Ultra-Hypofractionated Whole Breast Radiation Therapy Using a Novel Boost Regimen for Early-Stage Breast Cancer.

Purpose: The 5-year results of the FAST-Forward trial demonstrate non-inferiority of local tumor control using a 26 Gy in 5 fraction regimen compared to 40 Gy in 15 fractions for breast cancer patients receiving adjuvant whole breast radiotherapy (WBRT) with or without a sequential conventionally-fractionated tumor bed boost (2Gy per fraction). Here, we report our institutional experience using the FAST-Forward regimen with a novel sequential boost regimen of 5.2 Gy in one fraction or 10.4 Gy in two fractions.

Methods and materials: Patients with non-metastatic invasive breast cancer or ductal carcinoma in situ (DCIS) treated with adjuvant WBRT of 26 Gy in 5 fractions from 7/1/2019 to 6/1/2022 were identified from an institutional database. Clinical outcomes including adverse events, disease control, and patient-reported outcomes were collected. Survival outcomes were estimated using the Kaplan-Meier method. Associations between toxicities and clinicopathologic and treatment characteristics were assessed using logistic regression.

Results: A total of 311 consecutive patients were included; the use of a 1 or 2 fraction boost was left to the discretion of the treating physicians (54% 1-fraction, 8.7% 2-fraction, 38% no boost). Median follow-up was 32 months. Overall survival and local recurrence-free survival probabilities at 36 months were 96% (95% CI: 94-99) and 93% (95% CI: 90-97), respectively. Acute and late toxicities occurred at a higher rate in the 2-fraction vs. 1-fraction and no boost groups (37.4%, 10.8%, and 12.2% (acute) and 22.7%, 8.6%, and 7.9% (late), respectively). Boost receipt, greater boost volume, 15X energy, increasing breast V95%, and bolus use were associated with risk of acute grade ≥2 toxicities.

Conclusion: A 5-fraction ultra-hypofractionated WBRT regimen for early-stage breast cancer with either no boost or a single fraction boost of 5.2 Gy resulted in excellent disease control and acceptable toxicity. Increased toxicity was observed with a boost of 10.4 Gy in two fractions and is no longer used at our institution.