Room: AAPM ePoster Library
Purpose: Gamma Knife (GK) radiosurgery deliveries are uniquely collimated to 4-, 8-, or 16-mm diameter shots. Because inverse planning is rare for GK, collimator shot selection differs for each planner even if prescription isodose line (IDL) is identical. This work explores the impact on tumor control probability (TCP) arising from subtle differences in dose distribution which result from collimator choice.
Methods: Two reference plans were created using a single-shot of either 8- or 16-mm collimators. The resultant reference prescription isodose volume (PIV(R)) in each plan was then defined as the target volume for a comparison plan using multiple 4-mm collimator shots. The average dose (D?) was calculated. TCP was calculated assuming a Poisson distribution based on the linear-quadratic cell survival model, using a clinically realistic radiological parameter set for calculations of a=0.23 Gy?¹, a/ß=10.0 Gy, and N0=1x106.
Results: Comparison plans created with multiple 4-mm shots were able to effectively mimic reference single-shot 8- (16-mm) shot plans: prescription 50% isodose line (IDL) coverage was 99.2% (99.5%) and PIV(R)/ PIV(4-mm) ratio was 1.008 (1.005). For the 8-mm target, TCP and D? were 0.863 (0.826) and 27.4 (25.8) Gy for the single-shot 8-mm versus the multiple 4-mm. For the 16-mm target, TCP and D? were 0.868 (0.882) and 28.7 (24.7) Gy for the single-shot 16-mm versus the multiple 4-mm. Based on the TCP equation, creating a slightly larger margin of prescription dose around the 8-mm target would have improved the TCP dramatically, which is possible to do using more 4-mm shots.
Conclusion: Plans were created with multiple 4-mm collimators which were similar to large single-shot collimators. Yet, parameters such as TCP and average dose were dissimilar. GK planners should be aware that varying collimator size can result in broader prescription dose gradients around the target which would improve TCP and average dose.