Assessment of interfraction set-up uncertainties according to different anatomical regions with kV-kV IGRT for head-and-neck radiotherapy

Intensity-modulated radiotherapy (IMRT) generates highly conformal dose distributions with steep dose gradients. The benefits of IMRT are potentially large for patients with head-neck cancer where the tumor shape is complex and its location often close to organs at risk (OARs). Set-up errors, anatomy and posture changes, however, cause geometrical uncertainities that may lead to serious degradation of the IMRT plan. Set-up errors have both systematic and random components. Random errors lead to a blurring of the dose distribution, while systematic errors lead to a displacement of the dose distribution with respect to the CTV. To account for these set-up errors safety margins are applied. In our clinic, 5-10 mm margins are used for patients with head-neck cancer in combination with online set-up correction protocols. In this study local set-up accuracy was analysed for 10 patients, who had received intensity modulated radiotherapy with guided daily kV-kV IGRT. We defined eight regions in the planning CT: occiput bone, mandible, temporal bone, hyoid bone, vertebrae C2, vertebrae C3, vertebrae C4 and vertebrae C5. We applied the Amsterdam and Rotterdam margin recipe for rigid body set-up errors to each region. The largest systematic errors were found as 0.32 cm and 0.25 cm respectively for the hyoid bone and mandible in the LNG direction. The largest random errors were found as 0.29 cm and 0.23 cm respectively for the hyoid bone and mandible in the LNG direction. The largest CTV-PTV margins were found as 0.71 cm and 0.55 cm respectively for the Amsterdam and Rotterdam approximation in the LNG direction. As a conclusion the measurement results are in agreement with the literature for the other applications.