Optimization and auto-segmentation of a high risk cardiac zone for heart sparing in breast cancer radiotherapy

Breast radiotherapy is associated with a risk of ischemic heart disease. Limiting left anterior descending coronary artery (LADCA) exposure might possibly reduce coronary risk. However, its manual delineation is poorly reproducible and its auto-segmentation remains unreliable. This study aims to define and characterize a high-risk cardiac zone (HRCZ) as a LADCA surrogate and to implement its auto-segmentation.

Forty breast cancer patients treated with adjuvant IMRT were included. We delineated the LADCA and eight HRCZ, defined as 1 cm-thick cardiac wall segments centered on the LADCA with symmetrical lateral margins defining the HRCZ width (ranging between 1 and 8 cm). We retrieved mean and maximum doses to the LADCA and to the HRCZ and calculated relative dose variations. We constituted an atlas with the HRCZ contours of 20 patients. Based on this latter, a commercial atlas-based auto-segmentation software delineated HRCZ for the remaining 20 patients and performances were evaluated using distance metrics.

Relative maximum dose variations were systematically positive and increased with HRCZ width, rising from 7.2% to 112.8% for right-sided irradiation (with a sharp increase above 4 cm), and from 9.5% to 30.4% for left-sided irradiation. Auto-segmentation performances asymptotically improved with HRCZ width: Dice similarity coefficient values were 0.62 for a 3 cm width and 0.69 for an 8 cm width.

A 3.5 cm-wide HRCZ is a reliable LADCA surrogate for breast radiotherapy. Applying maximum dose constraints to the HRCZ could limit LADCA exposure. Auto-segmentation algorithms can reliably delineate HRCZ.