Room: Davidson Ballroom A
Purpose: To model spherical radiosurgical dose distributions utilizing standard-MLC in the treatment of trigeminal neuralgia and to examine the potential risk of normal-tissues toxicity due to isocenter positioning inaccuracy.
Methods: Differentially-weighted dynamic-conformal partial-arcs plans consisting of 4-couch-kicks with clockwise/counter-clockwise(pairs) having 8 collimator-angles for left and right trigeminal nerve-root (TNR) were generated. 90 Gy maximum point-dose was prescribed at middle of the TNR. Dose was calculated using Eclipse AAA with 1mm grid-size on 1mm cut cisternogram-CT scans. Plans were optimized to minimize brainstem dose avoiding beam-entry through eyes. Central leaf of standard-MLC of 5mm width was set to a variable gaps of 1.5-3mm(DLG=1.1mm) for Truebeam with 10MV-FFF(2400MU/min) beam. Collimators were fixed at 0.4cmÃ—0.4cm. Risk of positioning inaccuracy was investigated by re-calculating plans with isocenter-shift from Â±0.5 to Â±2.0mm along x-, y-, and z-direction (total, 48 plans;24 plans for each TNR). Maximal doses to brainstem and left/right temporal lobe were recorded.
Results: For both left/right TNR, the Linac/MLC-based plans provided spherical radiosurgical dose distributions of ~5mm(FWHM) and V45=0.1cc(similar to Gamma Knife-SRS). Average, total MU was 25,800; corresponding beam-on time ~11min (not including couch-kicks). In original plan, maximal doses to brainstem/temporal lobe were kept below 18Gy/16Gy. However, for left TNR, simulation results shows that brainstem could receive higher dose by a factors of 1.5, 2.0, 3.4, and 4.4 and temporal lobe by a factors of 1.3, 1.8, 2.3 and 3.4 with Â±0.5mm, Â±1.0mm, Â±1.5mm, and Â±2.0mm, respectively; consequently, prescription point-dose was degraded by ~10% (Â±1.5mm) and 30% (Â±2.0mm) due to isocenter-shifts. Similar results were found for right TNR.
Conclusion: Utilizing standard-MLC, small spherical radiosurgical dose distributions similar to GK radiosurgery was obtained. For Linac/MLC-based SRS, however, if uncorrected, a very small isocenter localization uncertainty of Â±0.5mm could potentially deliver very high dose to adjacent normal-tissues. Correction strategies for isocenter deviations will be discussed.