Abstract
This study aims to evaluate inter-fractional set-up errors in patients treated with distinct immobilization equipment (thermoplastic mask, knee-fix and feet-fix, wing board and vac-lok) for four anatomical regions including brain, head and neck (HN), thorax and pelvis. Data of randomly selected 140 patients who were treated for four anatomical regions were obtained using Hi-Art Helical Tomotherapy (HT) system. Pre-treatment planning was based on automatic registration readings of computed tomography (CT) and mega-voltage computed tomography (MVCT) on a daily basis. Distinct immobilization equipment was used for varying anatomical regions. Individual mean set-up error (M), systematic error (Σ), and random error (σ) values were calculated through daily translational and rotational deviation values. The size of translational, systematic and random error was 1.31 - 4.93 mm for brain, 2.28 - 4.88 mm for HN, 4.04 - 9.90 mm for thorax, and 6.34 - 14.68 mm for pelvis. Rotational values were as follows: 0.06˚ - 0.73˚ for brain, 0.42˚ - 0.6˚ for HN, 0.48˚ - 1.14˚ for thorax and 0.65˚ - 1.05˚ for pelvis. The highest translational, systematic and random error value was obtained from the pelvic regional. The highest standard and random error value in pitch and roll was produced in the rotational direction of the pelvis (0.05˚ and 0.71˚), while the highest error value in yaw was (1.14˚) produced from thorax. Inter-fractional set-up errors were most commonly produced in the pelvis, followed by thorax. Our study results suggest that the highest systematic and random errors are found for thorax and pelvis. Distinct immobilization equipment was important in these results. Safety margins around the clinical target volume (CTV) are changeable for different anatomical regions. A future work could be developed to new equipment for immobilization because of the reduced margins CTV.
Highlights
Radiation therapy (RT) is the treatment modality using beams of X-rays, electrons or protons to treat cancer cells
We aimed to evaluate inter-fractional set-up errors in patients treated with distinct immobilization equipment for four anatomical regions, including brain, head and neck (HN), thorax, and pelvis using the automatic registration values of pre-treatment planning computed tomography (CT) images and daily mega-voltage computed tomography (MVCT) images
The major systematic error occurred in X and Y directions in the pelvis (7.35 mm; 14.68 mm), the highest standard error at Z direction was in thorax (9.11 mm)
Summary
Radiation therapy (RT) is the treatment modality using beams of X-rays, electrons or protons to treat cancer cells. There are two major considerations in RT: 1) providing a more homogenous dose distribution for the tumor and its surrounding; 2) preserving more healthy tissues. Inappropriate positioning causes both reduced dose for the target volume and radiating to healthy tissues. Two-dimensional conventional RT has been replaced by three-dimensional conformal RT and intensity-modulated radiotherapy (IMRT) The latter offers a wide conformality for the dose distribution, reducing organs at risk (OAR) dose and toxicity [7]. Compared to conventional treatment modalities, geometric uncertainties are more critical, and detection and reduction of set-up errors are more invaluable in IMRT [8]. Appropriate IMRT use is based on minimizing set-up errors to provide true safety margins around CTV [10]
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