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Life sciences · Journal article
Frontiers in Oncology · November 4, 2026
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Background Photodynamic therapy (PDT) is a first-line treatment for actinic keratosis (AK), particularly in elderly patients. Merkel cell carcinoma (MCC) is a highly aggressive primary cutaneous neuroendocrine malignancy associated with UV exposure and immunosuppression. In skin with severe UV-induced field cancerization, it remains unclear whether PDT might activate occult highly aggressive tumors. Herein, we report an 89-year-old female who developed a CK20/CK7 double-negative MCC at the original AK site within a short period after successful PDT for AK and basal cell carcinoma (BCC), aiming to explore whether the association between PDT and subsequent MCC represents mere coincidence or a potential risk. Case presentation An 89-year-old female presented with a one-year history of an ulcer on the left nasal ala and an eight-month history of an erythematous patch on the left temporal region. Biopsy confirmed BCC on the left nasal ala and AK on the left temporal region, both treated with PDT (20% ALA under occlusion for 3 hours, 630 nm red light, 80–100 J/cm², three sessions). At the 3-month follow-up, both lesions had healed well. Eight months later, a firm, pale-red nodule developed at the original AK site. Histology revealed small round blue cells in the dermis; immunohistochemistry showed dot-like AE1/AE3 positivity, CD56 and NSE positivity, a Ki-67 index of approximately 75%, and negativity for CK20/CK7 and other markers. The diagnosis was primary cutaneous MCC (T2N0M0, Stage IIA) with positive surgical margins. Given advanced age and anatomical constraints, pembrolizumab was administered for 24 cycles without immune-related adverse events. Post-treatment biopsy showed no viable tumor cells. At the last follow-up in July 2026, the patient remained recurrence-free with a disease-free interval of approximately 12 months. Conclusions This report describes a case of CK20/CK7 double-negative primary cutaneous MCC. The temporal and spatial proximity between MCC and PDT suggests a possible causal association; however, pre-existing subclinical MCC before PDT or a coincidental second primary tumor in the context of field cancerization cannot be excluded. This report does not undermine the established role of PDT in AK management, but it emphasizes the need for individualized risk assessment and long-term surveillance in elderly patients with extensive field cancerization.