Absolute dose measurements for lung gated delivery stereotactic body radiation therapy

Purpose: To evaluate absolute dose between phase- and amplitude-gated modes for lung stereotactic body radiation therapy (SBRT), using the moving targets developed in-house. Methods: The moving targets were developed in-house, allowing measurements of absolute dose using Semiflex ion chamber, Pinpoint ion chamber and microDiamond detector. 24 plans were generated in the Pinnacle 16.2 treatment planning system (TPS). 720 partial dynamic conformal arcs (DCA), 120 for phase- and 120 for amplitude-gated mode per detector, were performed to compare dose discrepancy (planned vs measured doses), for all detectors. Single fraction of 15 Gy was delivered with TrueBeam Novalis STx, triggered on the stable phases (phase-gated mode) or near the end-exhalation (amplitude-gated mode), for a breathing period ranging from 2 s to 7 s, using flattened filter free beams (FFF). Results: The lowest discrepancy between measured and planned dose was obtained using the microDiamond detector in the phase-gated -0.1 (-0.2 to 0) % and -0.3 (-0.4 to -0.1) % in amplitude-gated mode (p < 0.001). Using PinPoint ion chamber, greater discrepancy was obtained in phase-gated -0.5 (-0.7 to -0.3) %, comparing to the amplitude-gated mode -0.1 (-0.2 to 0) % (p < 0.001). The greatest discrepancy was observed using the Semiflex ion chamber, with lesser differences between the phase-gated 1.5 (1.2-1.7) % and amplitude-gated mode 1.4 (1.2-1.5) (p = 0.134). Conclusion: Detectors with low discrepancy, PinPoint ion chamber and microDiamond detectors, can be used in routine quality assurance for lung gated delivery in SBRT treatment, for commercial phantom as for phantom developed in-house.

Automated summary

The study evaluated absolute dose in lung SBRT between phase- and amplitude-gated modes using in-house developed moving targets. Results showed that the microDiamond detector had the lowest dose discrepancy in phase-gated mode, while the Pinpoint ion chamber had lower discrepancy in amplitude-gated mode, and the Semiflex ion chamber showed the greatest discrepancy.