Small‐field measurement and Monte Carlo model validation of a novel image‐guided radiotherapy system

Abstract

The RefleXion™ X1 is a novel radiotherapy system that is designed for image-guided radiotherapy, and eventually, biology-guided radiotherapy (BgRT). BgRT is a treatment paradigm that tracks tumor motion using real-time positron emission signals. This study reports the small-field measurement results and the validation of a Monte Carlo (MC) model of the first clinical RefleXion unit. Methods: The RefleXion linear accelerator (linac) produces a 6 MV flattening filter free (FFF) photon beam and consists of a binary multileaf collimator (MLC) system with 64 leaves and two pairs of y-jaws. The maximum clinical field size achievable is 400 × 20 mm 2 . The y-jaws provide either a 10 or 20 mm opening at source-to-axis distance (SAD) of 850 mm. The width of each MLC leaf at SAD is 6.25 mm. Percentage depth doses (PDDs) and relative beam profiles were acquired using an Edge diode detector in a water tank for field sizes from 12.5 × 10 to 100 × 20 mm 2 . Beam profiles were also measured using films. Output factors of fields ranging from 6.25 × 10 to 100 × 20 mm 2 were measured using W2 scintillator detector, Edge detector, and films. Output correction factors k of the Edge detector for RefleXion were calculated. An MC model of the linac including pre-MLC beam sources and detailed structures of MLC and lower yjaws was validated against the measurements. Simulation codes BEAMnrc and GATE were utilized. Results: The diode measured PDD at 10 cm depth (PDD10) increases from 53.6% to 56.9% as the field opens from 12.5 × 10 to 100 × 20 mm 2 . The W2-measured output factor increases from 0.706 to 1 as the field opens from 6.25 × 10 to 100 × 20 mm 2 (reference field size). The output factors acquired by diode and film differ from the W2 results by 1.65% (std = 1.49%) and 2.09% (std = 1.41%) on average, respectively. The profile penumbra and full-width halfmaximum (FWHM) measured by diode agree well with the film results with a deviation of 0.60 mm and 0.73% on average, respectively. The averaged beam profile consistency calculated between the diode-and film-measured profiles among different depths is within 1.72%. By taking the W2 measurements as the ground truth, the output correction factors k for Edge detector ranging from 0.958 to 1 were reported. For the MC model validation, the simulated PDD10 agreed within 0.6% to the diode measurement. The MC-simulated output factor differed from the W2 results by 2.3% on average (std = 3.7%), while the MC simulated beam penumbra differed from the diode results by 0.67 mm on average (std = 0.42 mm). The MC FWHM agreed with the diode results to within 1.40% on average. The averaged beam profile consistency calculated between the diode and MC profiles among different depths is less than 1.29%.

First-Year Experience of Stereotactic Body Radiation Therapy/Intensity Modulated Radiation Therapy Treatment Using a Novel Biology-Guided Radiation Therapy Machine

“…Shi et al have measured the small-field dosimetry results and validated a Monte Carlo model of the X1 system. 7 Output correction factors for Edge detector ranging from 0.958 to 1 were reported. Pham et al have evaluated the treatment plan quality and delivery efficiency of the X1 system in comparison to 2 traditional linacs (TrueBeam and Trilogy) for 42 patient cases across 6 different cancer sites.…”

Treatment planning system commissioning of the first clinical biology‐guided radiotherapy machine

“…RefleXion X1 machine commissioning, Monte Carlo beam validation, and PET sub‐system commissioning are described in separate manuscripts. 8 , 9 , 10 The scope of the TPS commissioning tests was designed based on our clinical environment and the clinical implementation of the X1 TPS. As the current TPS version does not offer fusion and contouring capability, its integration with Eclipse™ (v15.6, Varian Medical Systems, Palo Alto, CA, USA) and MIM™ (v7.1.2, MIM Software Inc, Cleveland, OH, USA) was evaluated.…”

“…Prior to commissioning, acceptance tests were performed ensuring agreement within the vendor‐specified tolerances of the tested parameters. RefleXion X1 machine commissioning, Monte Carlo beam validation, and PET sub‐system commissioning are described in separate manuscripts 8–10 . The scope of the TPS commissioning tests was designed based on our clinical environment and the clinical implementation of the X1 TPS.…”

Commissioning of a novel PET‐Linac for biology‐guided radiotherapy (BgRT)

“…The recent upgrades increased the dose rate to 1000 MU/min. The beam commissioning of the RefleXion X1 for intensity-modulated radiotherapy (IMRT)/stereotactic body radiotherapy (SBRT) was evaluated by Han et al 8 and the small field measurements and Monte Carlo simulations were performed by Shi et al 9 The treatment planning system was evaluated previously for IMRT and SBRT 10,11 and for BgRT. 12 The PET system characterization, calibration and quality assurance of RefleXion X1 was previously reported by Hu et al 13 where NEMA NU2-2018 standards were used to assess the PET image sensitivity, spatial resolution, scatter, count normalization, and image quality, in addition to the results of the daily PET quality assurance results.…”

“…The beam commissioning of the RefleXion X1 for intensity‐modulated radiotherapy (IMRT)/stereotactic body radiotherapy (SBRT) was evaluated by Han et al 8 . and the small field measurements and Monte Carlo simulations were performed by Shi et al 9 . The treatment planning system was evaluated previously for IMRT and SBRT 10,11 and for BgRT 12 .…”