Main Session

Sep 28

SS 12 - Sarcoma 1: From Margins to Mainstream: Advances in our Understanding of Rare Bone, Cutaneous, and Connective Tissue Tumors

173 - A Prospective Single-Arm Trial of Postoperative 36 Gy in 10 Fractions for Merkel Cell Carcinoma

05:05pm - 05:15pm PT

Room 153

Presenter(s)

Devarati Mitra, MD, PhD - University of Texas MD Anderson Cancer Center, Houston, TX

D. Mitra¹, R. Lin², A. Farooqi¹, A. J. Bishop¹, A. K. Yoder¹, C. Pearlnath², S. Kol², R. Weiser², R. P. Goepfert², I. Glitza², M. I. Ross², M. K. Wong³, and B. A. Guadagnolo¹; ¹Department of Radiation Oncology, The University of Texas MD Anderson Cancer Center, Houston, TX, ²The University of Texas MD Anderson Cancer Center, Houston, TX, ³Roswell Park Cancer Center, Buffalo, NY

Purpose/Objective(s): Merkel cell carcinoma (MCC) is a radiosensitive neuroendocrine cutaneous malignancy for which standard adjuvant radiotherapy (RT) is delivered over >5 weeks. We sought to prospectively evaluate in-field disease control and toxicity of a radiobiologically similar 2-week RT course.

Materials/Methods: Between 2022-2025 we enrolled 30 patients with high-risk locoregionally confined MCC on a prospective, single-arm trial of hypofractionated post-operative RT, consisting of 36 Gy in 10 fractions. Neoadjuvant and adjuvant systemic therapy were permitted. The co-primary endpoints of the study are local control for patients receiving adjuvant RT to the primary tumor bed and nodal control for those receiving adjuvant RT to the draining nodal basin. Acute toxicity monitoring was performed for each patient.

Results: Enrolled patients had a median age of 69 (IQR 62-79) with 80% Non-Hispanic White and 60% male. Four (13%) were immunocompromised.

For the 28 patients with known primary tumors, the median tumor size was 13 mm (IQR 7-21) with diverse anatomic targets: 15 upper extremity (54%), 6 head and neck (21%), 4 lower extremity (14%) and 3 trunk (11%). Additional risk factors for recurrence included: 17 (61%) with lymphovascular invasion, 8 (29%) with <1 mm surgical margins, and 1 (4%) each with prior local recurrence or perineural invasion. Twenty-seven patients (90%) presented with clinically node negative disease, of which 23 (85%) underwent sentinel lymph node (SLN) biopsy and 11 (48%) had involved SLN. SLN+ patients do not undergo further nodal surgery, but all received nodal RT.

Six patients received neoadjuvant immunotherapy with 3 having >50% disease viability and 3 having 0-40% disease viability. Ten patients received adjuvant immunotherapy (33%). RT target sites were: primary only (n=16, 53%), nodal basin only (n=3, 10%), or both (n=11, 37%). The median time from resection to RT was 52 days (IQR 41-67).

We observed no grade 3+ RT-associated acute toxicities. Thirteen patients (43%) experienced grade 2 radiation dermatitis. Other acute toxicities were grade 1 and included dermatitis (n=16, 53%), fatigue (n=7, 23%), pain (n=5, 17%), and arthralgia (n=2, 7%). Late toxicity data are not mature.

With a median follow-up time of 15 months from resection, there have been no in-field recurrences (100% local control after primary site RT and 100% nodal control after nodal RT). Three patients who received primary site only RT developed nodal recurrence and one patient developed a distant metastasis. Two patients died 6 and 14 months after surgery (unrelated to MCC). The estimated two-year OS probability is 0.877 (95% CI 0.724-1.00) and two-year recurrence-free survival probability is 0.740 (95% CI 0.561-0.977).

Conclusion: Adjuvant HFRT for resected MCC, delivered as 36 Gy in 2 weeks, has a favorable disease control and toxicity profile at >1 year median follow-up.