An otherwise healthy 13-year-old girl presented with a firm nodule on the plantar right forefoot that was tender after cheerleading. Initial workup was unremarkable, but magnetic resonance imaging revealed a multilobulated mass surrounding the flexor hallucis longus tendon. Surgical resection revealed a tenosynovial mass without tendon infiltration. Pathologic examination was consistent with tenosynovial giant cell tumor. The patient resumed cheerleading and remained asymptomatic after 1 year.
As far as we know, this is the first report of a tenosynovial giant cell tumor of the flexor hallucis longus in a pediatric patient; it illustrates the importance of considering this lesion when diagnosing a pediatric plantar mass.