Author: William P. Donahue, Lauren Smith, Shu (Stella) Xing 👨🔬
Affiliation: Department of Medical Physics, Memorial Sloan Kettering Cancer Center 🌍
Purpose: To quantify the dosimetric uncertainty of end-exhale gating as a function of breathing pattern.
Methods: A Truebeam delivered AP square fields to EBT4 film placed in the Quasar MRI4D Motion Phantom (Modus Medical Devices), acquiring one static and multiple gated measurements. Gated deliveries used simulated breathing traces of a 12 bpm sinusoidal curve, 30mm peak-to-peak amplitude with breath-holds of various durations from 0 to 60s. The Varian RPM system was used for beam-hold gating with a 3mm gating window at end-exhale. The films were aligned using fiducial markers in the phantom and dose profiles were extracted using FilmQA Pro (Ashland).
Results: Gated deliveries displayed a dose shift superiorly (towards inhale direction) compared to the static delivery. The dose shift became more severe as the number of gates during delivery increased, measuring a maximum central axis shift of 5mm. The clinically relevant high dose region (95% isodose) was asymmetrically shifted, with the superior 95% isodose line shifting further than the inferior side as number of gates increased. The penumbra of the gated profiles widened compared to static profiles, and this difference increased dramatically as the number of gates increased.
Conclusion: These trends indicate that a breathing frequency resulting in an increased number of gates during treatment may result in a dose shift towards the direction of the gate window and widening of the penumbra. The dose shift can be explained by the residual motion of the film as it enters and exits the beam-on window, compared to full exhale. This effect may arise in clinical scenarios where patient is unable to maintain breath holds longer than 10s. Breathing frequency and breath-hold duration should be considered in the treatment planning process as it may impact dose; and is particularly concerning in scenarios where targets abut radiosensitive organs in the superior-inferior direction.