Relationship Between Pityriasis Lichenoides and Mycosis Fungoides: A Clinicopathological, Immunohistochemical, and Molecular Study

Background: Several cases of pityriasis lichenoides (PL) have been reported to evolve into mycosis fungoides (MF). Objective: To elucidate clues to this progression. Methods: Fifty-eight patients with PL between 2000 and 2013 (follow-up: 3–16 years, average: 8.3). Results: A total of 3 (5.2%) of the 58 patients with PL developed MF after 3–11 years of prolonged clinical course. Papules and small plaques characterized PLs, and patches and larger plaques subsequent MFs. A total of 35 of 41 (85%) followed up non-MF associated patients with PL reported lasting complete remissions. Histopathologically, apoptotic keratinocytes disappeared mostly or completely in subsequent MFs. The presence of epidermotropism, folliculotropism, and epidermal lymphocytic nuclear atypia in PLs was not predictive of MF. CD8+ cells were the dominant intraepidermal lymphocytes in the 3 PLs but remained so in only 1 subsequent MF. CD7+ lymphocytes decreased substantially in 2 MFs, and lymphocytic nuclear atypia increased markedly in 1. T-cell receptor gene rearrangement studies demonstrated clonal populations in 1 of 2 studied PLs and in all 3 subsequent MFs. Conclusions: A few PLs may evolve into MF. Prolonged clinical course, appearance of patches and larger plaques, markedly increased lymphocytic nuclear atypia, marked diminution of apoptotic keratinocytes and CD7+ and CD8+ lymphocytes, and clonal T-cell receptor gene rearrangement may serve as clues.

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