Comparison of Phase-Gated and Amplitude-Gated Dose Delivery to a Moving Target using Gafchromic EBT3 Film.

Introduction: This study compared phase-gated and amplitude-gated dose deliveries to the moving gross tumor volume (GTV) in lung stereotactic body radiation therapy (SBRT) using Gafchromic External Beam Therapy (EBT3) dosimetry film.

Materials And Methods: Eighty treatment plans using two techniques (40 phase gated and 40 amplitude gated) were delivered using dynamic conformal arc therapy (DCAT). The GTV motion, breathing amplitude, and period were taken from 40 lung SBRT patients who performed regular breathing. These parameters were re-simulated using a modified Varian breathing mini phantom. The dosimetric accuracy of the phase- and amplitude-gated treatment plans was analyzed using Gafchromic EBT3 dosimetry film. The treatment delivery efficacy was analyzed for gantry rotation, number of monitor unit (MU), and target position per triggering window. The time required to deliver the phase- and amplitude-gated treatment techniques was also evaluated.

Results: The mean dose (range) per fraction was 16.11 ± 0.91 Gy (13.04-17.50 Gy) versus 16.26 ± 0.83 Gy (13.82-17.99 Gy) ( < 0.0001) for phase- and amplitude-gated delivery. The greater difference in the gamma passing rate was 1.2% ±0.4% in the amplitude-gated compared to the phase gated. The gantry rotation per triggering time (tt) was 2° ±1° (1.2°-3°) versus 5° ±1° (3°-6°) ( < 0.0001) and MU per tt was 10 ± 3 MU (6-13 MU) versus 24 ± 7 MU (12-32 MU) ( < 0.0001), for phase- versus amplitude-gated techniques. A 90 beam interruption in the phase-gated technique impacted the treatment delivery efficacy, increasing the treatment delivery time in the phase gated for 1664 ± 202 s 1353-1942 s) compared to 36 interruptions in the amplitude gated 823 ± 79 s (712-926 s) ( < 0.0001).

Conclusion: Amplitude-gated DCAT allows for better dosimetric accuracy over phase-gated treatment patients with regular breathing patterns.