SU-F-T-624: Dose Sparing to Normal Brain Tissue for Stereotactic Radiosurgery Utilizing Negative Margins.

PURPOSE: The volume of normal brain tissue exposed to doses above 12 Gy (V12) strongly predicts the risk of radionecrosis. A negative margin MLC aperture results in fields smaller than the PTV. The aim of this study is to investigate the relationship between V12 and the negative margin MLC while keeping adequate PTV coverage with proper dose normalization.

METHODS: Spherical PTVs were contoured with a center of mass in the frontal and left temporal lobe ranging in size from 1 cm to 4 cm in diameter. A field arrangement was determined for each PTV location using five dynamic conformal arcs for the frontal PTV and three dynamic conformal arcs for the left temporal PTV. Treatment plans for each PTV were generated using a MLC aperture with a positive margin of 1 mm and dose normalized so that the prescribed 18 Gy covers 95% of the PTV. New plans were generated with MLC aperture margins reduced in 1 mm increments, until the max point dose reached is greater than 200%. The volume of dose outside the PTV to 12, 9 and 5 Gy was analyzed.

RESULTS: Depending on the PTV size, the greatest negative margin planned was between -4 mm and -2 mm. Negative margins of -1 mm yielded the lowest significant volumes to all doses. Reductions in V12 in relation to the PTV volumes were between 15% and 31% for PTV sizes of 4 and 1 cm, when compared to the 1 mm positive margin plan.

CONCLUSION: Doses to normal tissue were reduced most with margins of -1 and -2 mm for all PTV sizes. In general the -2 mm margin was minimally superior, so a -1 mm margin can be followed in most circumstances to limit maximum PTV dose.