries in left-sided breast cancer radiotherapy and to clarify whether the mean heart dose (MHD) reflects the doses to the other cardiac substructures. Material and Methods Fifty women with left-sided breast cancer were prospectively evaluated. Patients received 3D-conformal RT (40 Gy delivered in 15 fractions of 2.67 Gy +/- boost of 13.35 Gy). Whole heart, cardiac chambers and coronary arteries (left main (LM), left anterior descending (LAD), circumflex coronary artery (CX), right coronary (RCA)) were contoured on radiotherapy CT-planning scans according to the cardiac contouring atlas of F.Duane. Dose distribution were generated for all cardiac substructures. Dosimetric associations were calculated. Results All patients met the dose constraint of the heart (mean MHD=3.08 Gy ; mean V20 heart=3.8%). The mean D mean and D max to the LAD were 11.45 and 29.5 Gy, respectively. The D max to the LAD ranged from 2.33 to 39.8 Gy. Low doses were delivered to LM, CX, and RCA ( D mean ≤1.3 Gy; D max ≤1.6 Gy). Considering the other cardiac chambers, left ventricle (LV) was the most exposed