Keratin-Positive Giant Cell-Rich Tumor of Bone Harboring an HMGA2::NCOR2 Fusion: Two Cases, Including a Patient With Metastatic Disease, and Review of the Literature

Novel HMGA2::COL14A1 Fusion Identified in Xanthogranulomatous Epithelial Tumor/Keratin-Positive Giant Cell Tumor

Xanthogranulomatous epithelial tumor (XGET)/Keratin-positive giant cell tumor (KP-GCT) represents a spectrum of recently described neoplasms characterized by a proliferation of distinctive mononuclear cells expressing keratin within a background of osteoclast-like giant cells, mixed inflammatory cells, and a variably prominent xanthogranulomatous component. Recent studies demonstrated a recurrent HMGA2::NCOR2 fusion in many cases. We herein describe a case of XGET/KP-GCT arising in the right femoral head of a 19-year-old male harboring a rare novel HMGA2::COL14A1 fusion.

Giant Cell Granuloma of the Jaws and Keratin-Positive Giant Cell-Rich Tumor of Bone and Soft Tissue

BACKGROUND

Different giant cell-rich tumors may occur in the jaws. Recently, a new condition known as keratin-positive giant-cell rich tumor harboring recurrent HMGA2::NCOR2 fusions has been described. Interestingly, the mononuclear cells of this tumor are immunoreactive with the AE1/AE3 keratin. Considering the similarities of central and peripheral giant cell granuloma of the jaws with the keratin-positive giant cell-rich tumor of the soft tissue and bone, we hypothesized whether the keratin-positive tumors could also occur in the maxillary bones.

METHODS AND RESULTS

An immunohistochemical investigation of AE1/AE3 in a cohort of 16 cases of peripheral and central giant cell granuloma of the jaws was carried out. None of the cases was keratin-positive.

CONCLUSIONS

Although no immunopositivity for keratin was observed in the present giant cell granulomas cohort, we cannot completely exclude the possibility of keratin-positive giant cell-rich tumors occurring in the jaws. Therefore, oral pathologists should be aware about this condition and further studies using cohorts from different laboratories are necessary.

Keratin-Positive Giant Cell-Rich Tumor: A Review and Update

Keratin-positive giant cell-rich tumor (KPGCT) is an extremely rare and recently described mesenchymal neoplasm that occurs in both soft tissue and bone, frequently found in young women. It has locally recurrent potential if incompletely excised but low risk for metastasis. KPGCT is histologically similar to conventional giant cell tumors of soft tissue but shows the presence of keratin-positive mononuclear cells. Interestingly, KPGCT also shares some morphological features with xanthogranulomatous epithelial tumors. These two tumors have recently been shown to harbor an HMGA2-NCOR2 fusion, arguing in favor of a single entity. Surgery is the treatment of choice for localized KPGCT. Therapeutic options for advanced or metastatic disease are unknown. This review provides an overview of the current knowledge on the clinical presentation, pathogenesis, histopathology, and treatment of KPGCT. In addition, we will discuss the differential diagnosis of this emerging entity.

Giant Cell Tumor of Soft Tissue: An Updated Review

Giant cell tumor of soft tissue (GCTST) is a locally aggressive mesenchymal neoplasm of intermediate malignancy that predominantly occurs in the superficial soft tissue of the extremities. It is histologically similar to a giant cell tumor of bone (GCTB) and shows a mixture of round to oval mononuclear cells and osteoclast-like multinucleated giant cells. Currently, immunohistochemistry plays a very limited role in the diagnosis of GCTST. Primary or secondary malignant GCTST has recently been described and tumors exhibiting high-grade histological features demonstrate higher rates of distant metastasis. GCTST lacks the H3-3A gene mutations that are identified in the vast majority of GCTBs, suggesting a different pathogenesis. Surgery is the standard treatment for localized GCTST. Incomplete surgical resection is usually followed by local recurrence. Radiation therapy may be considered when the close proximity of critical structures prevents microscopically negative surgical margins. The systemic treatment options for advanced or metastatic disease are very limited. This review provides an updated overview of the clinicoradiological features, pathogenesis, histopathology, and treatment for GCTST. In addition, we will discuss the differential diagnosis of this peculiar neoplasm.

Histones and their practical application in bone tumors: Do I always need them?

Historically, the diagnosis of giant cell-rich neoplasms arising in bone has been challenging owing to overlapping clinical and radiographic findings resulting in the difficult separation of several neoplasms, particularly when biopsy material is limited. However, with the discovery of the driver histone mutations in giant cell tumor of bone (GCTB) and chondroblastoma, as well as USP6 rearrangements in aneurysmal bone cyst, pathologists now have objective ancillary tools to aid in the separation of several histologically similar giant cell-rich neoplasms. Furthermore, the recognition of histone mutations has allowed pathologists to revisit several entities, such as "malignant chondroblastoma," and furthered our understanding of phenomena such as "aneurysmal bone cyst-like change," formerly recognized as "secondary aneurysmal bone cyst." Herein, the evolution of testing for histone mutations in bone tumors is considered; the sensitivity and specificity of the histone antibodies is reviewed; and a practical guide for the use of these ancillary tests is offered.

CSF1 expression in xanthogranulomatous epithelial tumor/keratin-positive giant cell-rich tumor

"Xanthogranulomatous epithelial tumor" (XGET) and "keratin-positive giant cell-rich soft tissue tumor" (KPGCT), two recently described mesenchymal neoplasms, likely represent different aspects of a single entity. Both tumors are composed of only a small minority of tumor cells surrounded by large numbers of non-neoplastic inflammatory cells and histiocytes, suggesting production of a paracrine factor with resulting "landscape effect," as seen in tenosynovial giant cell tumor. Recent evidence suggests that the paracrine factor in XGET/KPGCT may be CSF1, as in tenosynovial giant cell tumor. We hypothesized that CSF1 is overexpressed in XGET/KPGCT. To test our hypothesis, we performed quantitative real time PCR (qPCR) for CSF1 expression and CSF1 RNAscope chromogenic in situ hybridization (CISH) on 6 cases of XGET/KPGCT. All cases were positive with CSF1 CISH and showed increased expression of CSF1 by qPCR. Our findings provide additional evidence that the CSF1/CSF1R pathway is involved in the pathogenesis of XGET/KPGCT. These findings suggest a possible role for CSF1R inhibition in the treatment of unresectable or metastatic XGET/KPGCT.