2487 - Efficacy of Stereotactic Body Radiation Therapy (SBRT) to Prolong Systemic Therapy (ST) for Patients with Oligoprogressive (OP) Metastatic Breast Cancer (MBC)

A. A. LeVee¹, H. Young¹, S. Yoon², S. M. Glaser², J. Mortimer¹, and J. G. Bazan Jr²; ¹Department of Medical Oncology, City of Hope National Medical Center, Duarte, CA, ²Department of Radiation Oncology, City of Hope National Medical Center, Duarte, CA

Purpose/Objective(s): Stereotactic body radiation (SBRT) in oligoprogressive (OP) metastatic breast cancer (MBC) has been explored to help patients remain on their current line of systemic therapy (ST). However, studies show conflicting results. In the CURB study that included all breast cancer subtypes, SBRT did not improve progression-free survival (PFS). On the other hand, in the AVATAR trial that included only HR+/HER2- patients, ablating OP lesions with SBRT was successful. Given the lack of clarity with regards to the role of SBRT in OP MBC, we conducted a single-institution study to test the hypothesis that SBRT would prolong time on ST for =6 months (mos). Materials/

Methods: We identified 365 patients that received radiation therapy in our institution at a dose per fraction of =500 cGy. In order to be considered, patients had to have: MBC and receive extracranial SBRT to 1-4 lesions. All patients who received radiation therapy to the brain were excluded. Amongst patients that received extracranial SBRT to 1-4 lesions, records were reviewed to confirm OP disease, defined as progression and/or new metastases in 1-4 lesions. Clinical and treatment data were collected. Our primary endpoint was the proportion of patients that remained on their line of ST from time of SBRT for = 6 mos. Secondary endpoints were the proportion of patients that remained on their line of ST from time of SBRT for =12 mos, time to next line of ST, PFS, and overall survival (OS).
Results: 51 patients met the inclusion criteria with a median age at time of SBRT of 62 years (IQR, 50-66 yrs) treated with a median 1 line of ST (IQR, 1-2). The median time from diagnosis of MBC to development of OP was 31.4 mos (10.0-52.6 mos). There were 35 patients with HR+/HER2- disease, 10 with HER2+ disease, and 6 with TNBC. SBRT was delivered to 1 lesion in 39 patients, 2 lesions in 9 patients, and 3 lesions in 3 patients. Follow-up data for disease outcomes was available in 46 of 51 patients with a median follow-up time of 15.5 mos (IQR, 9.4-24.4 mos) post-SBRT. Overall, 52% (24/46) of patients remained on their line of ST at 6 mos, including 58% of HR+/HER2- (19/33), 44% of HER2+ (4/9), and 25% of TNBC (1/4) patients (p=0.41). The rates of continuation of ST for =6 mos were similar in patients that had previously received 1 line of ST (56%, 15/27), 2 lines of ST (43%, 3/7) or >2 lines of ST (50%, 6/12) [p=0.82]. The median time to ST change and median PFS were 6.4 mos and 6.1 mos, respectively. Median OS was not reached with 1-year OS post-SBRT of 90.3%. In the 30 patients that had reached a follow-up of at least 1-year post-SBRT, 53.3% (16/30) remained on their line of ST from time of SBRT.
Conclusion: More than half of patients in our institution who received SBRT for OP MBC remained on their line of ST for at least 6 additional mos. These results are in line with the AVATAR trial in that HR+/HER2- breast cancer patients benefit from SBRT for OP, and with the CURB trial in that TNBC patients may not benefit. These data support our ongoing phase II study of SBRT for OP in HR+/HER2- MBC.