Assessment of three-dimensional set-up errors in head and neck cancer patients treated by intensity modulated...

Set-up errors are an inherent part of radiation treatment process. Coverage of target volume is a direct function of set-up margins, which should be optimized to prevent inadvertent irradiation of adjacent normal tissues. The aim of this study is to evaluate set-up errors and propose optimum margins for target volume coverage in head and neck radiotherapy.

Twenty six head and neck cancer patients received intensity modulated radiation therapy (IMRT) included in the study. The weekly portal images taken after correction of the systematic error -if any- were evaluated. The systematic error tested and corrected by taking portal images in the 1st 3 days of treatment by using the VARIS offline review system. Three hundred sixty four portal images matched anatomically with anterior and lateral digitally reconstructed radiographs (DRRs). Five hundred forty six points used to evaluate isocenter displacement in antero-posterior direction (AP), supero-inferior direction (SI) and right-left direction (RL).

The mean isocenter displacement in AP, RL, and SI directions were 1.5 +/- 1.6 mm, 1.3 +/-1.4 mm and 2.13 +/- 1.6 mm. Ninety six percent of the isocenter deviations were within 4 mm in all three directions. The displacement more than 4 mm (negative or positive) was 4% in the vertical direction, 7% in the longitudinal direction and 1.6% in the lateral direction. There is insignificant increase of the isocenter shift in the last weeks of radiotherapy especially in the vertical and longitudinal directions.

The current setup for irradiating head and neck cancer patients using IMRT in our department is accurate. The 4 mm CTV-PTV margin is enough.