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.