Room: AAPM ePoster Library
Purpose: To assess the impact of scintillator size when coupled to an organic photodiode for dosimetric measurements such as output factors and PDDs.
Methods: An organic bulk heterojunction photodiode was fabricated on an ITO-coated glass substrate by spin coating a blend of P3HT and PCBM, and depositing aluminum top contacts. Eljen Scintillators (~5x5x5 mm³; EJ-204, EJ-208, and EJ-260) or Saint-Gobain scintillators (~3x3x2 mm³; BC-400 and BC-412) were placed on the opposite side of the glass. Output factors of a Varian TrueBeam accelerator were measured using a 6 MV photon beam, 5 cm depth in solid water, an SSD of 95 cm and varying the field size from 1x1 cm² to 25x25 cm². The non-scintillation contribution (i.e. Compton current induced in the wires) to the signal was measured with the same setup, but no scintillator was placed on the photodiode. The output factor measurements were compared with an Exradin A12 ion chamber for field sizes of 4x4 cm² to 25x25 cm² and with a semiflex 31010 micro ion chamber for smaller field sizes. PDDs were measured by placing solid water on top of the detectors with an SSD of 100 cm.
Results: The larger scintillators (Eljen) had less than 1% deviation from the ion chamber for output measurements for field sizes of 3x3 cm² to 25x25 cm², but exhibited an under-response for smaller field sizes. The smaller scintillators (Saint-Gobain) output factors deviate from the ion chamber measurements. However, after subtracting the non-scintillation signal the agreement with the ion chamber greatly improved. The corrected signal was within 1% of the ion chamber from field sizes 1x1 cm² to 20x20 cm². All the devices matched the PDDs measured with an ion chamber within 2%.
Conclusion: The smaller sized scintillators exhibited better output factors for small field sizes after correcting for the non-scintillation signal.
Funding Support, Disclosures, and Conflict of Interest: Project funded by NSERC Discovery Grant (Award # RGPIN/06113 - 2014) and NSERC postgraduate scholarship-doctoral (PGS-D)