Primary cutaneous CD4⁺ small/medium T-cell lymphoma accounts for ∼2% to 3% of all primary cutaneous T-cell lymphomas.² It has no clear gender predilection. Most patients are elderly, in their sixth and seventh decades of life, but it can occur at any age, including in children.^{2,5,6,7,8,9,10,11,12,13,14,15}

Involvement is generally confined to the skin, but rare aggressive cases with systemic disease have been described.¹⁰ It is unclear whether the latter cases represent previously undiagnosed mycosis fungoides or peripheral T-cell lymphoma, not otherwise specified (in particular, follicular helper T-cell lymphoma), with (secondary) cutaneous tumor formation and unrecognized systemic disease or whether a small subset of primary cutaneous small/medium CD4⁺ T-cell lymphoma can indeed evolve to systemic involvement. Nevertheless, most patients present with an isolated erythematous or violaceous cutaneous papule, plaque, or nodule, involving the head and neck region, trunk, or upper extremities (Fig. 16-1). Involvement of the lower extremities is infrequent.³ Rarely, patients will present with multifocal disease or large tumor nodules.^{1,2,3,4,5,7,10,11,14,16,17} The lesions are usually asymptomatic; however, pain and/or pruritus has been described in some cases.^3,9,11,13,14 Their clinical course is variable: lesions may be of sudden onset or may exhibit a more protracted course.^{6,9,10,12,13,14,16} Occasional lesions may resolve spontaneously.^11,18 Rarely, patients have been reported to present with alopecia,¹³ purpuric,¹⁴ poikilodermic,³ ulcerative,³ or annular lesions.¹⁹

These lymphomas typically demonstrate involvement and effacement of the dermis with either a nodular or diffuse distribution and lack significant epidermotropism, although focal epidermal involvement has been described (Figs. 16-2A, 16-3A–D, and 16-4A).¹⁵ Surface ulceration may be seen in larger lesions.^2,7,9,14,17 The tumor may involve adnexal (Fig. 16-2B)⁷ and vascular structures and may show angiodestruction.¹⁶ The lesions are typically “top heavy” in the dermis, but may extend into the deep dermis and subcutaneous tissue (Fig. 16-3D,F).^2,11 The majority of neoplastic cells are of small-to-medium size (Figs. 16-2B, 16-3F, and 16-4B); however, a minor subset (<30%) may be large and exhibit cytologic atypia.^2,6 Similar to other T-cell lymphomas, the tumor cells may be accompanied by a reactive mixture of histiocytes, eosinophils, polytypic plasma cells (Fig. 16-3E),^10,11 small B lymphocytes, and immunoblasts^{3,4,5,7,8,10,11,14,17} as well as rare neutrophils.¹¹ An associated granulomatous reaction may be seen, mostly noted around follicular and adnexal structures. Tumor necrosis is not seen,^7,19,20 and there is no secondary germinal center formation.^3,15