Baseline ultrasound and FDG-PET/CT imaging in Merkel cell carcinoma

Merkel cell carcinoma (MCC) is a cutaneous tumor with a high tendency to metastasize, and a significant proportion of patients have metastases at first presentation. This study aims to determine the value of baseline ultrasound (US) and ¹⁸ fluorodeoxyglucose-positron emission tomography/computed tomography (¹⁸ FDG-PET/CT) imaging in both patients with clinically localized MCC (Stage I/II) and patients who present with palpable lymph nodes (Stage III).

This retrospective cohort included 135 MCC patients who underwent baseline US (with fine needle aspiration cytology (FNAC)) and/or FDG-PET/CT imaging between 2015 and 2021.

Of the 104 patients with clinically localized disease, 48% were upstaged to Stage III and 3% to Stage IV by imaging or sentinel lymph node biopsy (SLNB). FDG-PET/CT imaging identified regional metastases in 23%, while US with FNAC identified regional metastases in 19%. SLNB was performed in 56 patients, of whom 57% were upstaged to Stage III. Of the 31 patients who presented with palpable lymph nodes, 16% were upstaged to Stage IV by FDG-PET/CT imaging.

Baseline imaging frequently upstages Stage I/II MCC patients to Stage III, both by US and FDG-PET/CT, Stage IV disease is rarely identified. Patients who present with palpable nodes are frequently upstaged to Stage IV by FDG-PET/CT imaging.