Monte Carlo calculated correction factors for the PTW microDiamond detector in the Gamma Knife-Model C

Accurate dose measurements in small fields require correction factors when sufficient CPE is not present inside of the field. These factors adjust for perturbation, volume averaging, and other effects; as such, they are field size, detector, and phantom dependent. In this work, Monte Carlo (MC) methods were used to calculate correction factors for PTW's microDiamond detector in Elekta's Gamma Knife Model-C unit. These correction factors allow for accurate measurement of output factors-even in the smallest field sizes where CPE is not present.

The small field correction factors were calculated as kQclin,Qmsr (fclin,fmsr) correction factors according to the Alfonso formalism. The MC model of the Gamma Knife was built with the EGSnrc code system, using BEAMnrc and DOSRZnrc user codes. Efforts were made to validate the MC model against experimental measurements. Using the model, field output factors and measurement ratios for each of the four helmet sizes were simulated for an ABS plastic phantom and validated against film measurements, detector measurements, and treatment planning system (TPS) data. Once validated against the available ABS phantom, the model was applied to a more waterlike solid water phantom. Using MC results from the solid water phantom, the final k correction factors were determined relative to the machine specific reference field-the 18 mm helmet, which is the largest field size available on the unit.

When validating against experimental measurements using the ABS phantom, all MC methods agreed with experiment within the stated uncertainties: MC determined field output factors agreed within 0.6% of the TPS and 1.4% of film; and MC simulated measurement ratios matched physically measured ratios within 1% for all helmet sizes. kQclin,Qmsr (fclin,fmsr) for the PTW microDiamond in the solid water phantom approached unity to within 0.4% ± 1.7% for all the helmet sizes except the 4 mm; the 4 mm helmet size over-responded by 3.2% ± 1.7%, resulting in a kQ4mm,Q18mm (f4mm,f18mm) of 0.969.

Similar to what has been found in the Gamma Knife Perfexion, the PTW microDiamond over-responds in the smallest 4 mm field. The over-response can be corrected via the Alfonso formalism using the correction factors determined in this work. Using the MC calculated correction factors, the PTW microDiamond detector is an effective dosimeter in all available helmet sizes.