Critical neurological structure sparing radiosurgery of vestibular schwannoma: dosimetric comparison of different techniques and dose prescription methods.

Abstract

To investigate potential sparing of critical neurological structures (CNSs) during radiosurgery of vestibular schwannoma (VS) employing different techniques and dose prescription methods. Fused CT and MRI datasets of eight patients with unilateral VS representing a wide range of target volume (0.48 to 12.08 cc; mean = 3.56 cc), shape and proximity to CNSs such as cochlea, trigeminal nerve and brainstem were re-planned employing static conformal field (SCF), dynamic conformal arc (DCA) and intensity modulated radiosurgery (IMRS) techniques. For every patient, five plans were created for a fixed margin dose of 12 Gy prescribed at 80% in three plans (SCF_80%, DCA_80%, and IMRS_80%) and 50% in another two plans (SCF_50% and DCA_50%). All plans were compared using standard dosimetric indices. Primary goal of every plan to cover ≥99% of target volume with 12 Gy was fulfilled for all patients with minimum significant dose to target (D₉₉) ≥11.99 Gy. Best conformity index (CI Paddick = 0.62 ± 0.12) was observed in SCF_80% and DCA_80% plans whereas; sharpest dose gradient index of 3.40 ± 0.40 was resulted from DCA_50%. All five plans resulted similar maximum dose to brainstem (11.04 ± 2.23 to 11.53 ± 1.10 Gy), cochlea (9.02 ± 1.79 to 10.15 ± 1.26 Gy) and trigeminal nerve (11.55 ± 1.38 to 12.19 ± 2.12 Gy). Among 80% prescription plans, IMRS_80% reduces mean and D₅ (P < 0.05) to all CNSs. Prescription of dose at 50% isodose sharpened the dose gradient and significantly (P < 0.05) reduced mean dose and D₅ to all CNSs at the cost of target conformity (P = 0.01). Mean dose to cochlea and trigeminal nerve were least at 4.53 ± 0.86 and 6.95 ± 2.02 Gy from SCF_50% and highest at 6.65 ± 0.70 and 8.40 ± 2.11 Gy from DCA_80% plans respectively. This dosimetric data provides a guideline for choosing optimum treatment option and scope of inter institutional dosimetric comparison for further improvement in radiosurgery of Vestibular Schwannoma (VS).