Dosimetric Comparison of Volumetric-Modulated Arc Therapy and Proton Therapy for Postmastectomy Radiation Using the New ESTRO-ACROP Breast Reconstruction Contouring Guidelines

Abstract

In 2019, ESTRO-ACROP released new consensus guidelines for postmastectomy radiation therapy (PMRT) in the setting of breast reconstruction. In particular, the chest wall (CW) target was redefined to exclude the breast prosthesis and underlying tissue. In this study, we assess the impact of these changes on treatment planning and compare the dosimetric outcomes using both volumetric-modulated arc therapy (VMAT) and pencil-beam scanning (PBS) proton therapy. We performed a treatment planning study of 10 women with left sided breast cancer who received PMRT after implant-based breast reconstruction. All target structures and organs-at-risk (OARs) were delineated first using current RTOG breast contouring guidelines and then separately delineated using the updated ESTRO-ACROP consensus guidelines. Four sets of plans were then generated; 1) VMAT using RTOG contours, 2) VMAT using new ESTRO-ACROP contours, 3) PBS using RTOG contours, and 4) PBS using new ESTRO-ACROP contours. All plans prescribed a dose of 50.4 Gy or GyRBE to the CW and 45 Gy or GyRBE to the regional lymphatics, inclusive of the internal mammary nodes. Similar target coverage objectives and normal tissue (OAR) constraints were utilized, applying the concept of ALARA. Dosimetric goals between the RTOG and updated ESTRO-ACROP contoured plans and across treatment modalities were compared. Utilizing the updated ESTRO-ACROP contouring guidelines, VMAT and PBS plans were able to generate equivalent coverage for all targets. The mean CW D95 was 49.9 Gy for VMAT vs. 48.4 GyRBE with PBS, and the mean regional lymphatic D95 was 45.1Gy (VMAT) vs. 42.8 GyRBE (PBS). Despite the more limited CW volume, proton PBS plans still demonstrated superior OAR metrics compared to VMAT: ipsilateral lung V20 of 16.1% (VMAT) vs. 5.3% (PBS), mean heart doses of 4.6 Gy (VMAT) vs. 0.51 GyRBE (PBS), LAD 1cc dose of 7.43 Gy (VMAT) vs. 5.95 GyRBE (PBS). Compared to plans generated with RTOG guidelines, the updated guideline plans were able to achieve equivalent target coverage with similar, but modestly improved sparing of most OARs: ipsilateral lung V20 7.3% (RTOG PBS) vs. 5.3% (ESTRO PBS) and 24% (RTOG VMAT) vs. 20% (ESTRO VMAT), mean heart dose 0.49 GyRBE (RTOG PBS) vs. 0.51 GyRBE (ESTRO PBS) and 4.95 Gy (RTOG VMAT) vs. 4.82 Gy (ESTRO VMAT). Notably, contralateral lung doses with VMAT were higher using the updated ESTRO-ACROP guidelines compared to RTOG guidelines with a mean V10 of 19.6%. PMRT with reconstruction utilizing the updated ESTRO-ACROP consensus contouring guidelines is feasible with both VMAT and PBS proton therapy. Treatment plans utilizing the updated contouring guidelines provided equivalent target coverage compared to conventional guidelines, while modestly further sparing cardiopulmonary structures. Care must be taken with new contouring guidelines when utilizing VMAT to minimize undue contralateral lung exposure.