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Purpose: The relative biologic effectiveness (RBE) rises with increasing linear energy transfer toward the end of proton tracks. Presently, there is no consensus on how RBE heterogeneity should be accounted for in breast cancer proton therapy treatment planning. Our purpose was to determine the dosimetric consequences of incorporating a brachial plexus (BP) biologic dose constraint and to describe other clinical implications of biologic planning., Methods and Materials: We instituted a biologic dose constraint for the BP in the context of MC1631, a randomized trial of conventional versus hypofractionated postmastectomy intensity modulated proton therapy (IMPT). IMPT plans of 13 patients treated before the implementation of the biologic dose constraint (cohort A) were compared with IMPT plans of 38 patients treated on MC1631 after its implementation (cohort B) using (1) a commercially available Eclipse treatment planning system (RBE = 1.1); (2) an in-house graphic processor unit-based Monte Carlo physical dose simulation (RBE = 1.1); and (3) an in-house Monte Carlo biologic dose (MCBD) simulation that assumes a linear relationship between RBE and dose-averaged linear energy transfer (product of RBE and physical dose = biologic dose)., Results: Before implementation of a BP biologic dose constraint, the Eclipse mean BP D0.01 cm 3 was 107%, and the MCBD estimate was 128% (ie, 64 Gy [RBE = biologic dose] in 25 fractions for a 50-Gy [RBE = 1.1] prescription), compared with 100.0% and 116.0%, respectively, after the implementation of the constraint. Implementation of the BP biologic dose constraint did not significantly affect clinical target volume coverage. MCBD plans predicted greater internal mammary node coverage and higher heart dose than Eclipse plans., Conclusions: Institution of a BP biologic dose constraint may reduce brachial plexopathy risk without compromising target coverage. MCBD plan evaluation provides valuable information to physicians that may assist in making clinical judgments regarding relative priority of target coverage versus normal tissue sparing., (Copyright © 2019 The Authors. Published by Elsevier Inc. All rights reserved.)

Purpose: The feasibility of proton postmastectomy radiation therapy in patients reconstructed with expanders has not been previously reported, limiting treatment options. We analyzed the dosimetric impact of the metallic port contained within expanders on intensity modulated proton therapy (IMPT) and report our techniques and quality control for treating patients in this setting., Methods and Materials: Twelve patients with the same expander model underwent 2-field IMPT as part of a prospective registry. All planning dosimetry was checked with an in-house graphic processing unit--based Monte Carlo simulation. Proton ranges through the expander were validated using a sample implant. Dosimetric impact of setup metallic port position uncertainty was evaluated. Pre- and posttreatment photographs were obtained and acute toxicity was graded using the Common Terminology Criteria for Adverse Events, version 4.0., Results: Nine patients had bilateral skin-sparing mastectomy with bilateral tissue expander reconstruction, and 3 patients had unilateral skin-sparing mastectomy and reconstruction. The left side was treated in 10 patients and the right side in 2. Target coverage and normal tissue dose uncertainties resulting from the expander were small and clinically acceptable. The maximum physician-assessed acute radiation dermatitis was grade 3 in 1 patient, grade 2 in 5 patients, and grade 1 in 6 patients., Conclusions: Postmastectomy IMPT in breast cancer patients with expanders is feasible and associated with favorable clinical target volume coverage and normal tissue sparing, even when taking into account treatment uncertainties; therefore, these patients should be eligible to participate in clinical trials studying the potential role of proton therapy in breast cancer. We caution, however, that institutions should carry out similar analyses of the physical properties and dosimetric impact of the particular expanders used in their practice before considering IMPT., (Copyright © 2017 American Society of Radiation Oncology. Published by Elsevier Inc. All rights reserved.)