Symplastic/pseudoanaplastic giant cell tumor of the bone

Objective: Giant cell tumor of bone (GCTB) is a locally aggressive primary bone tumor. Its malignant counterpart is quite rare. Rarely, a conventional GCTB shows marked nuclear atypia, referred to as symplastic/pseudoanaplastic change, which can mimic sarcomatous transformation. Recently, somatic driver mutations of histone H3.3 exclusively in H3F3A have been described in GCTB. We report a series of 9 cases of GCTB with symplastic/pseudoanaplastic change, along with analysis of H3F3A variants. Materials and methods: Nine cases of GCTB with symplastic change were identified. Clinico-radiological features, morphological features, and immunohistochemical stain for Ki-67 stain were reviewed. H3F3A variants were also analyzed using Sanger sequencing. Results: Histologically, conventional giant cell tumor areas with scattered foci of markedly atypical cells were seen in all of the cases and all showed rare if any Ki-67 labeling. One patient had received denosumab treatment and another radiation therapy. Radiological features were characteristic of conventional GCTB. Mutation in H3F3A (p.Gly34Trp [G34W]) was found in 6 of the 7 cases. Clinical follow-up ranged from 6 to 208 months. Local recurrences were seen in 4 cases (44 %). Conclusions: GCTB with symplastic/pseudoanaplastic change is an uncommon variant of conventional GCTB, which can mimic primary sarcoma or sarcomatous transformation. These tumors possess the same missense mutation in histone H3.3 as conventional GCTB.