Methods (Differential diagnosis, investigations and treatment):
A needle core biopsy was taken from the lesion. Microscopic examination showed a prominent proliferation of xanthomatous histiocytes and smaller fibrohistiocytic cells. There are also isolated epithelioid cells with moderate nuclear atypia. Few osteoclasts giant cells were also noted. No marked pleomorphism, necrosis, or atypical mitosis was identified. No features of an overt inflammatory process nor other mesenchymal components (Figure 5a ).
Immunohistochemical studies showed focal positive keratin expression in the epithelioid cells. (Figure 5b ). However, cells were negative for low molecular weight keratins such as CK7, CK8/18, and CK CAM5.2. Xanthomatous cells showed diffuse positivity for CD68 and factor XIIIa. While smooth muscle, vascular, and neural differentiation markers were negative. INI-1 (SMARCB1) immunohistochemistry retained its nuclear positivity.
The initial suggested morphological differential diagnoses were fibrohistiocytic lesion of bone, non-ossifying fibroma, and exuberant reaction to adjacent neoplasm. However, focal keratin positivity and the absence of other inflammatory features ruled out those possibilities. Thus, Xanthogranulomatous Epithelial Tumor (XGET) emerged as a working diagnosis. Given its rarity and scant literature, the case underwent central review at a referral center, which concurred with our diagnosis. Our case underwent a sarcoma-targeted gene fusion panel analysis, yet no fusion was identified.
The patient initiated Denosumab therapy as part of a trial to mitigate the need for extensive surgery. The patient is currently under regular follow-up to monitor the efficacy and safety of this novel treatment approach.