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

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.