PURPOSE: Trastuzumab Deruxtecan (T DXD), a new antibody drug conjugate is a new treatment option for 2nd line metastatic breast cancer (MBC) for HER2 + or HER2 low tumors. Palliative or ablative radiotherapy (RT) may be required in patients who are being treated with T-DXd. However there is a lack of evidence regarding the safety profile of combining T-DXd with RT. TENDANCE study aimed to evaluate safety and efficacy of combined T-DXd and RT. MATERIALS AND METHODS: This retrospective multicenter study included 54 patients treated concurrently with T-DXd and RT for HER2+ and HER2 low MBC between February 2021 and December 2023. All data were collected from a web-questionnaire, centralized after medical records and validation of the protocol by the local ethical committee. Primary endpoint was the safety of combined therapy. RESULTS: Median age was 60 years. Patients who received T-DXD were further categorized into HER2+ (40.7 %), HER2 low/hormonal receptors HR+ (40.8 %) or HER2 low/HR- (18.5 %). In the HER2+ patients, T-DXd was administered as 2nd (18.2 %) or 3rd (31.8 %) or 4th (50 %) line therapy. RT was delivered using palliative (72.2 %) or ablative doses (27.8 %). Indications consisted mostly of palliative bone irradiation (46.3 %) and stereotactic radiosurgery (SRS) (25.9 %). With the median follow-up of 9 months, 22.2 % of patients had a complete response and 77.8 % had either a partial response or stable disease. Grade 1 or 2 asthenia was observed in 51.8 % of patients, while only 16.6 % experienced other grade 1 or 2 adverse effects. There was no T-DXd therapy discontinuation related to RT. CONCLUSION: To our knowledge, TENDANCE is the largest study evaluating concurrent T-DXd and RT. This preliminary report suggests the feasibility of the combination of RT and T-DXd with manageable toxicity rate. Longer follow-up and further prospective studies are required to confirm these results.