Clinical Evaluation of Shot-Within-Shot Optimization for Gamma Knife Radiosurgery Planning and Delivery

OBJECTIVE: Shot-within-shot (SWS) optimization is a new planning technique that relies on various combinations of shot weighting and prescription isodose line (IDL) to reduce beam-on time. The method differs from other planning techniques that incorporate mixed collimation, multiple stereotactic coordinates, and traditionally low prescription IDLs (50%. Planning metrics were then compared among the original plan and the 2 replans. RESULTS: More than a third (39%) of the brain metastases were eligible for the SWS technique. For these lesions, the differences between the original plan and reference SWS plan were as follows: ΔV(12Gy) < 0.5 cc in 93% of cases, ΔV(12Gy) < 0.5 cc in 100% of cases, Δselectivity < 0.1 in 79% of cases. Negligible differences were seen between the 2 replans in terms of Δselectivity and ΔV(12Gy); ΔGI < 5% in 99% of cases. After optimization, beam-on time was reduced by 25%–30% in approximately 40%–50% of eligible lesions when compared with the reference SWS plan (ΔT(max) = 42%). In comparison with the original plan, beam-on time was reduced even further, ΔT > 50% in 20% of cases (ΔT(max) = 70%). CONCLUSIONS: This work demonstrates clinically that optimization using the shot-within-shot technique can reduce beam-on time without degrading treatment plan quality.