Abstract
PurposeBeam gating with deep inspiration breath hold (DIBH) has been widely used for motion management in radiotherapy. Normally it relies on some external surrogate for estimating the internal target motion, while the exact internal motion is unknown. In this study, we used the intrafraction motion review (IMR) application to directly track an internal target and characterized the residual motion during DIBH treatment for pancreatic cancer patients through their full treatment courses.Methods and MaterialsEight patients with pancreatic cancer treated with DIBH volumetric modulated arc therapy in 2017 and 2018 were selected for this study, each with some radiopaque markers (fiducial or surgical clips) implanted near or inside the target. The Varian Real‐time Position Management (RPM) system was used to monitor the breath hold, represented by the anterior‐posterior displacement of an external surrogate, namely reflective markers mounted on a plastic block placed on the patient's abdomen. Before each treatment, a cone beam computed tomography (CBCT) scan under DIBH was acquired for patient setup. For scan and treatment, the breath hold reported by RPM had to lie within a 3 mm window. IMR kV images were taken every 20° or 40° gantry rotation during dose delivery, resulting in over 5000 images for the cohort. The internal markers were manually identified in the IMR images. The residual motion amplitudes of the markers as well as the displacement from their initial positions located in the setup CBCT images were analyzed.ResultsEven though the external markers indicated that the respiratory motion was within 3 mm in DIBH treatment, significant residual internal target motion was observed for some patients. The range of average motion was from 3.4 to 7.9 mm, with standard deviation ranging from 1.2 to 3.5 mm. For all patients, the target residual motions seemed to be random with mean positions around their initial setup positions. Therefore, the absolute target displacement relative to the initial position was small during DIBH treatment, with the mean and the standard deviation 0.6 and 2.9 mm, respectively.ConclusionsInternal target motion may differ from external surrogate motion in DIBH treatment. Radiographic verification of target position at the beginning and during each fraction is necessary for precise RT delivery. IMR can serve as a useful tool to directly monitor the internal target motion.
Highlights
For locally advanced unresectable pancreatic cancer, conventional doses of radiation are not effective to improve long‐term survival, and stereotactic body radiotherapy or hypofractionated ablative radiotherapy has shown promising local control with an acceptable rate of adverse events.[1]
There are two data pairs that are more than 5 mm away from the group‐averaged positions, which were from intrafraction motion review (IMR) images taken outside of the deep inspiration breath hold (DIBH) gating window
The treatment beam was gated by the Real‐time Position Management (RPM) signal, currently there was no interlock between the TrueBeam IMR imaging system and RPM gating system
Summary
For locally advanced unresectable pancreatic cancer, conventional doses of radiation are not effective to improve long‐term survival, and stereotactic body radiotherapy or hypofractionated ablative radiotherapy (in 15–25 fractions) has shown promising local control with an acceptable rate of adverse events.[1] For these types of treatment, since the target doses significantly exceed the tolerance of the surrounding normal tissues, proper organ motion management is crucial to avoid severe complications. Gierga et al reported a study of seven patients using fluoroscopy to observe the motion of fiducial clips.[2]. Feng et al showed that the tumor border motion was much larger than normally expected, based on 17 cine MRI studies.[4] They reported that the magnitude of motion for pancreatic tumors, though variable, can be as much as 4 cm in SI direction. Analyzed was the displacement vectors from the initial setup position to the positions observed in IMR images
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
