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.

Keywords