Impact of rectum and bladder anatomy in intrafractional prostate motion during hypofractionated radiation therapy

The objective was to determine the magnitude of the prostate intrafractional motion relative to bony pelvis anatomy, and to evaluate the relationship between this displacement and some clinical and anatomical variables.

The prospective study consisted of 544 images (375 pre-treatment CBCT and 169 post-treatment CBCT) from 15 prostate adenocarcinoma patients that were used for intrafractional prostate motion determination. In addition, two radiation oncologists re-contoured the bladder and rectum on each CBCT according to the patient's anatomy of the day. ANOVA and correlation analysis followed by linear regression analysis were performed to identify clinical or anatomical variables that predict large prostate intrafractional motion.

Prostate shift increased in patients with hormone therapy (p ≤ 0.02). The regression analysis showed that patients with large bladder intrafractional filling (p < 0.01) and a large bladder volume difference from planning CT were more likely to experience bigger longitudinal prostate motion (> 3 mm). Recommended bladder size values: anterior-posterior size ≤ 10 cm and anterior-posterior/cranio-caudal ratio ≤ 1.7, both parameters measured in the midsagittal prostate plane, were defined.

The treatment margin should not be reduced for those patients who were treated with hormone therapy and/or whose rectum or bladder was far from complying the preparation protocol conditions.