Two Challenging Dermatopathology Cases: Combined Merkel-Squamous Carcinoma and Pediatric Nodular Hidradenoma

Conclusions

• The first case suggests that combined Merkel cell and squamous cell carcinoma is a rare but highly aggressive cutaneous tumor that can be diagnostically confusing, especially when TTF1 is positive and the patient already has metastatic squamous cell carcinoma elsewhere.
• Because combined Merkel cell/squamous tumors may arise from UV-related pathways rather than true pulmonary or classic dermal neuroendocrine origins, diagnosis must rely on the full clinicopathologic context, not any single marker.
• For suspected combined Merkel cell/squamous carcinoma, aggressive management with nodal evaluation, polyomavirus testing, and serial surveillance imaging is warranted.
• The second case shows that an apparently benign, well-circumscribed adnexal tumor in a child can still behave malignantly when deep infiltration, vascular invasion, higher mitotic activity, and pleomorphism are present.
• Atypical nodular hidradenoma appears to be a borderline entity that overlaps with hidradenocarcinoma, and classification should be based on the overall pattern of aggressive features rather than one isolated finding such as intravascular invasion.
• Hydradenocarcinoma is extremely rare but can recur locally and metastasize, so complete excision, nodal assessment, and close follow-up are important despite limited formal guidelines.
• The presentation emphasizes that rare skin tumors often require cautious expert review because histopathologic discordance is common and pediatric cases deserve especially careful interpretation and management.