Fast Beam Scanning and Accurate Output Factor Measurements for Small-Field Dosimetry Using a Novel Scintillation Detector
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Background
The most used instruments for small-field dosimetry have notable limitations, including the need for correction factors, limited scanning speeds, and challenges in alignment for percentage depth dose (PDD) measurements, particularly for extremely small-fields. However, plastic scintillation detectors (PSDs) are an attractive alternative for small-field dosimetry due to their correction-free nature, linear dose response, and fast response time.
Purpose
This study evaluates the robustness and accuracy of the dosimetric measurements using a new water-equivalent PSD in small-field dosimetry. The study also aims to report an indirect method for measuring PDD in small-fields, with a scanning time that is 5 to 10 times faster than traditional methods.
Method
PDDs, profiles and output factors were measured on a Varian TrueBeam 6XFFF photon beam for the field size of 0.5×0.5 cm 2 , 1×1 cm 2 , 2×2 cm 2 , 3×3 cm 2 , 4×4 cm 2 using a new PSD from Blue Physics (BP-PSD). These measurements were compared with those obtained using a well-established PSD (Standard Imaging W2), micro-diamond (TN60019, PTW-Freiburg, Germany), and micro-silicon detectors (TN60023, PTW-Freiburg, Germany). Owing to its fast response, the BP-PSD enabled the collection of beam profiles at 31 depths, which were used to derive the PDD while avoiding detector misalignment along the beam path. Data was collected in a water tank controlled by the PTW BeamScan software. The pulse-by-pulse raw data from BP-PSD were converted to respective dosimetry data using in-house software.
Result
The BP-PSD demonstrated excellent agreement with other detectors for small-field output factors (FOFs), with a maximum variation of 1.6%. The BP-PSD also showed strong agreements in PDD measurements with an ion chamber (TN31013) for both 3×3 cm 2 and 10×10 cm 2 field sizes, achieving a 98% gamma passing rate (gamma criteria: 1mm,3%). For the profile measurements, the BP-PSD showed consistency with both the micro-diamond and micro-silicon diode detectors, with less than 1% variation in measured penumbra length. At a 3×3 cm 2 field size, the measured penumbra length (4 mm) agreed with previously published data (3.86–4.2 mm). Additionally, for field size less than 3×3 cm 2 the indirect PDD measurements derived from profiles showed significant improvement compared to the direct measurements using various detectors, using TPS-calculated PDD as a reference.
Conclusion
The BP-PSD has proven to be a robust and reliable detector for small-field dosimetry. It exhibits excellent agreement with other detectors in measuring small FOFs and provides accurate measurements with significantly faster scanning speeds in a water tank. The fast response feature enables the indirect PDD measurement method
