Program Information
Silicon Diode Measurements of Output Field Size Factors for Proton Pencil Beam Scanning with a Dedicated Nozzle
J McGlade*, L Lin , T Solberg , University of Pennsylvania, Philadelphia, PA
Presentations
SU-E-T-695 (Sunday, July 12, 2015) 3:00 PM - 6:00 PM Room: Exhibit Hall
Purpose: Proton irradiation degrades solid state detectors, producing high recombination and leakage currents and ultimately rendering them unusable for dosimetry. In this work we evaluate the performance of a new p-type silicon diode under proton beam irradiation.
Methods:
A novel p-type silicon diode with improved radiation hardness has recently been developed (IBA Dosimetry, Schwarzenbruck, Germany). The dosimetric properties of the devices were assessed for application in pencil beam scanning (PBS) proton irradiation. Using PBS fields with 4 mm spaced, uniform intensity spots, dose and dose rate linearity were evaluated over a clinical range. Output factors were measured at energies of 100, 180, and 225 MeV and for field sizes ranging from 4x4 – 20x20 cm2. Measurements were performed at the center of the square fields at varying depths upstream from the Bragg peak and compared with previously published results.
Results:
At shallow depths, the diode exhibits linear behavior (0.1%) over the clinical range of doses and dose rates; the sensitivity of the diode in this region is ~6.0 nC/Gy. As the depth of measurement approaches within 2 cm of each Bragg peak, the deviation from linearity becomes greater (±2-6%). The diode shows less response with small fields and - over responds with large fields.
Conclusion
At depths more than 2 cm upstream from the Bragg Peak, the diode shows very good agreement with ion chamber and calculated results for output factors. This demonstrates that diodes can be used for relative measurements at shallow depths, but should be used with caution at deeper depths and for higher energy. Measurements at all depths exhibit an energy dependence that is characteristic of silicon diodes and requires correction.
Funding Support, Disclosures, and Conflict of Interest: Detectors provided by IBA Dosimetry GmbH, Schwarzenbruck, Germany
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