Histopathological changes in primary extramammary Paget disease with emphasis on syringomatous structures and syringocystadenocarcinoma papilliferum in situ-like changes.

Primary extramammary Paget disease (PEMPD) is a rare adenocarcinoma, and is histopathologically characterised by intraepithelial proliferation of carcinoma cells with abundant amphophilic cytoplasm. Minor histopathological findings in PEMPD, including syringomatous structures and syringocystadenocarcinoma papilliferum (SCACP) in situ-like changes, have rarely been reported. The present study aimed to analyse histopathological changes in PEMPD. Consecutive patients with PEMPD who underwent surgical resection were included. Melanin within carcinoma cell cytoplasm, syringomatous structures and SCACP in situ-like changes were assessed on haematoxylin and eosin-stained slides. Overall, 55 patients (16 female and 39 male patients) were included; 23 patients (41.8%) had an invasive carcinoma component and the remainder had only in situ lesions. Melanin pigment within carcinoma cell cytoplasm was observed in 85.5% of patients, while 17.4% of patients with invasive PEMPD exhibited melanin pigment in invasive carcinoma cells. Syringomatous structures, characterised by ductal, tadpole-like or small cystic structures composed of bland epithelial cells, were observed in 9.1% of patients (31.3% of all female patients), including in areas outside the PEMPD lesions. SCACP in situ-like changes, characterised by papillary epidermal hyperplasia with replacement of carcinoma cells, were also observed in 9.1% of PEMPD cases. These findings demonstrated that a high frequency of the presence of melanin pigment within carcinoma cell cytoplasm, including in invasive carcinoma cells, could lead to misdiagnosis as malignant melanoma. Additionally, syringomatous structures (especially in female patients) and SCACP in situ-like changes may lead to the misdiagnosis of syringoma and SCACP in situ, respectively. Therefore, recognition of these histopathological changes in PEMPD is required for accurate diagnosis.