Radiologic and histopathologic changes after Gamma Knife radiosurgery for acoustic schwannoma.

Gamma knife radiosurgery (GKRS) is a widely used treatment option for acoustic schwannomas, 3 cm in diameter or less. Between May 1990 and February 1998, 102 acoustic tumors in 101 patients were treated with GKRS. There are 77 patients with a follow-up period of more than six months (mean 55, range 7 to 90 months). Seventy (91%) of these tumors have remained unchanged or reduced in volume. After GKRS there was an increase in volume in seven cases. In four the volume increase affected solid tumour. Among these, three patients were in stable condition and are being observed. One of these patients developed brain stem compression symptoms and was operated. In another three cases, cysts with multiple septa developed medial to the tumor and compressed the brain stem and fourth ventricle, thus necessitating post-GKRS surgery. In these three patients, MRI had shown loss of central contrast enhancement followed by its return. Histological findings at surgery before and after GKRS were compared for these four tumours. In spite of the MRI changes, there were no definite histological findings after GKRS which could be attributed to radiation induced changes. The development of cysts occurred after the treatment of larger tumors.