We performed an analysis of the National Cancer Database to determine optimal doses of conventionally-fractionated adjuvant radiotherapy for patients with stage I/II or III Merkel cell carcinoma.
The cohort included 2735 patients with resected Merkel cell carcinoma of the head and neck, trunk or extremities receiving radiotherapy. Exclusion criteria included doses of radiotherapy <30 or >80 Gy, or dose per fraction >200 or <180 cGy. Recursive partitioning analysis and spline models were used to select dose thresholds. Multivariable Cox regression was performed to validate thresholds with respect to overall survival.
Recursive partitioning analysis models defined a threshold of 57 Gy for stage I/II Merkel cell carcinoma, above which 3-year overall survival rate was decreased (P < 0.0001). The 3-year overall survival rate for patients receiving 50.0–57.0 Gy (81.2%) was greater compared to doses of 30.0–49.9 Gy (75.3%) or >57.0 Gy (70%, P < 0.0001). Doses > 57.0 Gy were associated with an increased hazard of death (1.31, confidence interval 1.07–1.60) with respect to doses of 50.0–57.0 Gy. Doses < 50.0 Gy for stage III Merkel cell carcinoma were associated with worsened 3-year overall survival (P < 0.0001) and increased hazard of death (2.01, confidence interval 1.43–2.82) with respect to doses between 50.0 and 57.0 Gy.
Our results support doses of 50–57 Gy for most patients with stage I/II Merkel cell carcinoma receiving conventionally-fractionated adjuvant radiotherapy. In contrast to a prior National Cancer Database analysis, our results suggest doses ≥ 50 Gy should be strongly considered for patients with stage III Merkel cell carcinoma regardless of anatomic subsite.