2666 - Reconstructive Outcomes and Complications after Mastectomy and Proton Therapy for Breast Cancer

M. A. Chakraborty¹, A. Lozano², C. Stern³, M. Kim², L. Z. Braunstein⁴, Q. LaPlant⁴, B. McCormick⁴, A. J. Xu⁴, A. Hanlon², S. N. Powell⁴, A. J. Khan⁴, and I. J. Choi^4,5; ¹Rutgers New Jersey Medical School, Newark, NJ, ²Center for Biostatistics and Health Data Science, Department of Statistics, Virginia Tech, Roanoke, VA, ³Plastic Surgery Service, Department of Surgery, Memorial Sloan Kettering Cancer Center, New York, NY, ⁴Department of Radiation Oncology, Memorial Sloan Kettering Cancer Center, New York, NY, ⁵New York Proton Center, New York, NY

Purpose/Objective(s): Proton beam therapy (PBT) is increasingly used for non-metastatic breast cancer given its dosimetric advantages. The unique properties of the Bragg peak and increased relative biological effectiveness of protons at the distal beam edge have raised concerns for increased fibrosis-related complications, particularly among those receiving PBT after mastectomy and breast reconstruction (RC), as RC-related outcomes and complications (RC-X) may be impacted by soft tissue toxicities. We hypothesize that PBT following mastectomy and breast RC is associated with acceptable rates of toxicities and RC-X. Materials/

Methods: We identified all patients with non-metastatic breast cancer who received mastectomy and breast RC followed by pencil beam scanning (PBS)-PBT from 8/9/2019 to 12/31/23 at our institution. Patient demographics, tumor characteristics, treatment details, adverse events (AEs), RC outcomes, and disease control outcomes were collected. Rates of toxicities and RC-X were calculated and compared by patient, tumor, and treatment characteristics using two-sample t-tests and Fisher’s exact tests.
Results: A total of 66 consecutive patients were identified, with a median age of 40.0 years. Tissue expander reconstruction was in place at the time of PBT in 50 (75.8%) patients (47% pre-pectoral, 27.3% subpectoral, 1.5% unspecified); implant in 12 (18.2%), and flap RC in 4 (6.0%). 88.5% of those undergoing implant-based RC had two-stage RC, and 11.5% single-stage RC. Patients were treated with PBS-PBT to 50.4 Gy in 28 fractions (87.9%) or 50.0 Gy in 25 fractions (12.1%), with a 10-16 Gy boost in 12 patients (18.2%). Median follow-up time was 15.3 months. Nineteen patients (28.8%) experienced acute (occurring =90 days after end of PBT) grade 2 (G2) radiation dermatitis; no grade 3 (G3) acute AEs were observed. Twelve G2 and five G3 late AEs were observed (G3: 1 (1.5%) chest wall discomfort, 3 (4.5%) arm lymphedema, 1 (1.5%) shoulder stiffness). Two (3.0%) late rib fractures occurred. Eighteen patients (27.3%) experienced a major R-CX: 12 developed G3 capsular contracture, 5 developed infection, and 1 had an implant failure due to cancer recurrence. Seven (10.6%) of these patients required surgical intervention, resulting in implant replacement in 3 patients and implant failure in 4 patients. No statistically significant associations between implant location, RC type, or single-stage vs. two-stage RC with the risk of major RC-X were observed.
Conclusion: PBS-PBT after mastectomy and breast reconstruction results in minimal significant toxicity events and favorable rates of implant failure or replacement when compared to recent reports of photon- and proton-based techniques. Continued follow-up will provide valuable information on longer-term impact on cosmesis and RC outcomes.