“Online correction of intrafraction motion during volumetric modulated arc therapy for prostate radiotherapy using fiducial-based kV imaging: A cohort study quantifying the frequency of shifts and analysis of men at highest risk”
Background (extract): We sought to characterize setup corrections in our practice informed by the TrueBeam Advanced imaging package, and analyze factors associated with intrafraction motion (IFM).
Methods (extract): 132 men received RT for prostate cancer with VMAT. All patients underwent planning CT immediately following transrectal placement of 3 fiducial markers (Gold Anchor). The most common RT course was 20 fractions (range: 17–44). Triggered kV images were acquired every 15 seconds over 2–3 full arcs using an onboard imaging system. Intrafraction motion (IFM) correction was considered when if any two fiducial markers in a single kV image were observed to be outside beyond a 3 mm tolerance margin. A manual 2D/3D match was performed using the fiducial markers from the single triggered kV image to obtain a suggested couch shift. Shift data for three (x, y, z) planes were extracted from the record and verify system and expressed as a single 3-dimensional translation. Shift percent (SP) was defined as the number of instances of an intrafraction correction divided by the total number of fractions for a given patient.
Results (extract): Over 2659 fractions of radiation, IFM was observed and corrected for 582 times across 463 (17%) fractions, and at least one shift was made over the course of treatment in 77% of men. On multivariate analysis, only rectal volume and width were associated with IFM.
Discussion (extract): The magnitude of IFM was 3.6 mm on average, and was ≥5 mm in 36% of patients thereby potentially resulting in underdosing of the intended clinical target volume that had been expanded by 5–8 mm for set up error. Clinicians should be aware of this potential for dosimetric uncertainty at the periphery of the treatment volume, and either correct for IFM in real time, adjust the PTV accordingly, or accept the risk of under-dosing the target.
Conclusion (extract): Men treated with shorter courses of therapy, such as SBRT, or men at high risk for IFM (e.g. larger rectal size) may warrant more careful consideration regarding the implications of IFM.
Article from Journal of Applied Clinical Medical Physics, 2024