A Case of Lymphomatoid Papulosis With CD15- and CD30-Positive Reed-Sternberg-Like Cells in CD8-Positive Mycosis Fungoides: Differential Diagnosis of Secondary Hodgkin Lymphoma and Large Cell Transformation of Mycosis Fungoides.

Mycosis fungoides (MF) is the most common type of cutaneous T-cell lymphoma, and cases with large cell transformation (LCT) are generally associated with a poor prognosis. Lymphomatoid papulosis (LyP) is a cluster of differentiation (CD)30-positive lymphoproliferative disorder characterized by recurrent eruptions and spontaneous regression, typically following a benign clinical course. Histopathologically, LyP occasionally contains Reed-Sternberg (RS)-like large cells, making it important to distinguish LyP from cutaneous involvement of Hodgkin lymphoma (HL) and MF with LCT. We report a case in which erythematous papules developed within MF lesions. Skin biopsy of the papule revealed RS-like large cells positive for CD15 and CD30. No abnormalities were detected by imaging studies or flow cytometry, and the papules regressed spontaneously, leading to a diagnosis of LyP. Treatment with oral etretinate and narrowband ultraviolet B phototherapy resulted in improvement of the MF lesions, and no recurrence of the erythematous papules has been observed. As LyP may morphologically resemble cutaneous involvement of HL or MF with LCT, comprehensive evaluation, including clinical course, imaging findings, and immunophenotypic analysis, is essential for accurate diagnosis.