Objective:To investigate the clinicopathological characteristics of giant cell tumor of bone (GCTB) in children.Methods:A total of 35 cases of GCTB diagnosed at Shanghai Sixth People′s Hospital Affiliated to Shanghai Jiaotong University School from 2016 to 2023 were collected, and a retrospective analysis of clinicopathological features and imaging findings was conducted.Results:Pediatric GCTB accounted for approximately 4.6% of total GCTB cases during the study period. There were 11 males and 24 females. The onset age ranged from 9 to 18 years (mean age 15 years, median age 16 years), with 8 cases (8/35, 22.9%) experiencing postoperative recurrence. Twenty-eight cases (28/35, 80%) primarily affected long bones, while 7 cases involved small or irregular bones. Imaging revealed osteolytic changes as the predominant feature, with 3 cases exhibited open physis, one of which had the tumor primarily at the diaphysis without crossing the physis. Histologically, pediatric GCTB resembled adult cases, characterized by mononuclear cells and osteoclast-like giant cells. Seven cases with denosumab treatment demonstrated degrees of giant cell disappearance, increased fibrous tissue and reactive bone proliferation in the stroma. One case was diagnosed as pediatric multicentric GCTB, and three cases as pediatric primary malignant GCTB, with malignant transformation into osteosarcoma. In all 35 cases, mutations in the H3F3A gene were identified, comprising 32 cases with H3.3 p.G34W mutations, one case with H3.3 p.G34V mutation, and 2 cases with H3.3 p.G34L mutations. Notably, the former two categories were successfully validated at the protein level through immunohistochemical staining, utilizing highly specific antibodies tailored for these mutation types: H3.3 p.G34W antibody and H3.3 p.G34V antibody. However, immunohistochemical staining was not available for the last category.Conclusions:Pediatric GCTB predominantly affects females and occurs primarily in long bones, mainly around the knee joint, the majority of tumors predominantly arise in the epiphysis and extend into the metaphysis; however, in cases where the epiphyseal plates are still unclosed, the tumors may be restricted to the metaphysis. Detection of H3F3A gene mutation is crucial for the diagnosis and differential diagnosis of pediatric GCTB.

Purpose To elucidate the clinicopathological characteristics of primary malignant giant cell tumor of bone(PMGCTB)with mainly osteosarcoma-like morphology.Meth-ods Clinicopathologic features of 7 cases of PMGCTB were ret-rospectively analyzed.Results Among 7 patients with PMGCTB,there were 4 females and 3 males,aged between 9 and 66 years(mean age 39.5 years,median age 35 years).The distal femur emerged as the most frequent site to be involved(3/6).The main clinical manifestations included pain and swelling at the original site of the tumor.Radiological findings indicated osteolytic lesions,often combined with sclerotic areas;most ca-ses showed cortical bone destruction and soft tissue masses(5/7).Histologically,the majority of tumors exhibited typical mor-phological features of osteosarcoma with a few or without osteo-clast-like multinucleated giant cells.Positive immunoreaction with H3F3A G34W was confirmed in 6 cases and with H3F3A G34V in 1 case.SATB2 and p63 were positive in all cases,p53 was proved to be wild type,the Ki67 proliferation index ranged approximately from 10％to 50％.H3F3A p.G34W mutation was detected in 6 cases and only 1 case harboring H3F3A p.G34V mutation.Conclusion PMGCTB is exceedingly rare and difficult for accurate diagnosis,especially for those with atypical morphological features.A comprehensive analysis involving ra-diological,immunophenotypic,and molecular detection is neces-sary to rule out other high-grade sarcomas.