Delayed yet effective response of an endolymphatic sac tumor to radiosurgery: case report focusing on its radio-biological behavior.

Endolymphatic sac tumors (ELST), though benign are locally invasive lesions. Owing to its vascularity, complete surgical resection is often not possible and adjuvant gamma knife radiosurgery (GKRS) is advocated to control tumor growth. These lesions do not uniformly respond to radiation therapy in the initial phase and their early radiobiological course after GKRS is less understood. We discuss a case of residual ELST where a mild increase was noted at 36 months following GKRS and then regressed completely after a decade. This report possibly has the longest follow-up revealing the true efficacy of GKRS in these tumors. ELST shows a variable response in the early years after GKRS. They may remain static, regress or increase in size. One should be aware of these patterns of early radiological responses and a long term follow up is warranted as some lesions may show radiosurgical effectiveness after a long latent period.