Purpose: With Icon’s capability to treat a patient fixated with a mask and the ability of temporally suspending the treatment as soon as the patient moves out of a pre-set tolerance, it is vital to verify the leakage since the sources are parked at the Block position for a maximum of 5 times per shot, each time could be as long as 30 seconds during treatment suspension.
Methods: Three methods were employed to measure leakage dose on three Icon machines. All methods calculate the leakage percentages using the ion chamber readings with all sectors On and the readings with some sectors Off. Method-A turns off some combinations of sectors. Method-B starts with turning Sector-1 on, and adds one consecutive sector at a time. Method-C turns on only one sector at a time.
Results: The results from machines 1, 2, and 3 for Method-(A, B, C) are (0.08%, 0.02%, 0.10%), (0.18%, 0.13%, 0.15%) and (0.14%, 0.09%, 0.14%), respectively.
Conclusion: Since GammaPlan requires a minimum shot time of 8.8 seconds for a shot containing 16-mm collimation (5.5 sec for 4-mm or 8-mm shots), a specification of 0.2% leakage at the Block position translates to 3.4-5.5% extra dose to the patient in the worst case. Our results show that we have met this spec.