Investigation of positional and geometric variations of organ-at-risk (OAR) in head-and-neck (HN) radiotherapy (RT) is greatly limited by the poor soft tissue contrast of X-ray-based imaging. We investigated the positional variation of parotid glands (PG) in the immobilized HN-RT treatment position on healthy volunteers using high-spatial-resolution 3D-MRI at 1.5T. The results showed that PGs generally had sub-millimeter and sub-degree group mean error, systematic error and random error under immobilization. Although PGs generally showed insignificant positional variations from global HN, individual left and right PG might move independently and resulted in insignificant positional correlations with each other and global HN.
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