ESTRO 2025 - Abstract Book

S561

Clinical - Breast

ESTRO 2025

(67%) of patients. The median total RT dose was 23Gy (range 8–64.2Gy), with a median of one fraction (range 1–30). The median EQD2 dose was 50Gy (range 12-104Gy), and the median BED was 60Gy (range 14.4–219Gy). The brain was the most frequently treated site (64%, n=49/77), followed by bone (14.3%, n=11/77). RT was delivered with ablative intent in most cases (83%, n=64/77), while palliative treatment was administered in 13 cases. Regarding T-DM1-specific toxicities in combination with RT, only two patients experienced acute skin toxicity (one grade [G] 3 and one G4) following treatment for a local breast recurrence. These cases required local treatment but resolved within three months without necessitating T-DM1 dose reduction or discontinuation. Among patients treated for brain metastases, one case of asymptomatic radionecrosis (G1) was reported, while no cases of pneumonitis were observed. For late toxicities, two cases of G2 haematologic toxicities (thrombocytopenia and anaemia) and one case of G2 ejection fraction reduction were noted, all occurring in heavily pretreated patients. None required T-DM1 dose modification, though cardiological follow-up was initiated.

Conclusion: These findings suggest that concurrent T-DM1 and RT are well-tolerated, with no significant increase in severe acute or late toxicity rates. Further studies involving larger patient cohorts are needed to validate these results.

Keywords: T-DM1, concomitant radiation therapy