Room: Room 209
Purpose: To determine the efficacy and utilization rate of shot-within-shot (SWS) optimization for Gamma Knife radiosurgery planning.
Methods: 35 patients previously treated for metastatic disease in the brain were selected. For applicable lesions, new plans were created using the SWS technique which co-locates two shots having different collimator sizes. Two SWS plans were created, one prescribed to the 50% IDL and another prescribed to an IDL optimized to reduce beam-on time but maintain the same dose gradient to a 50% IDL plan. The following metrics were quantified and compared between SWS plans and with the treated plan: Selectivity, Gradient Index, V12Gy normal brain, and Beam-on time.
Results: 100 lesions were assessed. Of these, 59 were suitable for shot-within-shot optimization (utilization rate = 59%). Those excluded were lesions capable of being treated with a single 4 mm shot and those not capable of being encompassed by a single 16 mm shot. Among applicable lesions, SWS optimization reduced beam-on time in comparison to the original plan according to the following distribution: Î”T10% = 48 lesions (81%), Î”T20% = 44 lesions (75%), Î”T30% = 34 lesions (58%). The median difference in V12Gy of normal brain was 0.01 cc. In only 4 lesions was the difference larger than 1.0 cc (max = 2.8 cc). In comparison to the SWS plan prescribed at the 50% IDL, the distribution was as follows: Î”T10% = 41 lesions (69%), Î”T20% = 30 lesions (51%), Î”T30% = 19 lesions (32%). As the SWS optimization technique is designed to maintain the same dose gradient as when utilizing the 50% IDL, the difference in V12Gy of normal brain was negligible.
Conclusion: In this retrospective study, SWS optimization would have reduced beam-on time by more than 30% in more than a third of brain met cases with no detriment in plan quality.