The relation of treatment parameters to the phenomenon of vestibular schwannoma swelling after Gamma Knife surgery

Michael Torrens, PA1, Christos Stergiou2, Pantelis Karaiskos2, Alexandra Trampeli3, Vasilis Vasdekis3

1Department of Neurosurgery, Hygeia Hospital 2Hygeia Hospital, Athens, Greece 3University of Economics and Business, Athens, Greece

Keywords: vestibular schwannoma, radiosurgery, gamma knife, radiation injury, Imaging

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     Vestibular schwannomas can transiently expand as part of the radiosurgery response.
Our purpose was to identify dosimetric factors that predispose to the specific and problematic phenomenon of early swelling of vestibular schwannomas (VS) after radiosurgery.
     76 patients were studied.
     Information gathered prospectively on 78 patients with VS treated by Gamma Knife 4C and followed for 2-6 years has been analyzed. 3 patients required a repeat Gamma Knife treatment because of VS swelling. Treatment parameters recorded include data on dosimetry, coverage, Paddick conformity, gradient index, collimator size and number, beam on time, total treatment time (which includes APS and helmet change) and source output. Tumour volume was assessed objectively on T2 MR images using GammaPlan® 5.2 both at treatment and follow up. Patients were divided into 2 groups - enlarged and controlled. Statistical analysis included the Mann Whitney test and Spearman Rank correlation.
     Primary volumetric follow up was at 2 years (25.4 ± 2.6 months). At this time 31 (40%) lesions enlarged and 47 (60%) were controlled. On the group analysis using the Mann Whitney test, enlarged tumour volume at 2 years was significantly associated with lower Paddick conformity index (P=0.017), a larger prescription dose (P=0.029) and greater average time per shot (P=0.029). Spearman Rank correlation showed a significant association between enlarged volume and greater average time per shot (0.249, P=0.028), smaller size of collimators (-0.277, P=0.014), a lower Paddick conformity index (-0.299, P=0.008). A trend towards significance was found with a prescription dose of 12 Gy rather than 11Gy (0.207, P=0.069), lower age (P= 0.079) and lower source output (P=0.113). Number of collimators, other dosimetric factors, treatment time and % cover were not significantly related to volume change. Factors such as conformity and size of collimators did not change significantly over the study time.
     This was a retrospective study.
     The fact that a greater average time per shot, smaller size of collimators and lower source output are all associated with the phenomenon of VS swelling suggests that lower dose rate may be a factor in its genesis, confirming a previous analysis on a smaller series of patients.
     We encourage others to evaluate these parameters.


Project Roles:

M. Torrens (), C. Stergiou (), P. Karaiskos (), A. Trampeli (), V. Vasdekis ()