SU‐E‐T‐703: Brain Dose From Gamma Knife Depends Primarily On the Treated Volume and Not On the Number, Shape Or Location of the Lesions

Purpose: To assess the hypothesis that the volume of brain parenchyma that receives a certain dose level in Gamma‐Knife is dependent on the treated volume and not on the number, shape or location of the lesions. This would help a physician validate the suitability of GammaKnife based stereotactic radiosurgery (GKSR) prior to treatment. Methods: Simulation studies were performed to establish the hypothesis for an oblong, a spherical lesion of various sizes and multiple spherical lesions. A similar study was performed on forty patients who underwent GKSR with mean age 54 (range 7–80), mean number of lesions 2.5 (range 1–6) and mean lesion volume at presentation 4.4cc (range 0.02cc–22.2cc). Following recommendations of QUANTEC, V12 of brain (VB12) was measured and a power‐law based relation is proposed here for estimating VB12 from the volume of target treated to 50% of maximum dose (VT50%). Results: In the simulation study, the volume of brain irradiated by 50% (VB50%), 10% (VB10%) and 1% (VB1%) of maximum dose was found to have linear, linear and exponentially increasing dependence on VT50%, respectively. In the retrospective study on forty GKSR patients, a similar relationship was found to predict the brain dose with a Spearman correlation coefficient >0.9 and the corresponding p‐value from Students T‐test <1*10‐4 for all the above 3 dependences. The relationships between clinical VT50% and the number and aspect ratio of lesions were statistically insignificant. The measured VB12 of brain agrees with the calculated value of VB12 to within 1.7%. Conclusion: The simulation and the retrospective clinical studies indicate that the volume of brain irradiated by a percentage of maximum dose is dependent on the treated volume and not on the number, shape or location of the lesions.