Trastuzumab deruxtecan treatment for HER2-low metastatic scrotal extramammary Paget's disease with bone marrow involvement: a case report.

Scrotal Extramammary Paget's disease (EMPD) is a rare cutaneous adenocarcinoma commonly confined to the intraepidermal region, with potential for dermal invasion and metastasis. This report describes a rare case of metastatic scrotal EMPD with bone-marrow involvement. The patient presented with localized pruritic scrotal lesions accompanied by systemic symptoms resulting from distant metastases to the lymph nodes and multiple bones, as well as compromised hematopoiesis caused by tumor cell infiltration of bone marrow. The diagnosis was confirmed by histopathological examination of biopsy specimens from the scrotal lesion, local lymph nodes, and bone marrow. Immunohistochemistry (IHC) revealed HER2 status as 2+ with a negative fluorescence in situ hybridization result. Although the patient presented with a heavy tumor burden involving the bone marrow, he appeared to derive benefit from trastuzumab deruxtecan (T-DXd), a HER2-directed antibody-drug conjugate, and observed disease stability during the treatment period. This case suggests the potentially aggressive biological behavior of EMPD and demonstrates that targeted therapeutics, such as T-DXd, may achieve disease control in cases with low HER2 expression. These findings emphasize the need for further research to establish optimal treatment strategies for metastatic EMPD.