Partial Breast Radiation Therapy in Low-Risk Breast Cancer Using a Magnetic Resonance Imaging–Guided Adaptive Approach; Results of the Phase 2 PARLOB Study

Purpose: The majority of patients with breast cancer are treated with breast-conserving surgery followed by postoperative whole or partial breast irradiation (PBI). Previous studies on PBI demonstrated similar ipsilateral breast tumor recurrences compared with whole breast irradiation, however, conflicting data on toxicity and cosmetic outcomes were reported. This study investigates the feasibility of stereotactic magnetic resonance imaging-guided adaptive radiation therapy (SMART) for PBI, with emphasis on toxicity and cosmetic outcomes. Methods and Materials: Patients with low-risk breast cancer suitable for PBI as defined by European Society for Radiotherapy and Oncology guidelines were included. SMART was delivered in supine position in a 2-week course of 5 fractions of 6 Gy, or 6.5 Gy in case of high risk characteristics. Patient- and physician-reported toxicity and cosmetic outcomes were scored at 3, 9, 18, and 36 months postoperatively. Results: Between 2017 and 2020, 50 patients were included and 45 patients were treated with SMART. Median age was 66 years, median tumor size was 1.2 cm. Most tumors were Bloom Richardson grade 1 (55.6%) and estrogen receptor positive (95.6%). The majority did not receive adjuvant systemic therapy (80%). The highest grade acute toxicity was grade 2 in 20%, mostly fatigue. At 18 months, 4 patients (9%) reported grade 2 toxicity, consisting of fibrosis and fatigue. No grade ≥3 toxicity was recorded. At 9 and 18 months, 86.4% of patients were (very) satisfied with the cosmetic outcome. The cosmetic outcome was good-excellent compared with the contralateral breast at 18 months in 90.9% and 73.8% as scored by patients and physicians, respectively. Conclusions: Stereotactic magnetic resonance imaging-guided adaptive PBI for patients with low-risk breast cancer was feasible and resulted in low rates of toxicity. Good cosmetic outcomes were reported by patients after a follow-up of 18 months.