Radiotherapy for Merkel cell carcinoma: recommendations from the DEGRO Dermatooncology Working Group.

Merkel cell carcinoma (MCC) is a rare malignancy with an annual incidence in Germany of 5.2 per million for men and 3.8 per million for women, predominantly affecting sun-exposed areas in older adults. Despite a frequently unfavorable prognosis, outcomes are multifactorial and depend on tumor site, disease stage, performance status, patient age, sex, and immune competence. The current standard of care prescribes surgical resection followed by adjuvant radiotherapy of the tumor bed, ideally initiated within 8 weeks after surgery to optimize clinical outcomes. In case of lymph node involvement, lymph node dissection and/or radiotherapy of the involved nodal basin are recommended. Furthermore, new evidence supports the effectiveness of shorter radiotherapy approaches, with moderate hypofractionated or even ultrahypofractionated schedules serving as effective options for certain patients, particularly those with frailty or comorbidities, or when it is clinically necessary to minimize the treatment burden. In cases of inoperability, refusal of surgery, or significant frailty, definitive local radiotherapy can achieve sufficient disease control. The synergistic potential of combining immunotherapy with radiation in advanced cases remains a key area for ongoing research.