SU-E-T-176: Improved Collimator Scattering Factor (Sc) Measurements for Small Fields Using Build-Up Caps in Robotic Radiosurgery.

PURPOSE The Monte Carlo calculation algorithm in the MultiPlan (Accuray, Palo Alto, CA) treatment planning system used for CyberKnife (Accuray) robotic radiosurgery requires in-air measurements. In this study, results were compared for the impact of build-up caps using a diode detector for small field in-air measurements. METHODS Two acrylic caps custom-made for the SFD diode detector (IBA, Germany) of two thicknesses were compared against free-in-air measurements. The 1.5 and 5 cm thicknesses correspond to Dmax and a depth beyond the range of electron contamination, respectively. A Blue Phantom (IBA) was used to position the diode 80 cm SAD. Measurements were made for all 12 fixed cones ranging from 5 to 60 mm. RESULTS For the 5 mm cone, there is a 15.6% and 20.0% difference in the Sc factor between the 1.5 cm and 5 cm buildup caps, respectively, versus the free-in-air measurement. For the 7.5 mm cone, the difference is 6.3% and 10.4% for the 1.5 cm and 5 cm buildup caps, respectively, versus the free-in-air measurement. While generally decreasing, the Sc factor for either buildup cap does not agree within 2% to free-in-air measurements until the cone used is larger than 40 mm. Overall, the two buildup caps yield similar Sc factors and the differences observed are attributed to electron contamination. CONCLUSIONS The Monte Carlo calculation algorithm used by the CyberKnife planning manual does not refer to any build-up caps. This study has shown that, particularly for small fields, there are large differences in Sc factors measured with and without buildup caps. In general, for fields smaller than 10 mm, build-up caps should be carefully chosen, especially in commissioning data.