Abstract PS15-08: Definition of a high risk cardiac zone on non-contrast computed tomography (CT) scans for indirect optimization of left anterior descending coronary artery (LADCA) dosimetry for breast radiotherapy
Authors: Loap Pierre , Tkatchenko Nicolas , Nicolas Eliot , Kirova Youlia 
Published: February 2021

Abstract
Background : Breast radiotherapy is associated with a cardiotoxicity risk which could be decreased by optimizing radiation dose to the left anterior descending coronary artery (LADCA). However, precise delineation of the LADCA on non-contrast computed tomography (CT) scans is complex and poorly reproducible.
Purpose : This study aimed to define a reproducible “high risk cardiac zone” (HRCZ) which could straightforwardly replace LADCA delineation on non-contrast CT scans for treatment planning optimization. Additionally, we compared standard dosimetric parameters (mean and maximum doses) of the proposed HRCZ and of the LADCA.
Materials/methods : Forty breast cancer patients treated with intensity-modulated radiation therapy (IMRT) were randomly selected from our institutional database. The LADCA was manually delineated on the non-contrast simulation CT scans according to ESTRO guidelines (Duane et al., 2017), to ensure reproducibility, and each contour was validated by a staff of three radiation oncologists. “High risk cardiac zones” (HRCZ) were defined as segments of the anterior cardiac wall, geometrically centered around the inter-ventricular groove where the LADCA anatomically lies (from top to bottom) with a constant 1cm-thickness and a symmetrical lateral expansion on both sides of the groove (HRCZ width). For each patient, eight “high risk cardiac zones” (HRCZ) were delineated differing by their width, ranging between 1 cm and 8 cm (by steps of 1cm). Mean and maximum doses to the LADCA and to the HRCZ were retrieved, and relative variations were calculated as follows, as a function of the HRCZ width:
Mean dose relative variation = [mean dose (HRCZ) - mean dose (LADCA)]/mean dose (LADCA)
Max. dose relative variation = [max. dose (HRCZ) - max. dose (LADCA)]/max. dose (LADCA)
Results : Mean and maximum dose relative variations between HRCZ and LADCA are reported in table 1 as a function of HRCZ width, for left-sided and right-sided irradiation. For a given HRCZ width, LADCA radiation exposure can be straightforwardly deducted from HRCZ dosimetric parameters, by applying the corresponding dose adjustments. HRCZ had higher maximum doses than LADCA, independently of the width. However, for a HRCZ width larger than 4 cm, dosimetric adjustments for the maximum doses drastically increased for right-sided irradiation with a potential risk of non-representativeness.
Conclusion : When planning breast radiotherapy on non-contrast CT scans, delineating a segment of the anterior cardiac wall centered on the inter-ventricular groove from top to bottom, with a 1cm thickness and a 3-4cm width, can replace LADCA uncertain contouring and can be conveniently used for indirect coronary sparing.
Table 1: maximum and mean dose relative variation between the LADCA and the proposed HRCZHRCZ width (cm)Max. dose var. (left-sided RT)Mean dose var. (left-sided RT)Max. dose var.(right-sided RT)Mean dose var. (right-sided RT)15.3%-13.0%7.8%-1.4%212.6%-19.1%16.3%-2.4%319.1%-22.6%26.4%-2.8%424.8%-26.0%38.6%-2.8%529.0%-28.5%52.2%-2.2%629.0%-32.3%71.3%-1.1%729.9%-36.0%92.4%0.5%829.9%-38.8%118.3%2.8%Table 1: maximum and mean dose relative variation between the LADCA and the proposed HRCZ, as a function of the HRCZ width. HRCZ: “high risk cardiac zone”. LADCA: Left anterior descending coronary artery.
Citation Format: Pierre Loap, Nicolas Tkatchenko, Eliot Nicolas, Youlia Kirova. Definition of a high risk cardiac zone on non-contrast computed tomography (CT) scans for indirect optimization of left anterior descending coronary artery (LADCA) dosimetry for breast radiotherapy [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 PS15-08.