Dosimetric Variations In Gastrointestinal Radiotherapy Using End-Expiratory Breath Hold: A Comparative Assessment of Alignrt and Identify Surface-Tracking Systems
Abstract
Purpose
End-expiration breath hold (EEBH) is commonly employed in GI radiotherapy to enable dose escalation while minimizing dose spillage to adjacent organs at risk. Surface-tracking systems are frequently used to support patient breath-hold stability. This study compares the dosimetric variation for EEBH GI treatment using AlignRT (Vision RT, London, UK) and Identify (Varian Medical Systems, Palo Alto, CA) surface-tracking systems.
Methods
Real-time surface-tracking data on 13 EEBH GI patients treated with either AlignRT on a c-arm linac or Identify with an Ethos system are analyzed retrospectively. First, we compared the systems’ localization performance using a human-shaped phantom designed to mimic patient surface variations in the GI region. Both AlignRT and Identify were evaluated across multiple forms of region of interest (ROI). Second, we investigated the dose variations according to real-time displacement readings on 3 translational directions. Using an average displacement at the breath-hold duration from AlignRT and Identify reports, the planning dose cloud is rigidly shifted according to the average residual displacements to compare the PTV coverage. During patient treatment, the AlignRT real-time delta tolerance was set to 3mm/3° in all 6 DOFs while the Identify system was limited to ±1mm only in the vertical direction.
Results
During system performance testing, AlignRT demonstrated 2% higher accuracy in surface-displacement prediction than Identify when identical ROI shapes were used. However, when a larger ROI was applied, Identify’s accuracy improved, exceeding AlignRT by 1.5%. AlignRT showed smaller average residual surface displacement than Identify (0.74mm vs. 2.87mm). The average PTV D95% dropped from 5197.37 to 4632.02cGy (10.8%) for Identify and from 5135.08 to 5120.14cGy (0.3%) for AlignRT due to residual surface displacement.
Conclusion
Both systems show reliability at patient treatment sessions. AlignRT is more flexible in selecting ROI than Identify while Identify shows larger dosimetric effect due to residual surface displacement.