Purpose: Involuntary motion due to patientâ€™s swallow or coughing is difficult to manage, however it may cause inaccurate dose delivery in hypofraction larynx SBRT. The goal of this study is motion control of larynx SBRT using surface image guidance.
Methods: Seven patients were recently treated with this phase 2 trial. Each patient had 5 treatments. Patients were simulated with long masks, where a small region was manually cut open at larynx level to allow surface tracking. Pretreatment and intrafractional CBCTs were acquired to verify internal anatomy. A new reference surface was captured after CBCT verification. Patient was monitored by surface guidance throughout the treatment. During treatment delivery, beam hold was initiated by Motion Management Interface. Gating window was customized for each patient and typically set at 1 mm above baseline motion due to respiration. Patient motion was recorded in log files and analyzed.
Results: The total treatment time that patient was in the room was 31.3Â±8.1 minutes. Frequency and length of swallow varied from patients and fractions. Swallow happened 7.1Â±5.3 times (ranging 0-24) per fraction. Single swallow took 2.28Â±0.9 seconds, however it is common for patients to have multiple swallows at one time, which last over 4 seconds. The magnitude of swallow was large, with peak amplitude at 6.4Â±4.0 mm above baseline and mostly at the longitudinal direction. Although patients were coached to not swallow during beam on (4.23Â±0.87 minutes), some patients experienced involuntary swallow, resulting in beam duty cycle as low as 76%. With surface gating, the average motion amplitude reduced from 1.3Â±2.2 mm to 0.7Â±0.8 mm(p<0.001).
Conclusion: We described a motion management workflow in treating larynx SBRT. It was found that swallow motion is common and frequent for larynx patients. Surface imaging gating is a novel application and is feasible in reducing the effect of involuntary motion.