Purpose: A biology-guided radiotherapy (BgRT) system - RefleXionᵀᴹ X1 was developed and equipped with PET imaging. This study reports the small field measurement results of the first clinical BgRT machine at Stanford.
Methods: The X1 linac produces 6MV flattening filter free beam and consists of a binary multi-leaf collimator (MLC) system with sixty-four 6.25mm-thick leaves and y-jaws switching between 1cm and 2 cm. All clinical beams are delivered with the narrow longitudinal beam geometry which resulted in a lack of charged particle equilibrium. Therefore, it is critical to ensure the accuracy of small field dosimetry for the system. We performed profile scans and relative output factor measurements of field sizes from 1.25x1cm² to 40x2cm² using a diode detector in a 3D water tank during the commissioning process. Subsequently, we repeated the tests using diode, film, and tissue-equivalent W2 scintillator in plastic water focusing on small fields down to 0.625x1cm².
Results: For the fields with 2cm Jaw, the mean differences of relative output factors measured using diode and scintillator were 0.57% and a maximum of 2.79% for the 0.625x2cm² field. The film results agreed better with the scintillator, with mean and max differences of 0.2% and 0.7%. For the fields with 1cm Jaw, the mean differences of relative output factors measured using diode and scintillator were 1.35% and a maximum of 3.15% for the 0.625x1cm² field. The film results maintained good agreements to the scintillator with mean and max differences of 0.44% and 0.99%. The FWHM differences calculated from the longitudinal and transverse profiles measured with diode and film were within 0.5mm across the depth from 1.5cm to 20cm.
Conclusion: This evaluation represents the first small field dosimetry of the clinical BgRT system serving as a future reference.