Evaluation of inter-fraction prostate motion using kilovoltage cone beam computed tomography during radiotherapy

The success of delivering the prescribed radiation dose to the prostate while sparing adjacent sensitive tissues is largely dependent on the ability to accurately target the prostate during treatment. Kilovoltage cone beam computed tomography (CBCT) imaging can be used to monitor and compensate for inter-fraction prostate motion, but this procedure increases treatment session time and adds incidental radiation dose to the patient. We carried out a retrospective study of CBCT data to evaluate the systematic and random correction shifts of the prostate with respect to bones and external marks.

A total of 449 daily CBCT studies from 17 patients undergoing intensity-modulated radiotherapy (IMRT) for localised prostate cancer were analysed. The difference between patient set-up correction shifts applied by radiation therapists (via matching prostate position in CBCT and planning computed tomography) and shifts obtained by matching bony anatomy in the same studies was used as a measure of the daily inter-fraction internal prostate motion.

The average systematic and random shifts in prostate positions, calculated over all fractions versus only 10 fractions, were not found to be significantly different.

The measured prostate shifts with respect to bony anatomy and external marks after the first 10 imaging sessions were shown to provide adequate predictive power for defining patient-specific margins in future fractions without a need for ongoing computed tomography imaging. Different options for CBCT imaging schedule are proposed that will reduce the treatment session time and imaging dose to radiotherapy patients while ensuring appropriate prostate cover and normal tissue sparing.