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.

Pang, Pei Ping Eric, Hendry, Julie, Cheah, Shie Lee, Soong, Yoke Lim, Fong, Kam Weng, Wee, Tien Seng Joseph, Tan, Wee Kiat Terence, Nei, Wen Long, Wang, Fuqiang, Wong, Ru Xin, Ng, Wee Loon and Chen, John (2014) 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. Radiotherapy and Oncology, 112(3), pp. 437-441. ISSN (print) 0167-8140

Official URL: http://dx.doi.org/10.1016/j.radonc.2014.09.008

Abstract

BACKGROUND AND PURPOSE: A planning margin ⩽3mm 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 1.1mm. Population mean (median) intra-fraction movement in the x-, y-, and z-planes were -0.1mm (0mm), -0.3mm (-0.3mm) and -0.2mm (-0.2mm) 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: 3mm-AL was not adequate to account for intra-fraction movement when the CTV-PTV margin was ⩽3mm and should be excluded from the routine imaging protocol and daily image-guided radiotherapy should be employed. Adjusting the action-level to 2mm would allow a more confident approach in delivery of the prescribed dose in head-and-neck IMRT cases.

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