Tumour-stage mycosis fungoides with large-cell transformation and partial CD30 expression clinically mimicking primary cutaneous anaplastic large cell lymphoma

Mycosis fungoides (MF) can progress to tumour stage with expression of CD30 antigens, creating a diagnostic overlap with other CD30 positive lymphoproliferative disorders, particularly cutaneous anaplastic large cell lymphoma (c-ALCL). Accurate distinction between these entities is essential, as they differ significantly in prognosis and management. 
We report a 69-year-old man who presented with rapidly progressive, generalised cutaneous nodules with ulceration over a period of two months, on a background of chronic pruritic dermatosis. Histopathological examination of skin biopsies revealed sheets of large, atypical lymphocytes with diffuse CD3 and CD4 positivity and partial CD30 expression involving approximately 30 – 40% of tumour cells.
In the absence of uniform CD30 expression (>75%), and in the context of a preceding chronic dermatosis with widespread multifocal lesions, a diagnosis of tumour-stage MF with features suggestive of large cell transformation was favoured over c-ALCL following clinicopathological correlation. 
This case highlights the diagnostic challenge posed by CD30-positive cutaneous lymphoid infiltrates and the importance of integrating clinical evolution, lesion distribution, histopathological features and quantitative immunophenotypic findings in establishing an accurate diagnosis.