Locoregional Ablative Radiotherapy for Breast Cancer Patients Unsuitable for Surgical Resection.

PURPOSE: Breast cancer patients unsuitable for surgical resection are typically managed with palliative systemic therapy alone. We report outcomes of 5-fraction ablative radiotherapy for non-resected breast cancers.

METHODS AND MATERIALS: Retrospective analysis of an institutional registry of breast cancer patients unsuitable for resection who underwent 35-40 Gy/5 fractions to the primary breast tumour and/or regional lymph nodes from 2014-2021. Primary outcomes were cumulative incidence of local failure (LF) and grade ≥3 toxicity (CTCAE v5.0).

RESULTS: We reviewed 57 patients who received 61 treatment courses (median age of 81 years, range 38-99). Unresectable tumour (10%), patient refusal (18%), medical inoperability (35%), and metastatic disease (37%) were the causes of not having surgery. Five patients (8%) had previously undergone adjuvant locoregional radiotherapy. Fifty-four percent (n=33/61) of treatment courses targeted the breast only, 31% (n=19/61) both the breast and lymph nodes, and 15% (n=9/61) the lymph nodes only. Sixty-seven percent (n=35/52) of the courses that targeted the breast were delivered with partial breast irradiation and 33% (n=17/52) with whole breast radiotherapy (median dose of 25Gy in 5 fractions) ± simultaneous integrated boost to the primary tumour. Most primary tumours (65%, n=34/52) and target lymph nodes (61%, n=17/28) were treated with a dose of 35 Gy in 5 fractions. Most treatments (52%) were delivered with intensity-modulated radiotherapy (IMRT). Radiotherapy was delivered daily (20%), every other day (18%), twice weekly (36%) or weekly (26%). The 2-year cumulative incidence of LF was 11.4% and grade≥3 toxicity was 15.1%. The grade ≥3 toxicity was 6.5% for IMRT treatments, versus 7.7% for non-IMRT treatments targeting partial breast and/or lymph nodes (HR 1.13, P=0.92), versus 38.9% for non-IMRT treatments targeting the entire breast (HR 6.91, P=0.023). All grade ≥3 toxicity cases were radiation dermatitis. No cases of brachial plexopathy were observed.

CONCLUSION: 35-40 Gy in 5 fractions is a safe and effective breast stereotactic body radiotherapy regimen and may be an attractive option for patients who are not surgical candidates. Highly conformal techniques (i.e. IMRT or partial breast irradiation) were associated with a reduced risk of toxicity and should be the preferred treatment approaches.