Quality Assurance of Varian On-board Imager

Accurate and repeatable setup of patients is a requisite in radiotherapy. In the treatment of head-and-neck tumors, accurate setup is particularly important because of the proximity of the treatment volume to critical structures such as the spinal cord, brainstem, and parotid glands. When intensity-modulated radiotherapy (IMRT) is used, proximity of this kind between the target and normal organs often leads to highly inhomogeneous fluence profiles with steep dose gradients. Thus, repeatable and accurate patient setup is crucial to prevent the potential hazard of encompassing critical structures for example, the spinal cord in the high-dose region. Indeed, studies have shown that improper setup in the head-and-neck region can have significant effects on tumor coverage and normal-tissue sparing (Manning et al 2001, Samuelsson et al 2003, Astreinidou et al 2005). Improvement of head-and-neck setup precision and reproducibility therefore continues to be an active field of study.