Definitive radiotherapy for initial management of merkel cell carcinoma: Assessing a novel hypo-fractionated regimen

Merkel cell carcinoma (MCC) is a rare skin cancer. Standard therapy often includes surgery and post-operative radiation (PORT). Given the radiosensitivity of MCC, definitive radiotherapy (RT) may be effective when surgery is contraindicated due to morbidity, functional compromise, and/or delayed wound healing. Here we retrospectively analyzed the efficacy of definitive RT for patients (pts) who received hypo-fractionated RT (hRT; 8 Gy x 1-3 fractions) versus conventional RT (cRT; 1.8-2 Gy x 25-35 fractions). We also compared the efficacy of definitive RT to surgery + PORT. From the Seattle MCC registry, we identified 61 Stage I-III pts who received definitive RT without upfront surgery or systemic therapy and a comparison cohort of 922 pts who received surgery + PORT. Of 61 pts who received definitive RT, 20 received hRT (median 24 Gy, range 8-24 Gy) and 41 received cRT (median 60 Gy, range 50-70 Gy). With 2.9 years (range 0.25-15) of median follow-up for cRT and 1.3 years (range 0.1-3) for hRT, only 5 of 61 (8%) pts who received definitive RT recurred loco-regionally within the RT field (local, in-transit or nodal recurrences). Among 41 cRT pts, 2 had local recurrences and 1 had an in-transit recurrence. Of 20 hRT pts, 1 recurred locally and 1 regionally. Both regimens had a 95% in-field local control rate (39/41 cRT; 19/20 hRT), compared to 96% for combined treatment (885/922). Disease-specific survival was comparable among pts who received definitive RT (68%) versus surgery + PORT (73%). These findings suggest that hRT and cRT appear similar in efficacy. Use of hRT may offer a less toxic, more convenient alternative to cRT and warrants further exploration. Furthermore, definitive RT seemed effective in treating MCC and does not appear to compromise local control compared to surgery + PORT and may be an option when surgery is contraindicated.