The effect of rectal retractor on intrafraction motion of the prostate

Abstract

Rectal retractors (RR) are used in prostate radiotherapy to retract part of the rectal wall further from the prostate in order to lower the rectal dose and toxicity. The aim of this study was to investigate the effect of RR on intrafraction motion of the prostate. Intrafraction motion of the prostate with RR and without it was recorded with electromagnetic real-time tracking system. Intrafractional motion data of 260 RR fractions and 351 non-RR fractions from 22 patients was analyzed. 3D and unidirectional motion patterns between RR and non-RR fraction datasets were compared in terms of percentage time at displacement ≥1, 2, 3, 4, 5 and 6 mm over 6 and 10 min of tracking time. Temporal patterns of the prostate motion were evaluated by re-binning the motion data in 1 min time intervals. The percentage time at displacement was larger in RR data compared to non-RR data in every direction (except anterior) and for every motion magnitude considered. For non-RR fractions the percentage of time of ≥1, 2, 3, 4, 5 and 6 mm 3D displacements within 10 min of tracking time were 44.8%, 16.0%, 6.4%, 2.9%, 1.4% and 0.5%, respectively. For RR fractions the corresponding percentages were 69.6%, 32.8%, 15.3%, 7.4%, 3.7% and 2.2%, respectively. The difference in 3D motion between the datasets was statistically significant (p < 0.03). Largest increase in the motion was seen in inferior and posterior directions when the RR was used. Motion increased linearly as a function of elapsed tracking time in both RR and non-RR datasets but the increase was more rapid in RR fractions. The use of RR increases the intrafraction motion of the prostate which can lead to inaccurate treatment localization and delivery thus questioning the justification of its use.