Bowen's Disease Along With Intradermal Merkel Cell Carcinoma Occurring on the Dorsum of the Hand.

Bowen's disease (BD), also known as squamous cell carcinoma (SCC) in situ, is a precancerous skin condition that can potentially progress to invasive tumors. Merkel cell carcinoma (MCC) is a rare and aggressive neuroendocrine tumor typically found in sun-exposed areas of elderly Caucasians. The coexistence of BD and MCC is extremely rare, particularly when MCC is located in subcutaneous tissue rather than its typical epidermal or dermal layers. This case report presents an unusual instance of BD coexisting with subcutaneous MCC on the dorsum of the hand in an elderly Japanese male. An 87-year-old Japanese male with over 30 years of sun exposure presented with a progressively enlarging red tumor on the dorsum of his left hand. A biopsy confirmed BD, and the tumor was excised with a 5 mm margin followed by skin grafting. Histopathological examination revealed subcutaneous MCC along with BD, with MCC cells forming small nests in the dermal papillary layer. Immunohistochemistry showed positive staining for synaptophysin and CK20 in a perinuclear dot pattern, confirming the MCC diagnosis. Given the patient's advanced age and the absence of positive surgical margins, a watch-and-wait approach was adopted. The patient has been under close outpatient monitoring, and no recurrence has been observed after six months. This case highlights the rarity of subcutaneous MCC coexisting with BD, with only a few reported cases of such coexistence. The unusual subcutaneous presentation and the presence of multiple micro-nodules instead of large atypical cell clusters suggest an early-stage MCC beneath BD. The pathogenesis of this coexistence remains unclear but raises important questions regarding the relationship between sun exposure and viral factors like Merkel cell polyomavirus (MCPyV), which was not tested in this case. The findings underscore the need for comprehensive diagnostic evaluation when encountering complex or atypical skin lesions. This report emphasizes the rarity of subcutaneous MCC coexisting with BD and underscores the importance of comprehensive diagnostic assessment in unusual cases. Further research is warranted to better understand the underlying mechanisms and to guide optimal management strategies for such rare and challenging presentations.