2025 Volume 24 Issue 2 Pages 168-174
61-year-old male presented with a black macule onhis left great toe that had been present for 12 years. Recently, the lesion had enlarged and developed a central nodule. Dermoscopic examination revealed a parallel ridge pattern, and a skin biopsy confirmed the diagnosis of acral melanoma. Total excisionof the lesionwas performed with preservationof the affected toe, followed by skin grafting. Based on a diagnosis of stage IIB (pT4aN0M0) acral melanoma, postoperative adjuvant therapy with pembrolizumab (200 mg every three weeks) was initiated. At the second administration, the patient was aware of itching. At the sixth administration, he presented with irregularly shaped purplish-red papules on the trunk and extremities, which coalesced into plaques on the legs and feet. Dermoscopy demonstrated whitish striae. Histopathologically, the epidermis was irregularly thickened and accompanied by compact ortho-hyperkeratosis and hypergranulosis. The elongated rete ridges displayed a serrated pattern with hydropic degeneration. Civatte bodies were partially observed in the epidermis. A dense, band-like inflammatory infiltrate was present just beneath the epidermis. These findings led to a diagnosis of lichen planus, which was successfully treated with oral prednisolone. The patient was able to complete the 18 cycles of postoperative adjuvant therapy with pembrolizumab. The symptoms of lichen planus improved without discontinuation of amlodipine, suggesting that the eruption was a drug reaction induced by pembrolizumab. Although the significance of pembrolizumab-induced lichen planus remains unclear, the importance of maintaining primary disease treatment through appropriate management of immune-related adverse events is well recognized. Skin Research, 24 : 168-174, 2025
