Room: AAPM ePoster Library
Purpose: validate the relative collimator output factors (ROFs) of Leksell Gamma Knife® Icon™ (GKI) using diode detectors following the IAEA TRS-483 protocol.
Methods: diode detectors, PTW-TN60008, PTW-TN60016, PTW-TN60017 and PTW-60019 detector, were used to measure the ROFs of 4-mm and 8-mm collimators of the GKI unit at our institution, with PTW-UNIDOS E electrometer and LGK Solid Water Dosimetry Phantom. The effective point of measurement (EPM) of each detector, when inserted into the phantom, was determined using the CBCT method reported elsewhere. A GKI plan was generated to position the EPM at the unit center point of GKI for ROF measurements. The exposure time was one minute for each irradiation, averaged over three readings. According to TRS-483 report, ROFs is the ratio of the averaged readings for the 4-mm/8-mm collimator to that for the 16-mm collimator, multiplied by a field output correction factor for each detector to account for the small field effects.
Results: percentage variations of ROFs for 4-mm and 8-mm collimators were respectively 0.28% and 0.93% for PTW-TN60008, 0.81% and 0.19% for PTW-TN60016, 3.29% and 0.91% for PTW-TN60017 and, 0.52% and 0.87% for PTW-60019. In general, differences in measured ROFs were within 1% of the recommended values, except for TN60017 which has underestimated the ROF by 3.3% for the 4-mm collimator. Repeated CBCT and measurements for this particular diode showed that the deviation in ROF was reproducible and was not due to experimental errors.
Conclusion: IAEA TRS-483 protocol was successfully used to validate the ROFs for a GKI unit using four diode detectors. Good agreements were obtained between the measured OFs and manufacturer recommended values. The only exception was the measurement of 4-mm collimator OF using TN60017, possibly due to unknown discrepancy of the output correction factor, which needed further investigation.
Funding Support, Disclosures, and Conflict of Interest: We are grateful for the financial support from the Union for International Cancer Control (UICC), 31-33 Avenue Giuseppe Motta 1202 Geneva - Switzerland.