Histopathologic and immunohistochemical features.

(A) Mycosis fungoides before transformation: proliferation of small-sized lymphocytes in the upper dermis with epidermotropism. (B) The same case after transformation: the lymphoid infiltrate has become tumoral and polymorphous with more than 25% of large-sized lymphocytes (arrows). (C) Analysis of percentage of large lymphomatous cells (cytologic transformation) after CD68 immunostaining: between histiocytic components (CD68 positive cells), large lymphomatous cells (arrows) are CD68 negative. (D) The main differential diagnosis of transformed-MF: “histiocytic-rich” or granulomatous MF. Note that the lymphocytes between histiocytic granulomas (arrows) remain small. (E) Immunostaining with the Ber-H2 antibody in a case of transformed-MF: CD30 expression by more than 75% of large cells. (F) Immunostaining with the CD20 antibody (anti-B lymphocytes): nodular infiltrate of CD20-positive lymphocytes.