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This study aims to assess the safety and efficacy of fractionated SRT (FSRT) associated to systemic therapy in patients with breast cancer bone metastases.

Patients with breast cancer bone metastases who were treated with FSRT within 21 days of receiving systemic therapy were identified. All lesions were treated with LINAC-based FSRT. Treatment was delivered using IMRT or VMAT in supine position using vacuum-locked or other customized devices. All patients received systemic therapy in relation to molecular pattern. Local control was evaluated at least after two months after treatment completion by means of radiological exams, while pain responses assessed at the end of treatment and every three months thereafter. Acute toxicity were reported and graded as per standardized Common Toxicity Criteria for Adverse Events 4.0 criteria.

From January 2019 to March 2021,a total of 40 patients with breast cancer bone metastases were identified. The median age of patients at the time of RT was 54 years. All patients were treated with higher doses of radiotherapy delivered in three fractions according to the lesion’s site (Total dose 18 Gy, 21Gy or 27Gy). The most common treated site was axial skeleton (83%). 18 patients for a total of 29 lesions, were treated with FSRT and concurrent systemic therapy. Administrated sistemic therapy was as follow: 10 patients (55.6%) received CDK4/6 inhibitors (12 radiotherapy courses), 6 patients (33.3%) trastuzumab (12 radiotherapy courses), 2 patients (11,1%) Eribulin. The most common collateral effect observed was neutropenia, in particular in association with CDK4/6 inhibitors. All patients completed treatment without interruptions. During the follow-up the most used diagnostic exam was the PET/CT. With a median follow-up of 7 months (range: 1-24 months), we recorded: patients treated in association with CDK4/6 inhibitors had 2 complete response (CR), 4 partial response (PR), 5 stable disease (SD), 1 progression disease (PD); patients treated with trastuzumab 3 CR, 3 PR, 4 SD, 2 PD; all patients treated with Eribulin had SD. 13 treatments determined a pain relief, of which eight a complete pain response. Nobody developed pathologic fractures.

Highly hypofractionated radiation therapy is a feasible and tolerable treatment for bone metastases. Longer follow-up will be needed to accurately determine response and late effects.