Radiation Physics and Engineering
A peer-reviewed journal published by K. N. Toosi University of Technology

Dosimetric impact of immobilization devices in head and neck IMRT: A retrospective analysis

Document Type : Research Article

Authors

Laleh Rafat-Motavalli 1
Elie Hoseinian-Azghadi 1
Niloofar Rafat-Motavalli 2
Mahdieh Dayyani 2, 3
Hashem Miri-Hakimabad 1, 2
Leila Sobhkhiz-Sabet 4
Zahra Shahamat 4

1 Department of Physics, Faculty of Science, Ferdowsi University of Mashhad, Mashhad, Iran

2 Research and Education Department, Reza Radiotherapy and Oncology Center, Mashhad, Iran

3 Radiation Oncology Department, Reza Radiotherapy and Oncology Center, Mashhad, Iran

4 Medical Physics Department, Reza Radiotherapy and Oncology Center, Mashhad, Iran

https://doi.org/10.22034/rpe.2025.518360.1267
Abstract
Immobilization devices are critical for ensuring precise and reproducible patient positioning during radiotherapy. These devices, along with the treatment couch, can alter dose distribution by increasing skin dose, reducing target coverage, and introducing attenuation and scattering. Guidelines like TG-176 emphasize incorporating the dosimetric impact of devices into treatment planning system (TPS) calculations, but current practices often neglect or only partially consider these effects, usually modeling the treatment couch as low-density material. This study evaluated the impact of immobilization devices on dosimetric parameters for target volumes and organs at risk (OARs) in 20 head and neck IMRT plans. Five scenarios were compared, from no devices to full immobilization and a double-layer couch in TPS calculations. Two-tailed paired t-tests assessed significance, with p-values reported. The most significant effect was for parameter GTV D100, showing a maximum difference of 5.5% and an average of 2% (p<0.0001). OAR doses had smaller differences, ranging from 1% to 3% (p<0.0001). Pearson correlation analysis indicated a relatively strong correlation between the posterior beam contribution and dose parameters of the target region (r = 0.93 to 0.95). To enhance accuracy, include all immobilization devices and the carbon fiber treatment couch in external contour and TPS calculations. If CT simulation equipment differs from treatment setups, apply appropriate CT number overrides. Additionally, use the same immobilization devices as in the treatment room for CT imaging, ensuring the field of view (FOV) is large enough to capture all devices and integrate a complete couch model into the images.

Highlights

  • The study provides an analysis of the dosimetric effects of immobilization devices and the treatment couch in IMRT.
  • Neglecting immobilization devices in TPS calculations lead to underestimation of target and altered OAR doses.
  • Solutions such as integrating immobilization devices and the carbon fiber couch into TPS calculations is proposed.

Keywords

Immobilization devices
IMRT
Dose coverage
OARs (Organs at risk)


Copyright
RPE is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License (CC BY-NC 4.0).

Conflict of Interest

‎The authors declare no potential conflict of interest regarding the publication of this work‎.

Funding

‎The authors declare that no funds‎, ‎grants‎, ‎or other financial support were received during the preparation of this manuscript‎.

 
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Radiation Physics and Engineering
Volume 6, Issue 4
Autumn 2025
Pages 65-75

  • Receive Date 21 April 2025
  • Revise Date 02 September 2025
  • Accept Date 23 September 2025

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