Semester of Graduation

Summer

Degree

Master of Science (MS)

Department

Physics & Astronomy

Document Type

Thesis

Abstract

Purpose: To validate the performance of a respiratory gating system for the automated delivery of the deep inspiration breath-hold (DIBH) technique.

Methods: The gating system utilized an automatic gating interface (Elekta Response) which connected a marker-based respiratory motion monitoring system to the linear accelerator control system. The gating system was characterized dosimetrically and temporally using two distinct approaches. Central-axis output and energy constancy were evaluated across 8 beam-matched linear accelerators. Additionally, a representative set of 5 treatment plans were delivered both non-gated and gated to a 2D diode array (MapCHECK). The respiratory motion monitoring system optically tracked a reflective marker that was attached to a dynamic phantom (QUASAR). The phantom was programmed to replicate a typical DIBH breathing waveform. The passing rates between these modes of operation were evaluated using gamma analysis and a percent dose difference comparison. Modular and end-to-end approaches were used to quantify system latencies. The modular components evaluated were the streaming latency of the tracking camera, sampling rate of the tracking software, signal travel time, and latency of the linear accelerator. The end-to-end approach involved measuring the displacement of a target moving at known velocities during the during the gating process.

Results: Output and energy constancy were both within ± 0.5% for each beam energy and linear accelerator investigated. The average differences in passing rates between non-gated and gated modes of operation were within ± 0.4% using gamma analysis (2%, 1mm). Average passing rates between modes of operation were greater than 99% using a percent dose difference comparison (1%). The first gated segment was found to have significantly (p =.02) longer beam-on latency compared to the subsequent gated segment. End-to-end beam-on and beam-off latency for the subsequent gated segment was found to be 1.49 and 0.34 seconds, respectively, which was consistent with measured component totals.

Conclusion: The gating system was able to achieve dosimetric operating characteristics that are desirable for accurate delivery of the DIBH technique. The methodology presented can be generalized to other respiratory gating systems that utilize the automatic gating interface studied in this work.

Committee Chair

Fontenot, Jonas D.

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