A 69-year-old man had erythema on the right external heel for about 30 years. The erythema had gradually increased and became nodular after external stimulation 3 years previously. Dermoscopy showed a whitish-pink spore-like structure covered with a milky white reticular structure at the edge. White unstructured areas and hair-pin vessels were observed in the nodules. Eccrine porocarcinoma was suspected, and biopsies were performed from the marginal erythematous plaque lesion and the central nodule. The erythematous plaque showed an intraepidermal sweat duct-like luminal structure without dysplasia in the tumor islands and the nodule showed carcinoma composed of similar tumor cells with nuclear and structural atypia. YAP1::MAML2 fusion gene has been reported to be useful for classifying sweat gland tumors. In the present case, YAP1::MAML2 fusion gene was detected by RT-PCR and the nodule was diagnosed as eccrine porocarcinoma. Seven years previously, eccrine poroma was diagnosed by biopsy of the same lesion, and resection was planned, but the consultation was interrupted and YAP1::MAML2 fusion gene was also detected by RT-PCR from the eccrine poroma of the lesion. These findings suggest that eccrine poroma may be the basis for the development of eccrine porocarcinoma, and that large eccrine poroma should be resected.

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