Bruised nodule on the leg of a teen

A 17-year-old healthy, overweight girl presented with a two-year history of a single 5-cm non-healing, tender ecchymotic nodule with ill-defined margins on the posterolateral aspect of her left thigh (Figure 1). No history of trauma or medication intake was noted. Complete skin exam was negative for other lesions and there was no lymphadenopathy. Ultrasound revealed a nonspecific 4 x 2 cm subcutaneous heterogeneous mass without peripheral edema. A complete surgical excision of the nodule was performed. Histopathology is shown (Figures 2, 3, safranin stain). Histopathology revealed massive dermal (Figure 2) and subcutaneous infiltration with histiocytes exhibiting emperipolesis, as shown by arrows (Figure 3). Immunochemistry stains were positive for S-100, CD68, and CD163. Markers for CD1a, CD207 (langerin), and factor XIII were negative. Mixed reactive lymphocyte infiltrates showed CD3, CD4, CD45, and CD20 positivity. Rosai-Dorfman disease (RDD) is a rare non-Langerhans cell histiocytosis of unknown etiology classified as sporadic (classic nodal, extranodal, neoplasia-associated, and autoimmune diseases-associated subtypes), familial (syndrome H), and cutaneous. Cutaneous Rosai-Dorfman disease (CRDD) is a benign and distinct subset of RDD, but given that cutaneous manifestations occur in 10% of sporadic RDD cases, it is important to exclude systemic involvement in CRDD with positron emission tomography scan (PET) imaging.1 CRDD is usually not associated with lymphadenopathy or with lab abnormalities. Laboratory results raising suspicion for systemic RDD include hypergammaglobulinemia, hemolytic anemia, leukocytosis, and elevated sedimentation rate.1 CRDD presents usually as a painless slow-growing nodule in female adults, (median age 43.5 years).1 However multiple, papulonodular, tumor-like (more than 1.5-cm-diameter size), indurated, eruptive xanthoma-like, and acneiform presentations have been described.1 One study reported a predilection for the extremities.2 Although very rare in children, CRDD including multifocal forms3 has been described in pediatric patients.4, 5 Histopathology revealing histiocytic infiltration with emperipolesis is a useful non-pathognomonic clue for the diagnosis of RDD. Emperipolesis may be subtler in extranodal sites, such as in CRDD, for which typical features include significant fibrosis, plasma cells infiltration, and spindled histiocytes in a storiform pattern.6 RDD histiocytes will stain positive for CD68, CD163, and S-100, and negative for CD207 and CD1a excluding the more common juvenile xanthogranuloma and Langerhans cell histiocytosis, both in the differential diagnosis.1 Expert consensus recommends that all suspected cases of RDD and CRDD undergo testing for IgG-4-positive plasma cells to exclude an IgG4-related sclerosing disease.1 However, the association between RDD and IgG-4-related diseases remains debated.1 The current histiocytosis classification (L, C, R, M, H) is based in part on different mutations in the mitogen-activated kinase (MAPK) pathway.7 Molecular profiling, such as sequencing for MAPK gain-of-function mutations in relevant genes (e.g., NRAS, HRAS, KRAS, MAP2K1, BRAF, or ARAF) is recommended for refractory or severe disease in sporadic RDD, and other histiocytoses to identify potential targets for therapy—mainly BRAF V600E inhibitors in the Langerhans group.7, 8 CRDD is classified in the C group (cutaneous and mucocutaneous histiocytoses) and has not been associated with BRAF or MAP2K1 mutations. However, NRAS mutations in CRDD have been recently described and could led to further therapeutic options in the future.7 While spontaneous resolution of CRDD has been reported over a period of 1–3 months post-diagnosis,9, 10 localized disease can be treated with surgical excision, cryotherapy, topical imiquimod, topical corticosteroids, and intralesional corticosteroids with varying degrees of success.7 In refractory multifocal disease, dapsone, imatinib, methotrexate, and systemic glucocorticoids have shown efficacy.7 In our patient, complete blood cell count, liver function tests, lactate dehydrogenase, coagulation studies, C-reactive protein, and PET-Scan were negative excluding sporadic RDD. She underwent local surgical excision of her CRDD on the left thigh. At 1-year follow-up, her thigh remains disease free, but she recently presented with a new CRDD nodule on the lower back with a negative systemic workup. Surgical excision is planned with careful future follow-up. The authors declare no conflicts of interest.