Histiocytic disorders are a wide range of disorders arising from abnormal proliferation and infiltration of dendritic cells. The Histiocyte Society has arranged the disorders into five main groups: L, C, M, R, and H. We present a case in which an elderly woman presented with a solitary osseous lesion in her skull in the right anterior calvarium. Biopsy and histological studies were strongly positive for cyclin D1; positive for CD68, S100, and ZBTB46; weakly positive for OCT2; and equivocal for ALK1 and CD163. Genomic studies also identified KRAS and GPS2 mutations. KRAS-positive genomic analysis favors a diagnosis of histiocytoma, while the solitary calvarium and spontaneous resolution with remission favor a diagnosis of Langerhans cell histiocytosis (LHC). Despite the strong clinical evidence favoring LCH, our patient’s clinical and histologic features did not fit any of the five histiocytic categories and were classified as an atypical histiocytic disorder.

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