Gamma Knife® plan optimization with fixed beam on times.

treatment time affects the Biological Equivalent Dose (BED), and hence the potential outcome, of a LGK treatment even when the prescription dose is fixed. To investigate this, multiple treatment plans for the same target with varying treatment times and calibration doserates in the range 1.5-3.5Gy/min have been created using LGP Lightning. Standard metrics for the plans as and BED were calculated for each plan. Based on the results, decisions can be taken upon the treatment time to be used to achieve a specific BED while still maintaining a good overall plan quality. Materials and methods: Optimized plans for five acoustic schwannomas, with various tumour sizes 0.891cc to 7cc with a prescription dose of 12Gy were created. The cochlea was as a risk-organ outlined with a small margin and a maximum dose constraint of 4Gy. For each case the Paddick (PCI) and gradient indices (GI) were calculated. BED was calculated using the formula by Millar and Hopewell. Results: For fixed 45-minute treatments, varying the dose-rate between 1.5 Gy/min to 3.5Gy/min for all the tumour sizes, the average PCI increased from 0.83 to 0.90 and the average GI decreased from 2.96 to 2.57. Increasing the beam on time from 45 to 60 minutes for a fixed dose-rate 2Gy/min increased the average PCI from 0.83 to 0.89 and decreased the average GI from 2.96 to 2.62. Increasing either dose-rate or treatment time will improve the quality of the plan, but this is an exponential effect with diminishing returns. With Lightning generated plans, the prescription dose is also, to a good approximation, an iso-BED surface for a given treatment time. The BED calculated on the prescription isodose surface varies mainly with the total treatment time (10-80 minutes) between approximately 65Gy2.47 down to 51Gy2.47 and not on other details such as reference dose rate and number of shots. Furthermore, the variation of BED on the prescription iso-dose surface is less than 5% for almost all combinations of treatment time and dose-rate. [ABSTRACT FROM AUTHOR]