Dosimetric Impacts of Image Guided Radiation Therapy-Determined Clinical Shifts for Spine SBRT Patients
Abstract
Purpose
Stereotactic spinal treatments deliver a high radiation dose and utilize smaller target margins, on the order of 1 mm, to minimize dose to the spinal cord. Intrafraction tumor tracking is therefore essential to minimize dose outside of the target. A common approach to limit the impact of patient motion during treatment delivery is to perform cone-beam CTs (CBCT) imaging prior to delivery of each treatment arc. This work implemented a multiple CBCT workflow and tracked couch shifts during treatment. In addition, the impacts of these shifts on dose delivered was investigated.
Methods
143 spine SBRT treatments were delivered on Varian TrueBeam between November 2024 and January 2026. Patients (N=32) were immobilized using a vacloc with vacuum sealed shrink wrap. CBCTs were acquired and shifts were applied during initial patient setup, post couch shifts but prior to delivering the first arc, and between treatment arcs. To evaluate dosimetric impacts of these shifts, patients (N=10) with clinical shifts larger than 1 mm translationally underwent a treatment planning study. Treatment plans were split into individual fractions and recalculated with each arc shifted to mimic treatment delivery. PTV coverage and OAR dose was evaluated and compared with the original plan.
Results
The average magnitude of patient shifts [mm or degrees] in the vertical, longitudinal, lateral, rotation, pitch, and roll directions, respectively were [0.3, 0.3, 0.4, 0.14, 0.13, and 0.18] between arcs. When clinical shifts were applied to each fraction (N=42), mean PTV V100 decreased by 3.8%, and mean maximum dose to spinal cord, bowel, and esophagus increased by 24.5 cGy, 22.5 cGy, and 8.6 cGy, respectively.
Conclusion
After initial setup CBCTs, there was generally sub-millimeter and sub-degree motion. Although small, this work shows that not accounting for these shifts during treatment can cause decreases in PTV coverage and increases in OAR dose.