Diffuse type tenosynovial giant cell tumour in the sinus tarsi in a paediatric patient: a rare case report with radiological and histopathological analysis.

We present a unique case of diffuse tenosynovial giant cell tumour (TSGCT) in the tarsal sinus of a paediatric patient. TSGCTs are uncommon in the foot and ankle with potential misdiagnosis as more common lesions such as ganglionic cysts. There have only been three cases of sinus tarsi TSGCTs reported in the literature, and this is the first report describing the diffuse subtype (D-TSGCT) in this location. The patient presented with ankle swelling. Magnetic resonance imaging revealed a focal nodular lesion in the sinus tarsi, demonstrating low T1/T2 signals with striking blooming artefacts, and indistinct margins with infiltrative nodular deposits upon careful scrutiny. Histopathological examination showed distinctive features of D-TSGCT including haemosiderin deposition, multinodular infiltrative growth pattern and villous architecture. D-TSGCT is more extensive, aggressive, and with higher recurrence rates than the more common localised type (L-TSGCT). Despite the described morphological differences in the literature, imaging alone is not always discriminatory. The presenting case demonstrates the diagnostic challenge of an extra-articular mass-like D-TSGCT mimicking L-TSGCT, reviewing existing radiological knowledge translated from its pathological features and highlighting the importance of comprehensive imaging and histological evaluation. Assessing for the striking blooming of low T1 and T2 signals, lack of peripheral hypointense capsule, indistinct margins as well as multinodular infiltration along the synovium and adjacent soft tissue structures may help distinguish it from the localised type. This report also underscores the need to consider TSGCT as a differential even in atypical anatomical sites.