SU-E-T-115: Collimator Scatter Factor (Sc) Measurements for IRIS in CyberKnife Using Build-Up Caps

Abstract

Purpose: To investigate the impact of custom-made build-up caps for two different diode detectors in robotic radiosurgery radiation fields with variable collimator (IRIS) for collimator scatter factor (Sc) measurement. Methods: Two acrylic caps were custom-made to fit our SFD or PFD (IBA Dosimetry, Germany) diode detectors. The two caps have thicknesses of 1.5 and 5 cm, corresponding to the dmax of the 6 MV CyberKnife and a depth beyond electron contamination, respectively. A watertank (Blue Phantom, IBA Dosimetry) was used to position the detector at 80 cm source-to-detector distance. Measurements were performed with the SFD and PFD, with and without the build-up caps, for all 12 clinical IRIS settings ranging from 5 to 60 mm. Result: As expected, the biggest discrepancy was found in the smallest (5mm) IRIS field size. The Sc factor with 5 cm build-up cap was 6.6% lower than that without build-up using the SFD detector while the PFD differed by 13%. However, when the 1.5 cm build-up cap was used, the largest difference (4%) was found using the 10 mm IRIS field size using the SFD while the maximum discrepancy using the PFD was still using the 5 mm IRIS field size (7.6%). For IRIS field sizes larger than 10 mm, maximum discrepancies were less than 1.3% and 3.7% for SFD and PFD, respectively. Conclusion: Sc measurement data are critical components in advanced algorithms for treatment planning, such as Monte Carlo, in order to calculate the dose accurately. After incorporating build-up caps, we discovered differences of up to 6.6% and 13% in Sc factors in the SFD and PFD detectors, respectively, when compared against in-air measurements without build-up caps. These are significant differences and were obtained with IRIS settings routinely used for clinical treatment.