Clinicopathologic features of epidermoid cysts in the upper and lower extremities, including a case of malignant transformation in the palmoplantar region

Purpose: Epidermoid cysts are common benign skin neoplasms derived from the pilosebaceous apparatus that usually develop in hair-bearing regions such as the head and neck. Epidermal cysts rarely occur in the extremities, especially in the palmoplantar region. Therefore, they can be easily misdiagnosed as warts or calluses. Here, we present our experience treating epidermal cysts in the extremities, including a very rare case of malignant transformation into squamous cell carcinoma. Methods: This retrospective study enrolled all patients who underwent excision of epidermoid cysts in the upper and lower extremities from March 2006 to April 2021.Results: Among 249 patients, there were 10 (seven male and three female patients) who had epidermal cysts in the extremities (4.0%). All four plantar cysts were located in weight-bearing areas. One palmar epidermal cyst occurred 33 years after the trauma. There was one case of a highly recurrent epidermal cyst on the heel that was eventually diagnosed as squamous cell carcinoma arising from the cyst lining on excisional biopsy. After wide excision with a margin of 2 cm, the resulting defect was reconstructed using a free thoracodorsal artery perforator flap. Conclusion: Epidermoid cysts in the extremities, especially the palmoplantar region, are rare. Detailed history taking, including underlying diseases and trauma history, is helpful for diagnosis. Complete excision is necessary to avoid relapse and to confirm the final diagnosis, especially in cases suspected of malignant transformation arising from epidermoid cysts.