Evaluation of the PTW microDiamond in edge-on orientation for dosimetry in small fields.

PURPOSE: The PTW microDiamond has an enhanced spatial resolution when operated in an edge-on orientation but is not typically utilized in this orientation due to the specifications of the IAEA TRS-483 code of practice for small field dosimetry. In this work the suitability of an edge-on orientation and advantages over the recommended face-on orientation will be presented. METHODS: The PTW microDiamond in both orientations was compared on a Varian TrueBeam linac for: machine output factor (OF), percentage depth dose (PDD), and beam profile measurements from 10 × 10 cm2 to a 0.5 × 0.5 cm2 field size for 6X and 6FFF beam energies in a water tank. A quantification of the stem effect was performed in edge-on orientation along with tissue to phantom ratio (TPR) measurements. An extensive angular dependence study for the two orientations was also undertaken within two custom PMMA plastic cylindrical phantoms. RESULTS: The OF of the PTW microDiamond in both orientations agrees within 1% down to the 2 × 2 cm2 field size. The edge-on orientation overresponds in the build-up region but provides improved penumbra and has a maximum observed stem effect of 1%. In the edge-on orientation there is an angular independent response with a maximum of 2% variation down to a 2 × 2 cm2 field. The PTW microDiamond in edge-on orientation for TPR measurements agreed to the CC01 ionization chamber within 1% for all field sizes. CONCLUSIONS: The microDiamond was shown to be suitable for small field dosimetry when operated in edge-on orientation. When edge-on, a significantly reduced angular dependence is observed with no significant stem effect, making it a more versatile QA instrument for rotational delivery techniques.

Characterization of a plastic dosimeter based on organic semiconductor photodiodes and scintillator.

BACKGROUND AND PURPOSE: Measurement of dose delivery is essential to guarantee the safety of patients undergoing medical radiation imaging or treatment procedures. This study aimed to evaluate the ability of organic semiconductors, coupled with a plastic scintillator, to measure photon dose in clinically relevant conditions, and establish its radiation hardness. Thereby, proving organic devices are capable of being a water-equivalent, mechanically flexible, real-time dosimeter. MATERIALS AND METHODS: The shelf-life of an organic photodiode was analyzed to 40 kGy by comparison of the charge-collection-efficiency of a 520 nm light emitting diode. A non-irradiated and pre-irradiated photodiode was coupled to a plastic scintillator and their response to 6 MV photons was investigated. The dose linearity, dose-per-pulse dependence and energy dependence was characterized. Finally, the percentage depth dose (PDD) between 0.5 and 20 cm was compared with ionization chamber measurements. RESULTS: Sensitivity to 6 MV photons was (190 ± 0.28) pC/cGy and (170 ± 0.11) pC/cGy for the non-irradiated and pre-irradiated photodiode biased at -2 V. The response was independent of the dose-per-pulse between 0.031 and 0.34 mGy/pulse. An energy dependence was found for low keV energies, explained by the energy dependence of the scintillator which plateaued between 70 keV and 1.2 MeV. The PDD was within ±3% of the ionization chamber. CONCLUSION: Coupling an organic photodiode with a plastic scintillator provided reliable measurement of a range of photon energies. Dose-per-pulse and energy independence advocate their use as a dosimeter, specifically image-guided treatment without beam-quality correction factors. Degradation effects of organic semiconducting materials deteriorate sensor response but can be stabilized.