Contributed by Rebecca Ocque, MD and Fiona Craig, MD
37 year-old woman with no significant past medical history presents to a dermatologist for a "mole" on her left calf. The lesion has been present for approximately 6 months and besides the appearance has not been problematic. The patient has been otherwise asymptomatic. The patient's physical exam was within normal limits except for a 2 x 2cm erythematous, indurated nodule on her left medial calf. The patient's laboratory studies, including a complete blood count, a basic metabolic panel, erythrocyte sedimentation rate, rheumatoid factor, and antinuclear antibodies, were all within normal limits. A CT scan of the chest, abdomen, and pelvis was normal. The lesion was excised.
The histiologic evaluation of the lesion demonstrated an evident cellular infiltrate within the dermis which spared the overlying epidermis (Figure 1). The cellular infiltrate was composed of large polygonal cells with abundant eosinophilic cytoplasm and enlarged vesicular nuclei and prominent nucleoli, admixed with plasma cells, neutrophils, and small lymphocytes (Figures 2 and 3). Occasionally, the large polygonal cells were seen to have intracytoplasmic lymphocytes (Figure 4). Special stains for fungi and mycobacterium were negative.
Immunohistochemistry demonstrated that the abnormal polygonal cells were CD68 positive (Figure 5), mostly S100 positive with some cells showing weak staining and some S100 negative cells (Figure 6), and CD1a negative (Figure 7). The S100 stain also highlighted lymphocytes within the polygonal cell cytoplasm (Figure 8). CD 20 and CD3 highlighted the background small B and T cell populations.