440P - Palliative radiotherapy in metastatic breast cancer: Does molecular subtype predict patient response to radiotherapy?

Background

This study aimed to determine whether molecular subtype does impact patient response to palliative radiotherapy in metastatic breast cancer.

Methods

All metastatic breast cancer patients referred for palliative radiotherapy (either for pain-relief or spinal cord/nerve decompression), between 2017 and 2022, were included in this study. Patient response to radiotherapy was evaluated at 3 weeks from radiotherapy. Re-irradiation need was assessed. Radiotherapy outcomes were statistically analyzed with regards to breast cancer molecular subtypes.

Results

Data of 137 metastatic breast cancer patients were evaluated. Ductal cell carcinoma was the most common histological type. Patients’ distribution according to molecular subtype was as follow: 17% of luminal A tumors, 72% of luminal B tumors, 6% of Her2 tumors and 5% of triple-negative tumors. All patients received single fraction palliative radiotherapy with a prescription dose of 8 Gy and 6 Gy respectively in 77 % and 13 % of cases. Forty-six patients were lost to follow-up and one patient didn’t complete treatment. Complete response to radiotherapy was reported in 21%, 54% and 50% of cases, respectively for luminal A, luminal B and Her2 tumors. No complete response was found in the triple-negative breast cancer subgroup. The “no response to radiotherapy” rate for luminal A and luminal B tumors, was 21% and 20% respectively. Re-irradiation of the same metastatic subsite was performed in 8 patients, all with luminal B molecular subtype. Although not statistically significant, luminal tumors tended to respond better to palliative radiotherapy when compared to non-luminal tumors, with a complete response rate of 48% versus 30% respectively (p=0.3). No statistically significant correlation was found between breast cancer molecular subtype and radiotherapy outcomes.

Conclusions

This study suggests that breast cancer subtypes do exhibit differential responses to palliative radiotherapy. Yet molecular subtype failed to reliably predict radiotherapy outcomes and identify patients who would derive greater than average benefit from palliative radiotherapy. Further studies are needed to identify reliable biomarkers predictive of likely radiotherapy sensitivity.

Clinical trial identification

Editorial acknowledgement

Legal entity responsible for the study

The authors.

Funding

Has not received any funding.

Disclosure

All authors have declared no conflicts of interest.