Purpose: Measure cone factors (CF) and commission a set of Stereotactic Radiosurgery (SRS) Cones for a Varian Edge machine with field sizes ranging from 17.5-4 mm diameter.
Methods: Cone Factors were measured using three detectors: GafChromic EBT3 Dose+ film, a Standard Imaging D1V diode, and an Exradin A16 microionization chamber. Measurements were repeated twice and averaged. Absolute dose output was determined by ‘daisy-chaining’ outputs in a 5x5cm2 field using a calibrated Exradin A1SL ionization chamber, and verified with absolute film dosimetry. CFs were measured at 100 SAD, 5cm depth. Ionization chamber measurements were completed using both polarities set to 300 V. EBT3 film was calibrated in 30 cGy intervals from 0-240 cGy and scanned on an Epson 11000XL flatbed scanner. Calibration curves were determined using a third-order polynomial fit to red-channel film measurements. The 17.5mm diameter cone was chosen as the machine-specific reference field. No small-field correction factors were applied. CFs were entered into BrainLab iPlan Treatment Planning System (v4.5.1), and compared against Varian Representative Beam Data (VRBD). End-to-end testing was performed using film embedded in a cranial phantom.
Results: Measured CFs were consistently below the VRBD, but within 5% for all field sizes. Film and diode measured CFs were within 2% for all field sizes. Microionization chamber measurements were far below film and diode measurements and were not included in calculating the final CF. End-to-end test results demonstrated <3% differences between predicted maximum dose.
Conclusion: Our measured CFs were consistently lower than the VRBD. This was expected as the VRBD was acquired with a SNC Edge shielded diode. Microchamber measurements were not sufficient to accurately quantify CFs for field sizes < 10mm diameter. Film and unshielded diode readings demonstrated agreement to within 2% for all field sizes. End-to-end testing demonstrated acceptable agreement between calculated and measured plans.
Commissioning, Small Fields, Stereotactic Radiosurgery