Ultrahypofractionation and Simultaneous Integrated Boost in Breast Cancer: Early Side Effects Analysis.

Purpose: The FAST-Forward study paved the way for ultrahypofractionation (UHF) in breast cancer. We prospectively registered and analyzed our case series receiving UHF + simultaneous integrated boost (SIB) to further reduce the treatment to a total of 5 days. The study aimed to present the 6-month early side effects results of the first patients treated with this scheme in 16 radiation oncology centers in Spain.

Methods And Materials: A total of 242 breast cancer patients received adjuvant radiation therapy between April and December 2020. The median age was 61 years (interquartile range, 53-70). All patients underwent breast-conserving surgery. Chemotherapy (QT) was administered to 27.7%, and endocrine therapy to 85.1%. A SIB of 29 Gy was applied to 60.7% of the patients, while 39.3% did not have a boost indication. Breast radiation therapy (RT) with SIB to the tumor bed and regional node irradiation was done in 5.4 % of patients.

Results: Most patients were treated with Volumetric modulated arc therapy (66.1%) and intensity modulated RT (30.6%). One patient received treatment by 3-dimensional techniques (0.4%) and 7 patients (2.9%) a combined intensity modulated RT-3-dimensional technique. Deep inspiration breath-hold was used in 16.9% of cases. At the end of treatment, erythema grade (G) 0 was presented in 56.1%, G1 in 43.1%, and G2 in 0.8%. G1 edema was observed in 14.6% and less than 1% had G2. After 6 months, 97% had G0 erythema, 3% G1, and 0% G2, while G1 edema was observed in 11.4% and G2 in 2.5%. No statistically significant impact on side effects was found for planning target volume breast and planning target volume boost volumes.

Conclusions: UHF with SIB of 29 Gy to the tumor bed in patients with early-stage breast cancer is clinically feasible, safe, and free of an excess of early side effects. Further analysis of late toxicity is needed.