An assessment of the magnitude of intra-fraction movement of head-and-neck IMRT cases and its implication on the action-level of the imaging protocol

Abstract

Background and purpose: A planning margin ⩽3 mm is employed in some head-and-neck IMRT cases due to the proximity of critical structures. This study aims to explore the need to redefine the action-level in the head-and-neck imaging protocol in consideration of the intra-fraction movement.

Material and methods: This is a local study of 18 patients treated using the same immobilisation system and setup protocol. Post-treatment orthogonal pair of kilovoltage X-ray images was acquired on the first three days of treatment. 106 sets of pre- and post-treatment kV X-ray images acquired over 53 fractions were analysed against the treatment planning DRR for calculation of intra-fraction movement.

Results: Individual mean intra-fraction movement in all directions ranged from -1.8 to 1.1 mm. Population mean (median) intra-fraction movement in the x-, y-, and z-planes were -0.1 mm (0 mm), -0.3 mm (-0.3 mm) and -0.2 mm (-0.2 mm) respectively. Intra-fraction movement in all three dimensions, x-, y- and z-planes were considered statistically significant (p<0.05). 7 out of 53 fractions (13.2%) were highlighted as the combined magnitude of the intra-fraction motion with the uncorrected pre-treatment setup errors had exceeded the boundaries of given margins.

Conclusions: 3 mm-AL was not adequate to account for intra-fraction movement when the CTV-PTV margin was ⩽3 mm and should be excluded from the routine imaging protocol and daily image-guided radiotherapy should be employed. Adjusting the action-level to 2 mm would allow a more confident approach in delivery of the prescribed dose in head-and-neck IMRT cases.

Keywords: Action-level; Correction strategies; Head and neck; Imaging protocol; Intra-fraction; Planning margins.