ESTRO 2024 - Abstract Book

S465

Clinical - Breast

ESTRO 2024

Purpose/Objective:

The term "oligometastatic disease (OMD)" was defined as a distinct cancer state between locally confined and systemically metastatic disease, and it is considered potentially curable. The tumor-agnostic SABR-COMET trial showed that stereotactic ablative radiotherapy (SABR) improved both overall survival (OS) and progression-free survival (PFS) rates, but the contribution of SABR could not be demonstrated in the breast cancer (BC)-specific NRG BR002 trial [1, 2]. In this study, we aimed to evaluate the treatment outcomes and associated prognostic factors in BC patients who had bone-only OMD. In particular, we tried to determine the subgroup that would benefit most from SABR according to the recent ESTRO-EORTC consensus recommendation of OMD.

Material/Methods:

We enrolled 47 primary-controlled patients with a total of 63 lesions with bone-only oligometastatic BC who underwent SABR for all bone lesions between July 2013 and March 2022 in our department. Cases with poly metastatic disease at the beginning but with bone-only disease after systemic treatment or with bone-only metastatic disease at diagnosis with up to 5 metastatic lesions that can be safely treated with SABR were included in this study. Kaplan-Meier curves were used for all time-to-event outcome measures by standard (non-stratified) log rank test and all statistical analyses were performed using SPSS 23.0 software (SPSS, Chicago, IL).

Results:

All patients were female, and the median age was 48 years (range, 30-77 years). The stratification of patients according to the ESTRO-EORTC consensus recommendation is presented in Figure 1. The median time to distant metastasis was 23.9 months (range, 0-183 months), and the median time between primary treatment and the SABR was 56 months (range, 0-291 months). A biopsy of metastatic foci was taken from 21 (44.7%) patients before SABR. In six of these cases, metastases developed at least two years after diagnosis. All patients had received at least one line of systemic therapy after the metastatic phase. The median number of the systemic treatment line was 2 (range, 1-4). Response assessment was performed in 42 patients (89.4%) at 3 to 6 months after SABR, and PET-CT was used in 33 (78%) of them. Nineteen (45.2%) patients obtained a complete response, 16 (38.1%) had a partial response, 5 (11.9%) had stable disease, and 2 (4.8%) had progressive disease. The median follow-up time was 34 months (range, 5-109 months) after SABR. For all patients, the 2-, 3-, and 5-year actuarial OS rates were 90%, 84%, and 66%, and the PFS rates were 49%, 40%, and 29%, respectively. The median OS was 83 months (SE: 18, 95% CI: 47 – 119), and the median PFS was 23 months (SE: 8.3, 95% CI: 6.6-39.5). The local control rate in the SABR-treated foci was 85%. In univariate analysis, OMD state (genuine vs. induced), de-novo OMD state (synchronous vs. metachronous), histology (luminal vs. HER-2 enriched), and repeat SABR were prognostic for OS; OMD state (genuine vs. induced) and repeat SABR were prognostic for PFS. Molecular subtype alterations were observed in 21 (42%) patients whose metastatic disease was confirmed by biopsy and 0% PFS was observed in 5 years in patients with phenotypic discordance, and 36% PFS was observed in patients without discordance and evaluated as borderline significant (p = 0.08). Factors affecting OS and PFS were evaluated in multivariate Cox proportional hazards models, and OMD state remained significant for OS, and OMD state and repeat SABR for PFS (p = 0.007 and p = 0.001, p = 0.012, respectively). HER2-enriched histological subtypes were also associated with improved LC (p = 0.01). There was no statistically significant difference between LC and BED4. SABR was well tolerated and only 6 (12%) patients had late side effects (4-vertebral compression fracture and 2- pain). There were no ≥grad 4 acute or late toxicities.