Fast and accurate measurement of small field dosimetry using a novel scintillation detector

The most used instruments for small-field dosimetry have notable limitations, including the need for correction of output 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.

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 accurate measurements of output factors, profiles, and an indirect method for measuring PDD in small fields, with a scanning time that is 5 to 10 times faster than traditional methods.

PDDs, profiles, and output factors were measured on a Varian TrueBeam 6XFFF photon beam for the field sizes of 0.5 × 0.5 cm², 1 × 1 cm², 2 × 2 cm², 3 × 3 cm², and 4 × 4 cm² 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.

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 agreement in PDD measurements with an ion chamber (TN31013) for both 3 × 3 cm² and 10 × 10 cm² field sizes, achieving a 98% gamma passing rate (gamma criteria: 1 mm, 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 cm² field size, the measured penumbra length (4 mm) agreed with previously published data (3.86-4.2 mm). Additionally, for an extremely small field size (0.5 × 0.5 cm²), the indirect PDD measurements derived from profiles showed significant improvement compared to the direct measurement without beam inclination correction.

The BP-PSD has proven to be a robust and reliable detector for small-field dosimetry. It shows excellent agreement with other detectors in measuring small FOFs, profiles, and PDDs, while achieving significantly faster scanning speeds in a water tank.