Verification of Small Field Conformal Arc Dose Distributions with SRS Mapcheck
Abstract
Purpose
SunNuclear SRS mapcheck diode array is a convenient option for 2D dose mapping combining small detector and relatively small 3.5mm detector grid size in axial direction (2.5mm in diagonal direction). We investigate the utility of SRS mapcheck in validating the dosimetry of sub-centimeter conformal arc fields.
Methods
Full conformal arcs 6x fields intended for treatment of spherical targets of 4-20mm diameters were created in Eclipse treatment planning system. Dose distribution was measured by localizing SRS Mapcheck array inside of SRS head phantom at plan isocenter by using combination of orhogonal KV, and CBCT imaging on Varian TrueBeam Edge (HD MLC). In addition, the isocenter dose was measured by placing Exradin A16 pinpoint chamber inside of the SRS head phantom. The obtained data was compared to Eclipse TPS AAA predictions. Eclipse TPS dose profiles were used to compute chamber corrections due to volume averaging effect.
Results
Mapcheck measurements shown good agreement with Eclipse TPS calculated dose distribution. Array central axis dose reading overestimated the dose at the isocenter. Small field correction feature of SRS Mapcheck software was used to reduce the discrepancies. With the correction on, the excess increased for smaller fields and varied from 0.4% for 20mm field, 3% for 10mm field to 10.5% for 4mm field. TPS dose predictions were in good agreement with pinpoint chamber dose measurements (maximum error 2.3% average error 1.1%). Chamber volume averaging correction was calculated via convolution of TPS calculated dose profiles with chamber geometry and varied from 0.999 for 10mm fields to 0.953 for 4mm fields. SRS Mapcheck long term stability was assessed by remeasuring 6mm field multiple times during the 8 week period. Measured max dose variations were less than 1.1%.
Conclusion
SRS Mapcheck can be utilized for small field validation (4mm+) with expected accuracy of no less than 4.4%.