In this retrospective study the authors studied the optimal interval following radiosurgery for differentiating between true growth of tumour versus pseudoprogression using serial volumetric data. A total of 118 patients with a median tumour volume of 0.74cm3 at Gamma Knife treatment and with a median follow-up of 4.1 years were included in the study. Patients with sporadic vestibular schwannoma were included and all NF2 patients were excluded. Fifteen patients had undergone prior surgical resection with subsequent clinical progression leading to radiosurgical treatment. The patients were followed up with T1 post gadolinium MR imaging at six months, one year and then annually thereafter. Volumetric analysis was performed on all pre and post treatment MR scans.
The authors observed transient enlargement of the tumour in 44% patients, starting at one year and attaining peak volume within three years following treatment. A tumour regression of 45% was observed by fourth year and 77% by sixth year.
The authors suggest that tumour enlargement within approximately 3.5 years should not be used as a sole criterion for salvage treatment. If there is space for the tumour to expand with significant risk to patient, then it may be worthwhile to wait and give the tumour a chance to regress.