Single-fraction stereotactic ablative body radiotherapy for sternal metastases in oligometastatic breast cancer: Technique and single institution experience

Introduction Due to size and close proximity to skin, the sternum is a complicated target for stereotactic ablative body radiotherapy (SABR). This is a retrospective case series of single-fraction SABR to sternal metastasis in patients with oligometastatic breast cancer. Methods Between June 2014 and June 2018, ten breast cancer patients received 20 Gy in 1 fraction to a solitary sternal metastasis. Eligible patients had Eastern Cooperative Oncology Group performance status of 0-2, oligometastatic disease (defined as 1-5 metastases) and a controlled primary site. Patients were treated with 3-dimensional conformal radiotherapy, each patient case comprising of> 6 coplanar beams and 2-6 non-coplanar beams. Local control, pain response and adverse events were retrospectively reviewed. Results The median planned target volumes were 84.75cc (range, 14.4-197.8cc). The median conformity index was 1.29 (range, 1.2-1.49). At a median follow-up of 32 months, nine patients achieved in-field control. Two patients had triple negative disease, one of them developed marginal recurrence, and the other had in-field recurrence. Seven patients had sternal pain prior to SABR, and within 3 months after SABR treatment, the pain improved (n = 3) or resolved (n = 2). Four patients developed acute grade 1 and 2 skin reactions, and two patients had late grade 1 skin reactions. There were no grade 3 or 4 toxicities. Conclusion Our case series demonstrates safety of SABR with associated disease control and analgesic benefit in selected patients with oligometastatic breast cancer. The marginal recurrence observed in this cohort suggests wider margins could be beneficial to account for microscopic disease.