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36. GIANT CELL FIBROBLASTOMA: A SOFT TISSUE TUMOUR ON A SPECTRUM WITH DERMATOFIBROSARCOMA PROTUBERANS. REPORT OF A CASE

Background: Giant cell fibroblastoma (GCF) is a soft tissue tumour which clinically, morphologically, immunohistochemically and cytogenetically resembles dermatofibrosarcoma protuberans (DFSP). Both GCF and DFSP occur preferentially in younger patients, have a male predominance and present as a slow growing painless mass in a truncal distribution in the dermis, subcutis or superficial skeletal muscle. Histologically, while DFSP contains hypercellular storiform areas, GCF is typically less cellular and contains pseudovascular spaces lined by giant cells, an onionskin-like perivascular lymphocytic infiltrate and fascial haemorrhage. Both tumours contain a t(17,22) translocation leading to generation of fusion protein COL1A1-PDGFRB. Both tumours have a predilection for local recurrence, and while DFSP rarely metastasises, GCF has not yet been reported to metastasise. Aim: To highlight the diagnostic features of GCF. Methods: We report a case of GCF in a 9-year-old boy who presented with an 11 cm non-tender slowly enlarging right axilla mass. Intra-operatively the lesion was within the pectoralis major muscle. We compare the morphology of GCF with that of DFSP and present supporting immunohistochemical and cytogenetic data. Results: The tumour macroscopically appeared fleshy and haemorrhagic. Histologically the tumour exhibited varied cellularity containing pseudovascular spaces lined by large atypical cells. Similar atypical cells were noted in solid areas of the tumour. Immunohistochemical staining for CD34 was positive. Karyotyping revealed the presence of a t(17,22) translocation. Conclusion: GCF is a soft tissue tumour that exists on a spectrum with DFSP. Awareness of the entity is important to avoid misdiagnosis as a malignancy.