(Sunday, 7/29/2018) 3:00 PM - 6:00 PM
Room: Exhibit Hall
Purpose: Ionization chamber based measurements have become the norm for SRS plan verification. Studies on small field dosimetry have demonstrated that an ionization chamber would underestimate the dose within the high dose region and overestimate the dose within the low dose region when measuring the profiles of small fields. This study aims to quantitatively investigate the dose response of a PinPoint chamber for point dose verification of CyberKnife fixed-cone collimator based SRS plans.
Methods: Targets with different volumes(ranging from 0.07cc to 4.78cc) were contoured on a CyberKnife stereotactic phantom. The MultiPlan TPS was used for SRS plan optimization. To generate the treatment plan, only a single fixed collimator was used at a time and three collimators were tested in this study (5mm, 7.5mm, and 10mm fixed cones). For testing purposes, the treatment beams were converted to the anterior-posterior direction while keeping the same beam offset. The point doses were then recalculated and compared to the measurement data.
Results: For the smallest target treated with a 5mm fixed collimator, 11% discrepancy between the calculated and measured point dose was observed while the discrepancy kept decreasing with the increasing of the target size and the cone size. The calculated doses were in good agreement with the measurements (less than 3%) for the target sizes larger than 0.68cc. For all the plans generated with 10mm cone, the discrepancies were always less than 5% for all the target sizes tested.
Conclusion: Clinically, selecting a combination of fixed collimators with the field size usually represents a compromise between the requirements of achieving high-dose conformality and that of minimizing the treatment time. The study quantitatively investigates the performance of a PinPoint ionization chamber for the measurement of CyberKnife fixed-cone collimator based SRS plans and provides very useful data for patient specific SRS plan verification.
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