Cardiac substructure exposure in breast radiotherapy: a comparison between intensity modulated proton therapy and volumetric modulated arc therapy.

INTRODUCTION: Proton therapy for breast cancer treatment reduces cardiac radiation exposure. Left-sided breast cancer patients with indication for internal mammary chain (IMC) irradiation are most at risk of radiation-induced cardiotoxicity. This study aims to evaluate in this situation the potential dosimetric benefit of intensity modulated proton therapy (IMPT) over volumetric modulated arc therapy (VMAT) at the cardiac substructure level. MATERIALS AND METHODS: Cardiac substructures were retrospectively delineated according to ESTRO guidelines on the simulation CT scans of fourteen left-sided breast cancer patients having undergone conserving surgery and adjuvant locoregional free-breathing (FB-) or deep inspiration breath-hold (DIBH-) VMAT with internal mammary chain irradiation. IMPT treatment was re-planned on the simulation CT scans. Mean doses to cardiac substructures were retrieved and compared between VMAT treatment plans and IMPT simulation plans. Pearson correlation coefficients were calculated between mean doses delivered to cardiac substructures using these two techniques. RESULTS: Mean doses to all cardiac substructures were significantly lower with IMPT than with VMAT. Regardless of the irradiation technique, the most exposed cardiac substructure was the mid segment of the left anterior descending coronary artery (LADCA). Pearson correlation coefficients between mean doses to cardiac substructures were usually weak and statistically non-significant for IMPT; mean heart dose (MHD) only correlated with mean doses delivered to the right ventricle, to the mid segment of the right coronary artery (RCA) and, to a lesser extent, to the LADCA. CONCLUSION: The dosimetric benefit of IMPT over conformal photon therapy was consistently observed for all cardiac substructures. MHD may not be a reliable dosimetric parameter for precise cardiac exposure evaluation when planning IMPT.

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

INTRODUCTION: 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. MATERIALS AND METHODS: 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. RESULTS: 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. CONCLUSION: 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.