Purpose or Case Report: Keratin-positive giant cell-rich tumor is a very rare subset of giant cell-rich tumors characterized by keratin-positive cells and HMGA2::NCOR2 gene fusion. First described in 2021, fewer than 40 cases have been reported in the English literature. Reported cases have shown a strong predilection for females in their 20s-30s and occur in both soft tissue and bone. The youngest reported case thus far has been a 13-year-old female.

A 4-year-old previously healthy male presented to clinic with leg pain and a limp after falling on his right leg. Initial X-ray showed a lytic lesion on the right proximal tibia. Follow up MRI showed a 2 x 2.5 x 4.3 cm solid lesion in the proximal tibial metaphysis extending into the epiphysis. The mass was T1 and T2 hyperintense and demonstrated homogenous enhancement after contrast. No periostitis was noted on radiograph and no subperiosteal collection was present. No soft tissue component was identified. CT chest, abdomen, pelvis was ordered to rule out other sites of disease and showed multi-station lymphadenopathy throughout the right groin. Biopsy revealed tumor cells negative for H3G34W and H3K36M histone markers, specific for giant cell tumor of bone and chondroblastoma respectively. The cells were focally positive for AE1/AE3, a stain for keratin. Next-generation sequencing revealed an HMGA2::NCOR2 fusion confirming the diagnosis of keratin-positive giant cell-rich tumor. He underwent complete curettage of the lesion as well as excision of 2 inguinal lymph nodes that were negative for disease.
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Conclusions: This case presents a unique primary bone tumor with conflicting imaging features inclusive of other primary bone tumor diagnoses. This case represents a new and rare lesion in the youngest known patient, broadening the demographic age range of this disease process. It contradicts existing literature that shows a predominance in young adult females. Most importantly, this case highlights the need to recognize the class of giant cell rich tumors that must be included in a differential diagnosis when patients present with atypical imaging findings.