ESTRO 2025 - Abstract Book

S2631

Physics - Detectors, dose measurement and phantoms

ESTRO 2025

Purpose/Objective: With the advancement of highly conformal techniques such as stereotactic radiosurgery and intensity-modulated radiation therapy, accurate dosimetry of small radiation fields has become increasingly critical. Scintillation detectors have proven to be a promising tool to characterise these fields. This study focuses on the determination of output factors(OF) for field sizes smaller than 4×4cm². Material/Methods: Two TrueBeam accelerators were used for comparison: MLC-HD120-STx(TrueBeam1) and the other with MLC120(TrueBeam2). Output factors(OF) for fields-defined by jaws were measured for 6FFF and 6WFF, at a depth of 10cm and using the intermediate field method with Exradin W2-scintillator(Standard-Imaging) 1x1mm(0.0008cm³) and 1x3mm(0.0024cm³) in a Beam-Scan(PTW). Cherenkov factors and gain, were obtained with a reference field 10x10cm²[1] Two sets of measurements in truebeam2 were included for fields smaller than 1×1cm². Those were taken by starting from the most open configuration(40×40cm²) and then adjusting to the target field size: positive configuration. The second set measurements began with the jaws set to 0.5×0.5cm² and were then adjusted to the selected field size: negative configuration. The results were compared to OF values obtained using the E-60017 diode(PTW). Results: For field sizes smaller than 2x2cm², it was necessary to measure in each of the configurations given potential hysteresis in the positioning of the jaws modifies the measurements by up to 2%(Table1). The OF measured using scintillators and diodes for the 6WFF beam and 6FFF beam demonstrate differences of <2% for field sizes ≥0.8×0.8cm², except for the 4x4 field where discrepancies of 4% and 6% were observed for the 1x1 and 1x3 scintillator, respectively. Specifically for 0.6×0.6 cm², these differences increase to 7% for Scintillator 1x3. The maximum deviation in OF between the two accelerators was found to be 5.5% for the diode and 7.4% for the scintillator, specifically for the 0.6x0.6cm² field at 6FFF(Table1). For both 6WFF and 6FFF, as the field size increases the deviations decrease, ranging from 4% to 0.2% for the diode and from 1.6% to 0.12% for the scintillator. The measurements on the scintillators at 6 MV energy agree well with the MonteCarlo results for field sizes equal to or larger than 1x1cm² <1%[2], although the measurements of the 0.8x0.8cm² field differ by 6%.