Novalis intensity-modulated radiosurgery: methods for pretreatment planning

The Novalis stereotactic radiotherapy system (BrainLAB, Heimstetten, Germany) allows for precise treatment of cranial base tumors with single-fraction radiosurgery. In some cases, however, proximity of the optic nerve and chiasm is a concern. In these cases, intensity-modulated stereotactic radiosurgery (IMRS) can be used to limit the dose to these structures. IMRS planning can be labor intensive, which poses a problem when it is performed on the day of treatment. We describe our methods and results of preprocedure planning for IMRS for patients with lesions in the cavernous sinus or parasellar regions in whom the dose to the optic nerve or chiasm might exceed our acceptable tolerance dose (8 Gy).

Patients whose lesions were more than 4 mm from the optic nerve and chiasm on standard magnetic resonance imaging scans but who were questionable candidates for radiosurgery because of concerns of dose to the optic nerve or chiasm were considered for IMRS. Preprocedure imaging (computed tomography and magnetic resonance imaging) was fused and analyzed using the BrainLAB BrainScan 5.3 treatment planning system. Dynamic conformal arc plans for stereotactic radiosurgery and IMRS were evaluated. Doses to the planning target volume and optic apparatus were assessed by dose-volume histograms and conformality index calculated to characterize the quality of the different plans. When IMRS was used, the preplan allowed for a rapid recalculation on the treatment day, minimizing the time patients were in the head frame before treatment.

We describe three patients with recurrent pituitary tumors and three with meningiomas. Doses were 1500 to 2000 cGy prescribed to the 80 to 96% isodose line delivered by eight to 22 fields. Tumor volumes ranged from 2.70 to 8.82 cm (mean, 5.7 cm). In five of the six patients, the dynamic conformal arc plan precluded delivery of therapeutic dose without exceeding optic nerve tolerance. On the basis of 95% coverage of target volume, maximum prescription doses of 7.7 to 20.64 Gy were possible with the dynamic conformal arc plans without exceeding 8 Gy to the optic apparatus. IMRS allowed maximum doses of 20 to 31 Gy using the same optic apparatus dose restriction. No complications have occurred, and all tumors have remained stable since treatment (mean follow-up period, 30 mo).

We believe this pretreatment technique streamlines the process for IMRS, allowing for better patient comfort and efficient physician time use.