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SU-F-T-612: Investigation of Acoustic Neuroma Planning for Stereotactic Radiosurgery Utilizing Linac-Based Cone Collimators.

PURPOSE
To assess the feasibility of designing clinically-acceptable stereotactic radiosurgery (SRS) plans utilizing linac-based cone collimators for patients presenting with acoustic neuroma.


METHODS
Five acoustic neuroma patients with gross tumour volumes (GTVs) of sizes from 1.3 to 2.7 cc were studied. The cranial-caudal extent of the GTVs range from 1.1 to 1.7 cm whereas the largest cross-sectional extent of the lesions varied from 2.0 to 2.4 cm. No PTV margin was added. The relevant organs-at-risk (OARs) were the brainstem, brain, lens, eyes and cochlea. The SRS planning system, ERGO (Elekta), was used to design treatment plans with non-coplanar arcs delivered using various stereotactic cone sizes on an Elekta Synergy unit. The prescription dose was 12 Gy to be delivered in a single fraction. The final dose distribution for each target was achieved with two to five isocenters, each consisting of up to five non-coplanar arcs.


RESULTS
The achieved GTV V12, V11.4, and V11 were 97.6-98.6%, 99.2- 99.8% and 99.6-100%, respectively. The penalty for using multiple isocenters for a single target was a relatively high maximum dose of up to 18 Gy, which equals 150% of the prescription dose. In all cases, the RTOG and Paddick conformity indices fell within the range 1.45-1.70 and 0.57-0.66, respectively. Point maximum dose to the brainstem varied from 12.4 to 14 Gy and its V12 was ≤0.12cc. The point maximum doses to the lens and eyes were ≤80 and ≤110cGy, respectively, and total body V10 was ≤6.2cc. Point maximum dose to ipsilateral cochlea was similar to the prescription dose.


CONCLUSION
Clinically-acceptable and deliverable dose distributions for acoustic neuroma cases can be achieved with linac-based stereotactic cones system. Up to five isocenters per target are required for GTVs of sizes ≤3cc. The treatment plans meet RTOG protocol requirement on conformity index.