The lichen aureus and lichenoid purpura variants are generally the variants that are challenging at times to differentiate from mycosis fungoides (MF). The lichenoid variant of PPD is uncommon. It typically occurs in middle-aged to older men on the lower extremities. It tends to be a chronic and occasionally pruritic dermatosis. Lesions are lichenoid purple to violaceous papules that fuse into plaques with a predominance for the hips, thighs, and legs (Fig. 38-1). Lichen aureus is also uncommon. It tends to occur as a solitary lesion or multiple lesions in a localized area and is asymptomatic. The lower extremity is the most commonly reported site of involvement, with some cases noting occurrence of lesions overlying perforating veins.

The typical biopsy specimen will demonstrate a moderate-to-dense lichenoid lymphocytic infiltrate with extravasated erythrocytes (Fig. 38-2A and 38-2B). There is controversy about whether to categorize PPD into the lymphocytic vasculitis category. Historically, the lack of fibrinoid change in the vessels has been included as criteria for PPD; however, many authors consider this a vasculitis despite the lack of fibrinoid change.^1,2 Although true vasculitis may be lacking, the endothelial cells are often swollen. Some vessels may appear occluded from the swelling. In typical cases of PPD, the majority of the T cells are CD4⁺, small in size, and lack cytologic atypia. Lymphocytes and erythrocytes may be seen in the epidermis. Focal areas of spongiosis can be seen, especially in the eczematous variant. Hemosiderin laden macrophages may be seen within or surrounding the areas of inflammation (Fig. 38-2B).