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 260RRfractions and 351 non-RR fractions from22patients was analyzed.3Dand unidirectional motion patterns betweenRR and non-RR fraction datasets were comparedinterms of percentage timeat displacement ≤1,2,3,4,5 and 6 mmover6and 10 min of tracking time. Temporal patterns of the prostate motion were evaluatedby re-binning the motion datain1 min time intervals. The percentage time at displacement was larger in RR data compared to non-RR datainevery 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 RRfractions the corresponding percentages were 69.6%, 32.8%, 15.3%, 7.4%, 3.7% and 2.2%, respectively. The differencein3Dmotion 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 timeinbothRRand non-RR datasets but the increase was more rapid in RRfractions. 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.