# A Diagnostic Dilemma and Classification Conundrum: Atypical Histiocytic Neoplasm Presenting as a Calvarial Mass

## TL;DR

An elderly woman's skull tumor presented with features not fitting standard histiocytic categories, leading to a classification as an atypical histiocytic disorder.

## Contribution

The paper presents a unique case of an atypical histiocytic neoplasm with conflicting diagnostic indicators.

## Key findings

- The patient's tumor showed cyclin D1 positivity and mutations in KRAS and GPS2.
- Clinical and histologic features did not align with any of the five established histiocytic categories.
- The case was classified as an atypical histiocytic disorder due to conflicting diagnostic evidence.

## Abstract

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.