Room: Exhibit Hall | Forum 3
Purpose: 1. Accurate measurement of output factors of small SRS cones using two independent detectors with corrections from an IAEA report #483. 2. Measurement of PDD for small SRS cones using a diode in a scanning water tank.
Methods: For cone output measurements, we used a PTW microdiamond detector and Sun Nuclear â€œEdgeâ€? diode. New correction factors were from the AAPM-IAEA joint working group. A scanning IBA water tank Blue Phantom allowed optimum positioning of the microdiamond diode detector to maximize the dose reading in a 1D water tank Norgren from IBA, using the light field guidance and small table adjustments. PDDs were measured using Edge diode in a scanning water tank. For small cone sizes, it is imperative that the scanning direction is parallel to the cone axis. To verify that, we performed four additional parallel scans, surrounding the main one with the distance of 1 mm from the beam axis. We used small distances between the scans to prevent the detector at depth to unnoticeably deviate from the beam axis due to misalignment. If all of the peripheral scans measured smaller PDD than the main scan, the latter was considered validated.
Results: Use of the new correction factors allowed to improve the agreement between the output factors measured by the two detectors from 1%-6% to 0.4%-1.2% (depending on the cone size) and changed the output factor measured by the diode by 1%-5%. All the peripheral PDD curves were lying below the main curve thus validating the measurement.
Conclusion: The practical methods described can be used for commissioning an SRS system with small cones. New correction factors significantly improve agreement between different detectors.