Author: Angela J Da Silva, Andrew Groll, Gopinath Kuduvalli, Thomas Laurence, David W. Meer, Manoj Narayanan, Mingshan Sun, Don Vernekohl, Ray Yang 👨🔬
Affiliation: RefleXion Medical 🌍
Purpose: To implement post-treatment review of delivered dose fractions on RefleXion X1 accounting for independent detection of transmitted beamlets and transient dose fluctuations of individual pulses.
Methods: During delivery of non-deterministic PET-guided radiotherapy (SCINTIX) on the RefleXion X1, beam images were acquired for each firing position using a megavoltage detector (MVD) in research-mode. Simultaneously, machine records for Dose Monitor Chamber (DMC) readings, gantry angle, couch position, and setup coordinates were algorithmically discretized to offline-reconstruct a unique delivery sinogram. Forward dose calculation (Collapsed-Cone-Convolution-Superposition) by deploying the reconstructed sinogram onto planning CT produced the 3D distribution of delivered dose. Compared to the product implementation of post-delivery dose review, we rely on MVD images rather than MLC sensor readings, and DMC recorded dose rate instead of nominal dose per pulse. The two methods of dose calculation are compared at the completion of treatment.
Results: Considering a phantom case study of SCINTIX delivery to a 22mm ball-target undergoing 10mm step-shift, 64,762 MVD images were acquired, dynamically thresholded to contain 16,577 frames of non-zero beamlet transmission, synchronized to 99,462 linac firings (spanning 0.0532-0.0565cGy per pulse). Overlaying the time-averaged treatment PET showed the system tracked the step-shift such that delivered dose distribution remained centered on the PET activity. Preliminary results demonstrated near equivalence between the inherent capability of RefleXion X1 post-delivery calculation using MLC sensors and nominal dose-per-pulse, compared the MVD-based parameterization and DMC measured transient dose described here.
Conclusion: For non-deterministic SCINTIX deliveries comprising 105-106 beamlets, accurate post-treatment dose reconstruction is possible by MVD detection of transmitted beamlets and monitor chamber measurement of dose per pulse. Further, when overlaid with time-averaged PET signal this establishes confidence that the delivered fraction dose intersects with the target.