Session Item

Several large randomized controlled trials have established superiority of moderate hypofractionation in maintaining cosmesis and limiting late effects without compromising locoregional control in breast cancer (BC), making moderate hypofractionation standard of care for breast cancer. The FAST Forward (FF) is a multicenter, phase 3, non-inferiority trial, randomising patients with pT1–3pN0–1M0 BC after breast-conserving surgery (BCS) or mastectomy to either 40 Gy in 15 fractions over 3 weeks, and 27 Gy or 26 Gy in five fractions over 1 week to the whole breast or chest wall. With a median follow-up of 71·5 months, the 26 Gy arm is demonstrated to being non-inferior to the standard of 40 Gy in 15 fractions over 3 weeks. A boost to the primary tumour bed was given to 24.3% of the patients.

The EORTC boost versus no boost trial showed a benefit for the local relapse (LR) risk, with the largest absolute benefit in young patients. We considered that high-risk patients could benefit from a simultaneous integrated boost (SIB) integrated in a 5 days treatment.

Between March and October 2020, 246 consecutive early BC patients were treated with a schedule of 5 fractions over one week to a total dose of 26 Gy. High-risk patients (n=147, 58,9%) received a SIB of 4 Gy, up to a total dose of 29Gy. Mean age was 61.8±11.3 years. IMRT/VMAT was used in 96.7%; 28.5% received primary or adjuvant chemotherapy and 84.6% adjuvant endocrine therapy. We evaluated the dose distribution to the target volume and the organs at risk, also breast edema and erythema (RTOG criteria) with a median follow up to 6 months.

Mean PTVbreast and PTVBoost volumes were 857.0±427.1cm3 and 60.2±36.2cm3. Mean PTVbreast and PTVBoost V95, V105 and V107 were 96.0%, 8.8%, 4.0% and 98.8%, 1.0% and 0.05%, respectively. Mean Dmean, V1.5 and V7 of heart were 129.2cGy, 21.7% and 1.25% and for anterior left descending artery Dmean was 349.4cGy and V8 was 5.82% in left breast.

At the end of treatment, erythema score was G0 for 55.7%, G1 for 42.3% and G2 for 0,8%, respectively. Oedema G1 was present in 14,25% and <1% G2. At 6 months, 94.7%, 2,8% and 0% had erythema G0, G1 and G2, respectively; oedema G1 was seen in 11.4% and G2 in 2.4% of patients. There was no statistically significant difference on side effects between PTVbreast and PTVBoost volumes. Slightly more SIB patients experienced oedema G1-G2 at the end of treatment (17.4% vs. 12.2% without boost), and at 6 months (16.8% vs. 8.8%), without statistical significance (p=0.302 and p=0.096, respectively).

Ultra-hypofractionation combined with a SIB up to 29Gy to the primary tumour bed in high-risk is a clinically feasible novel technique with very low added side effects at 6 months. We continue including and following patients and initiated discussions about a prospective randomized phase III study.