ESTRO 2020 Abstract Book

S98 ESTRO 2020

changes did not correlate with MHD and V5 indicating that whole heart dosimetry parameters do not predict physiological changes resulting from irradiation of cardiac

subvolumes. Conclusion

This is the first report demonstrating structural and functional consequences of subvolume targeting in the mouse heart. Our data are in close agreement with clinical observations indicating the base of the heart as a radiosensitive region and form the basis for further mechanistic investigations. OC-0197 Dose surface maps of the heart identify dose sensitive regions A. McWilliam 1,2 , L. Graham 1 , C. Dootson 1 , A. Abravan 1 , M. Van Herk 1,2 1 University of Manchester, Division of Cancer Science, Manchester, United Kingdom ; 2 The Christie NHS Foundation Trust, Radiotherapy Related Research, Manchester, United Kingdom Purpose or Objective It has become accepted that for lung cancer patients the radiation dose to the heart is an independent predictor of survival. Analysis of dosimetric effects on the heart have investigated volumetric dose statistics, however potential critical cardiac structures are present on the heart surface. Volumetric parameters may not be optimal due to the required inter-patient registration accuracy. In this work we present a methodology for creating cardiac surface dose maps and use these to identify cardiac surface regions where excess dose results in worse patient survival. Material and Methods 648 cardiac surface maps were successfully created with a polar coordinate system with the centre positioned at the centre of mass for the heart contour of each slice in turn. This accounted for the asymmetric nature of the heart and can be considered a modified cylindrical coordinate system. The radiotherapy dose for each patient was sampled on this surface with the described coordinate system. All hearts were normalised to the same superior-inferior dimensions for analysis. For validation of the mapping, and localisation of dosimetric effects, the cardiac chambers were manually delineated and mapped onto the surface maps for 20 patients. A point spread function (PSF) was fitted to the blurred edge of the mapped chambers to quantify uncertainty in the mapping process. To account for this uncertainty, dose maps were blurred by a 2D-Guassian function with width described by the PSF. Permutation testing was used to identify regions where excess dose resulted in worse patient survival with the T- map calculated. A threshold was set at the 99 th percentile of the T-map and the dose from the cardiac surface each patient received extracted for analysis in a multi-variable cox-proportional hazards survival model. Results All surface maps were blurred with 2D-Guassian filter of size σ φ =4.3° and σ y = 1.3 units. Permutation testing for patients dead and alive at 6,12,18 and 24 months showed significant differences, p<0.001. The T-map for 18 months is included as figure 1 highlighting the highest significance region at the base of the heart and extending into the right and left atria. The mean dose to the region defined by the 99 th percentile across all patients was 21.6Gy compared to the mean dose to the heart of 12.7Gy. Table 1 shows the multi-variable analysis where the dose to this region on the heart surface is significantly associated with survival, hazard ratio 1.014 per Grey, p=0.03, controlling for covariates including tumour volume.

Conclusion We successfully created a modified cylindrical polar coordinate system for mapping radiotherapy dose to the heart. Surface maps identified a region near the base of the heart where excess dose results in worse patient survival. This region extends over the left and right atria close to the path of the coronary arteries suggesting these sub-structures are driving this effect. OC-0198 Heart sparing radiotherapy in breast cancer: the importance of baseline cardiac risks S. Schönecker 1 , A. Gaasch 1 , C. Simonetto 2 , M. Eidemüller 2 , D. Reitz 1 , M. Pazos 1 , M. Rottler 1 , P. Freislederer 1 , M. Braun 3 , N. Harbeck 4 , M. Niyazi 1 , C. Belka 1 , S. Corradini 1 1 LMU Munich, Department of Radiation Oncology, Munich, Germany ; 2 Helmholtz Center Munich, Institute of Radiation Medicine, Munich, Germany ; 3 Red Cross Hospital, Department of Obstetrics and Gynecology- Breast Centre, Munich, Germany ; 4 LMU Munich, Department of Obstetrics and Gynecology- Breast Centre, Munich, Germany Purpose or Objective Patients with left-sided breast cancer have an increased risk of developing cardiovascular disease (CVD) after radiotherapy (RT). While the awareness of cardiac toxicity through irradiation has increased enormously over the last decade, the role of individual baseline cardiac risks has not yet been systematically investigated. Aim of the present study was to evaluate the impact of baseline CVD risks on radiation-induced cardiac toxicity. Material and Methods 210 patients with left-sided breast cancer treated in the prospective Save-Heart Study using a modern heart sparing technique (deep inspiration breath-hold, DIBH), were analysed regarding baseline risk factors for CVD. Three frequently used prediction tools (Procam, Framingham and Reynolds score) were applied to evaluate the individual CVD risk profiles. Moreover, 10-year CVD excess absolute risks (EAR) were estimated using the individual mean heart dose (MHD) of treatment plans in free breathing and DIBH. Results The individual baseline CVD risk factors had a strong impact on the 10-year cumulative CVD risk. The mean baseline risks of the non-diabetic cohort non-diabetic