02 Ten-Year Outcomes of Hypofractionated Breast/Chest Wall Radiotherapy With Simultaneous Integrated Boost (SIB) in Patients Under-Represented on Clinical Trials

Background

Hypofractionated breast radiotherapy is standard, yet pivotal trials under-enrolled patients with factors linked to higher toxicity risk (younger age, non-White race, large breast size, chemotherapy exposure, and regional nodal irradiation [RNI]). We performed a prospective pilot study to evaluate feasibility and long-term disease control of a condensed 15-fraction whole breast/chest wall ± RNI regimen with simultaneous integrated boost (SIB).

Materials and Methods

From 2011 to 2018, women with stage 0 to III breast cancer (excluding N3) treated with breast-conserving surgery or mastectomy were eligible if they had greater than or equal to 1 of prior chemotherapy, non-White race, age less than 50 years, large breast/chest wall separation, and/or planned RNI. Radiotherapy delivered at 39.9 Gy in 15 fractions to breast/chest wall (and RNI when indicated) with a concurrent SIB of 8.1 Gy (0.54 Gy/fraction) to the lumpectomy cavity or mastectomy scar (total 48.0 Gy). Primary end points were grade 3 cutaneous toxicity (CTCAE) for all patients and, among those receiving RNI, grade 3 or higher brachial plexopathy (RTOG) and greater than or equal to 20% increase in ipsilateral arm volume (lymphedema). Toxicities were assessed pre-radiation therapy (RT), on the last day of RT, and 3 weeks, 9 weeks, and 12 to 18 months post-RT. Long-term recurrences and vital status were abstracted from the medical record. Kaplan-Meier estimated locoregional recurrence-free survival (LRFS), distant metastasis-free survival (DMFS), and overall survival (OS) from RT completion.

Results

Seventy-four participants (75 treated breasts/chest walls) were enrolled; 59% received chemotherapy, 39% were Black, 33% were younger than 50 years, 13% received post-mastectomy RT, and 40% received RNI. Severe toxicity was uncommon: 1 patient developed grade 3 dermatitis (3 weeks post-RT) and 1 developed a grade 3 axillary infection; no grade 3 or higher brachial plexopathy or greater than or equal to 20% arm-volume increase was observed through 12 to 18 months among patients receiving RNI. With a median follow-up of 10.2 years (range, 2.0-13.9), 4/75 (5.3%) experienced locoregional recurrence and 4/75 (5.3%) developed distant metastases (1 after locoregional recurrence). There were 8 deaths (10.7%; 3 breast cancer-related). Five- and 10-year Kaplan-Meier estimates were LRFS, 98.7% and 93.1%, respectively; DMFS, 94.2% and 94.2%; OS, 95.8% and 89.2%.

Conclusions

Fifteen-fraction breast/chest wall RT with SIB (and RNI when indicated) demonstrated low severe toxicity and favorable 10-year disease control in a cohort enriched for higher-risk, under-represented patients, supporting broader use of condensed SIB regimens in routine practice.