Presentations at AAPM 2021 Further Validate the RefleXion X1 Machine

RefleXion participated in the American Association of Physicists in Medicine (AAPM) 2021 Annual Meeting. In collaboration with its clinical and academic partners, RefleXion presented 13 abstracts focused on the validation and performance of the X1 radiotherapy system. This post includes several of the highlights.

The RefleXion™ X1 machine is a novel ring-gantry-based radiotherapy machine that uses rapid rotation and a new pneumatic spring-based binary multi-leaf collimator that transitions at 100 Hz. The X1 machine may provide excellent dosimetric conformality, which is especially pertinent in very high-dose regimens like single-dose radiotherapy (SDRT). SDRT may reduce intrafraction error, treatment time, cost and patient inconvenience. One of RefleXion’s AAPM presentations focused on assessing the SDRT performance of the RefleXion X1 in multiple target scenarios using ArcCHECK (AC) analysis. Using a research version of the RefleXion treatment planning system (TPS), investigators carried out treatment plans for three multi-target lung cancer cases. All three patient plans met dosimetric coverage and organ-at-risk avoidance goals. With the treatment machine set at 850MU/Min, the average delivery duration was 46.19 mins (range: 37.47–59.98 mins). RefleXion plans for SDRT in the multiple target setting met dosimetric and physics quality criteria. These results demonstrate treatment planning robustness and treatment delivery feasibility for relevant clinical scenarios such as SDRT or oligometastatic disease— defined as a state of metastatic disease that is limited in total disease burden, usually by number of clinically evident or radiographic sites (either 1–3 or 1–5), that is not rapidly spreading to more sites.

These results demonstrate treatment planning robustness and treatment delivery feasibility for relevant clinical scenarios such as SDRT or oligometastatic disease.

Another presentation reported the preliminary TPS commissioning results for the first clinical installation of the RefleXion X1 machine at Stanford University. The TPS generates intensity-modulated radiation therapy (IMRT)/stereotactic body radiation therapy (SBRT) plans using the gradient-descent-optimization and Collapsed-Cone-Convolution algorithm (2.1mm grid) for a 6MV-FFF beam that is delivered axially via 50 firing positions with the couch advancing every 2.1mm. The test results agreed within the tolerances specified by TG-53, MPPG 5.a, TG-119, TG244, and TG-148. A subset of the commissioning tests has been identified as a baseline data for an ongoing quality assurance (QA) program.

Presenters shared the results of a lung phantom film dosimetry study performed under multiple motion conditions using the first clinical installation of the RefleXion X1 machine (also at Stanford University). The study aimed to determine the dosimetric effects of motion during RefleXion SBRT treatment delivery. The interplay of RefleXion SBRT delivery and target motion was measured for several clinically relevant lung cases. Clinically acceptable ITV coverage was obtained for smaller amplitudes of motion with 2cm-jaws, whereas significant under-dosing was observed for larger motion amplitudes with 1cm-jaws.

A separate presentation showcased the results from testing the QA and commissioning results of the first clinical installation of the RefleXion X1 machine adapted from the TG-148 protocol, a set of standards developed by the AAPM to provide QA guidelines for treatment delivery, imaging and treatment planning. The mechanical tests were performed to verify the centering, alignment and divergence of the MLC and y-jaw. The beam parameters were commissioned using a 3D water phantom and a diode detector. Beam quality, transverse and longitudinal profiles of each slice width were measured and compared with the beam model. The localization accuracies of the laser and kVCT to MV beam isocenter were checked using a ball cube phantom. This evaluation, which demonstrated that the X1 machine met compliance and performance standards, represents the first QA and commissioning evaluation of the clinical (BgRT*) system as a future reference.

This evaluation, which demonstrated that the X1 machine met compliance and performance standards, represents the first QA and commissioning evaluation of the clinical (BgRT*) system as a future reference.

Presenters reported on a study that compared treatment plan quality for IMRT cases between the X1 planning system without PET-guidance and a commercial volumetric modulated arc therapy (VMAT) planning system. The study included 19 patients previously treated with VMAT on C-arm linacs for this retrospective study, including 8 prostate cancer patients (80Gy/40fx), 6 lung cancer patients (66Gy/30fx) and 5 post-surgical parotid cancer patients (60Gy–66Gy/30fx). For each VMAT plan, a corresponding plan was generated on the RefleXion TPS using the researchers’ institutional planning constraints. All clinically relevant metrics in this study, including plan PTV D95%, PTV D1%, Conformity Index (CI), and organs at risk constraints were analyzed and compared between the VMAT and RefleXion plans using paired t-tests. They found that clinically acceptable plans were obtained with both techniques. For the prostate, lung, and head and neck sites, no statistically significant difference was observed in PTV D95%, PTV D1%, or CI between the VMAT and RefleXion treatment plans. The RefleXion TPS provided comparable plan quality to VMAT plans.

The RefleXion X1 machine has a dose monitor chamber (DMC) for measuring beam output from the linac. The DMC is located between a fixed primary collimator and a movable secondary multi-leaf collimator. Backscattered radiation from the secondary collimator may contribute to the total charges collected in the DMC and may result in an artifact in the measurement. One presentation focused on an investigation using Monte Carlo (MC) simulation to characterize the impact of backscattered radiation to the DMC measurements. A maximum backscatter contribution of 0.12%±0.1% was obtained for a simulated field of 10 cm (16 MLC leaves) with a 2 cm jaw opening. For a simulated field of 1.25 cm (2 MLC leaves), a maximum backscattering contribution of 0.27%±0.1% was recorded. On average, backscattering contribution decreased with an increase in the field size. Minimal variation in backscattering was observed for different jaw openings. The investigators found that the effect of backscattered radiation into the DMC using Monte Carlo simulation for the X1 linac photon beam is minimal, even with very small field sizes and is not correlated to jaw size. Accordingly, correcting for field-size dependence of DMC output to account for backscatter is not necessary for the RefleXion X1 design.

Another presentation demonstrated the results of a study that evaluated the focal spot size influence on treatment beam penumbra using a MC technique. The study revealed that the variation in X-ray beam spot size did not have a significant effect on the percent depth dose (PDD), with an average difference < 0.34% for PDD at depth of 100 mm. Penumbra width increases as X-ray beam spot size increases (R=1, P<0.01), while the field width is not affected by variation of X-ray beam spot size. The study authors concluded the beam spot size has a sizeable effect on penumbra width. This study shows that a small X-ray beam spot is preferred for small treatment fields due to the resulting narrow penumbra width.

Another presentation characterized the performance of the PET subsystem of the first RefleXion X1 machine installation using the NEMA NU-2 2018 standard. The study found that when the X1 machine was operating in imaging mode, spatial resolution and image contrast were equivalent to those of typical diagnostic systems, while sensitivity and count rate were lower. The design of the X1 system has a far smaller detector area than a typical diagnostic system. The clinical efficacy when used in BgRT mode has not yet been validated.

Lastly, RefleXion presented on the quantification of peripheral dose in the new ring-gantry RefleXion X1 machine, demonstrating that the X1 machine’s peripheral dose is low, even in scenarios where the scattering effect is pronounced.

These abstracts provide further validation of RefleXion’s X1 machine and demonstrate RefleXion’s commitment to achieving and surpassing industry standards for the delivery of quality radiotherapy treatments. They also show the future potential of BgRT.

These abstracts provide further validation of RefleXion’s X1 machine and demonstrate RefleXion’s commitment to achieving and surpassing industry standards for the delivery of quality radiotherapy treatments.

*The RefleXion X1 is cleared for SBRT/SRS/IMRT treatments. BgRT is limited by U.S. law to investigational use.

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