Tumor Response After Neoadjuvant Magnetic Resonance Guided Single Ablative Dose Partial Breast Irradiation.

Purpose: To assess the pathologic and radiologic response in patients with low-risk breast cancer treated with magnetic resonance (MR) guided neoadjuvant partial breast irradiation (NA-PBI) and to evaluate toxicity and patient-reported outcomes (PROs).

Methods And Materials: For this single-arm prospective trial, women with unifocal, non-lobular tumors with a maximum diameter of 20 mm (age, 50-70 years) or 30 mm (age, ≥70 years) and tumor-negative sentinel node(s) were eligible. Patients were treated with a single ablative dose of NA-PBI followed by breast-conserving surgery after an interval of 6 to 8 months. Target volumes were defined on radiation therapy planning computed tomography scan and additional magnetic resonance imaging. Prescribed doses to gross tumor volume and clinical target volume (gross tumor volume plus 20 mm margin) were 20 Gy and 15 Gy, respectively. Primary outcome was pathologic complete response (pCR). Secondary outcomes were radiologic response (on magnetic resonance imaging), toxicity (Common Terminology Criteria for Adverse Events), PROs (European Organisation for Research and Treatment of Cancer QLQ-BR23, Hospital Anxiety and Depression Scale), and cosmesis (assessed by patient, radiation oncologist, and BCCT.core software).

Results: Thirty-six patients were treated with NA-PBI, and pCR was reported in 15 patients (42%; 95% confidence interval, 26%-59%). Radiologic complete response was observed in 15 patients, 10 of whom had pCR (positive predictive value, 67%; 95% confidence interval, 39%-87%). After a median follow-up of 21 months (range, 12-41), all patients experienced grade 1 fibrosis in the treated breast volume. Transient grade 2 and 3 toxicity was observed in 31% and 3% of patients, respectively. Local recurrences were absent. No deterioration in PROs or cosmetic results was observed.

Conclusions: NA-PBI has the potential to induce pCR in a substantial proportion of patients, with acceptable toxicity. This treatment seems a feasible alternative to standard postoperative irradiation and could even result in postponement or omission of surgery if pCR can be accurately predicted in selected low-risk patients.