Merkel cell carcinoma (MCC) is a rare and aggressive neuroendocrine skin cancer with high risk of distant recurrence and mortality. Lesions characteristically arise within the dermis with variable involvement of the epidermis and subcutis.
We present five cases of subcutaneous MCCs, with minimal involvement of the deep dermis, no involvement of the superficial/mid-dermis or epidermis, and no overlying regression or ulceration.
Heterogenous presentations were observed with stage II to III MCCs located in the neck, limbs and gluteal region. Clinically none of the cases had superficial MCC primaries or evidence of distant metastatic disease on staging FDG PET/CT. All cases were diagnosed in Queensland and managed successfully with curative intent locoregional treatment, with 3 of 5 patients enrolled in an adjuvant immunotherapy trial (IMAT). 3 of 5 patients in this cohort had baseline immunosuppression. None of the cases have recurred to date, with variable length of follow-up.
All patients underwent whole exome sequencing, with 5 out of 5 having a negative UV signature; 4 out of 5 staining positive for Merkel cell polyomavirus (McPV) on immunohistochemistry. Although a limited sample size, a higher rate of McPV positive MCC occurred than commonly reported in QLD (~80% viral negative MCC population). The tumour mutational burden of these variants was low compared to MCCs of UV origin and lacked common driver mutations.
All of the cases have a favourable natural history and are successfully managed with surgery and/or radiation, suggesting that curative intent loco-regional treatment should be pursued for such cases in future.