Purpose: Cardiac toxicity after breast cancer (BC) radiation therapy is partly due to the large radiation doses to coronary arteries. The left anterior descending artery (LAD) is particularly exposed. A first step in achieving robust dose constraints to the LAD during treatment planning is homogeneous delineation based on guidelines. LAD delineation can be problematic due to heart movements. The aim of the study was to establish a safety margin for delineation of the LAD in patients with BC. Methods and Materials: We studied 45 patients with left-sided BC who had an indication for adjuvant radiation therapy between 2015 and 2018. They all underwent cardiac-gated computed tomography scan, as well as planning computed tomography scans with or without contrast agents, to assess LAD diameter and movements. Cardiac-gated computed tomography scan was performed during monitoring of the cardiac cycle. Acquisition was launched immediately after contrast injection (arterial sequence), with deep-inspiration breath hold and use of a beta-receptor blocking agent. By manually reviewing each scan, the LAD positions and diameter were defined at 20 different phases of the cardiac cycle at 5 different sites: ostium (OS), circumflex bifurcation (bfc), first diagonal bfc, second diagonal (D2) bfc, and apex (right coronary anastomosis). Results: Movement of the LAD is maximal at the ostium and then constant overall even when far from its origin. The diameter decreases with the distance from ostium: 4.9 mm (OS), 3.9 mm (circumflex), 3.5 mm (D2), and 3.1 mm (D2). Conclusions: We suggest using a safety delineation margin consisting of a cylinder with a diameter of 10 mm surrounding the LAD. These findings must be validated in independent series of patients treated for BC.

Background: Cardiac toxicity after breast cancer (BC) radiotherapy (RT) is partly due to the large radiation doses to coronary arteries. The left anterior descending artery (LAD) is particularly exposed. A first step in achieving robustly proven dose constraints to the LAD during treatment planning is homogeneous delineation based on guidelines. LAD delineation can be problematic due to heart movements.Purpose/Objective(s): The aim of the study was to establish a safety margin for delineation of the LAD in BC patients.Materials/Methods: We studied 45 left-sided BC patients who had an indication for adjuvant radiotherapy between 2015 and 2018. They all underwent cardiac-gated computed tomography scan (CG-CT), as well as planning CT scans (P-CT) with or without contrast agents, to assess LAD diameter and movements. CG-CT was performed while monitoring the cardiac cycle. Acquisition was launched immediately after contrast injection (arterial sequence), with deep inspiration breath hold and use of a beta-receptor blocking agent. By manually reviewing each scan, the LAD positions and diameter were defined at 20 different phases of the cardiac cycle at 5 different sites: ostium, circumflex (Cx) bifurcation (bfc), first diagonal (D1) bfc, second diagonal (D2) bfc and apex (right coronary anastomosis). Next step is to introduce it in softwear of automatic delineation to improve the atlas of cardiac structures.Results: Movement of the LAD is maximal at the ostium, then constant overall even when far from its origin. The diameter decreases with the distance from ostium: 4.9 mm (OS), 3.9 mm (Cx), 3.5 mm (D2) and 3.1 mm (D2).Conclusion: We suggest using a safety delineation margin consisting of a cylinder with a diameter of 10 mm surrounding the LAD. These findings must be validated in independent series of patients treated for BC. Citation Format: Eliot Nicolas, Nadjoua Khalifa, Sabrina Bouhroum, Cyril Laporte, Youlia Kirova. Safe margins for delineation of left anterior descending artery (LAD) using cardiac-gated CT-scan (CG-CT) in left side breast cancer patients [abstract]. In: Proceedings of the 2020 San Antonio Breast Cancer Virtual Symposium; 2020 Dec 8-11; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2021;81(4 Suppl):Abstract nr PS15-13.