3008 - Local Control of Merkel Cell Carcinoma Treated with Standard of Care Radiation Therapy Compared to Palliative Dose Radiation Therapy

C. N. Kersch^1,2, S. Karki³, R. A. Melson², and R. F. Thompson^1,2; ¹Department of Radiation Medicine, Oregon Health & Science University, Portland, OR, ²Portland VA Healthcare System, Portland, OR, ³Portland VA Research Foundation, Portland, OR

Purpose/Objective(s): Merkel Cell Carcinoma (MCC) is a rare, aggressive skin cancer of neuroendocrine origin. Standard of care (SOC) often includes surgical resection followed by adjuvant radiation therapy (RT) to 50-66 Gy, or less commonly definitive RT to 60-66 Gy. However, retrospective single institution studies suggest that even 8 Gy in 1 fraction may elicit objective response with durable local control of MCC (1,2). Anecdotally, our institutional experience aligns with these reports, observing excellent local response to palliative RT doses, with significantly reduced toxicity. We hypothesize that de-intensified RT will achieve equivalent local control of MCC compared to SOC. The primary objective of this study is to assess local control of MCC as a function of RT dose in the nationwide Veteran population. This study has the potential to lay the groundwork for dose de-escalation as a new SOC for MCC. Materials/

Methods: In this retrospective, IRB-approved study, we mined the national Veterans Affairs (VA) records database, Corporate Data Warehouse, for all patients treated for MCC. This is the largest retrospective cohort of MCC treated with RT to our knowledge. Using a combination of manual chart review and semi-automated data extraction from free-text notes, we collected information on MCC diagnosis, tumor site and size, treatment technique, and radiation dose and fractionation. We assessed local response, overall survival, and time to local and distant progression as outcomes.
Results: We identified 1535 patients with MCC who received radiotherapy. While continued data mining of the complete cohort remains underway, our preliminary results have identified zero cases of local recurrence of the index lesion with either SOC high dose RT or with palliative doses. Intact tumors displayed an objective complete or partial response in all cases.
Conclusion: Our preliminary results suggest that MCC is exquisitely radiosensitive, and that palliative doses may be sufficient for local control of even large tumors. Lower dose and shorter fraction treatments would have advantages including reduced radiation related toxicities, financial toxicity to patients, and time spent in the medical setting for often elderly patients. Ongoing work will evaluate this finding in the full VA patient cohort identified, as well as study distant control and overall survival. 1. JG Iyer et al (2015) Single-fraction radiation therapy in patients with metastatic Merkel cell carcinoma. Cancer Medicine. 4(8):1161-1170. 2. MM Cook et al (2020) Postoperative, single-fraction radiation therapy in Merkel cell carcinoma of the head and neck. Adv Radiat Oncol. 5(6):1248–54.