Evaluation of the Effect of Radiotherapy Timing on Toxicity in HER2-Positive Breast Cancer Receiving Trastuzumab Emtansine During the Adjuvant Period

Aim: Breast cancer is the second leading cause of cancer-related deaths in women.Approximately 20-25% of breast cancers express HER2 and are associated with poor prognosis.With the use of HER2-targeted therapies, there has been an increase in treatment success for breast cancer cells overexpressing HER2.Trastuzumab extensive (T-DM1) is the antibody-drug conjugate of trastuzumab and the cytotoxic agent extensive (DM1), a maytansine derivative and microtubule inhibitor.The aim of our study was to evaluate the effect of continuing T-DM1 therapy on toxicity in patients receiving adjuvant radiotherapy using real-world data.There is no study in the literature that evaluates the toxicity of adjuvant T-DM1 in real life.Methods: Patient files were examined retrospectively.The primary endpoint of the study was whether the timing of radiotherapy increased adjuvant T-DM1 toxicity.Patients were divided into two groups: those who continued T-DM1 during and after T-DM1 adjuvant radiotherapy.Results: A total of 50 patients were included in the study.Twenty (40%) of the patients received sequential radiotherapy with T-DM1, thirty (60%) continued T-DM1 during radiotherapy.No significant difference was detected in terms of toxicity in both groups. Conclusion:In our study, we observed that some physicians started T-DM1 after radiotherapy considering that toxicity might increase.However, we observed in the analysis that there was no significant increase in the toxicities with simultaneous use.We believe that as the use of antibodydrug conjugates in the clinic increases, more studies are needed to determine the timing of radiotherapy.