Evaluation of intra- and inter-fraction movement of the cervix during intensity modulated radiation therapy

Abstract

Background and purpose: To assess the degree of intra- and inter-fraction cervical motion throughout a course of intensity modulated radiation therapy (IMRT) for cervical cancer patients.

Materials and methods: A retrospective study of 10 women with stage 1B1-3B cervical cancer diagnosed from September 2007 to July 2008 was conducted. All patients were treated with chemoradiation using IMRT followed by intracavitary brachytherapy. Pretreatment, patients had 2 seeds placed at a depth of 10mm into the cervix. On-Board Imaging (OBI) was used to obtain anterior/posterior (AP) and lateral X-rays before and after each treatment. OBI images were rigidly aligned to baseline digitally reconstructed radiographs (DRRs), and movement of cervical seeds was determined in the lateral, vertical, and AP directions. Mean differences in cervical seed position and standard error of the mean (SEM) were calculated.

Results: A total of 922 images were reviewed, with approximately 90 images per patient. The mean intra-fractional movement in cervical seed position in the lateral, vertical, and AP directions were 1.6mm (SD±2.0), 2.6mm (SD±2.4), and 2.9mm (SD±2.7), respectively, with a range from 0 to 15mm for each direction. The mean inter-fractional movement in the lateral, vertical, and AP directions were 1.9mm (SD±1.9), 4.1mm (SD±3.2), and 4.2mm (SD±3.5), respectively, with a range from 0 to 18mm for each direction.

Conclusions: This is the first study to assess intra- and inter-fractional movement of the cervix using daily imaging before and after each fraction. Within and between radiation treatments, cervical motion averages approximately 3mm in any given direction. However, maximal movement of the cervix can be as far as 18mm from baseline. This wide range of motion needs to be accounted for when generating planning treatment volumes.