Aim. The aim of the study was to examine the dosimetric effect of rectal retraction, using a rectal retractor, by performing a comparative treatment planning study. Material and Methods. Treatment plans using volumetric arc therapy (VMAT) were produced for ten patients both with and without rectal retraction. A hypofractionation scheme of 42.7？Gy in seven fractions was used. The dose to the rectal wall was evaluated for both methods (with and without retraction) using four dose-volume criteria: , , , and . Results. The retraction of the rectal wall increased the distance between the rectal wall and the prostate. The rectal wall volume was reduced to zero for all dose-volume values except for , which was 0.2？cm3 in average when the rectal retractor was used. Conclusion. There was a significant decrease of , , , and when the rectal retractor was used without compromising the dose coverage of planning target volume (PTV). 1. Introduction The uncertainties in positioning together with movements of the prostate due to varying filling of the rectum and the bladder [1] imply the use of a margin from the clinical target volume (CTV) to the planning target volume (PTV) [2]. Rectal irradiation side effects are due to the large PTV margin and the proximity of the prostate and the rectal wall [3], which often leads to a compromise between target dose coverage and sparing of the rectal wall. Image guided radiotherapy (IGRT) with fiducials in the prostate used for daily verification prior to each treatment reduces the CTV to PTV margin, which decreases the dose to the rectal wall [4]. External beam radiotherapy (EBRT) of prostate cancer can be delivered with three-dimensional conformal radiotherapy (3D-CRT) or intensity modulated radiotherapy (IMRT) [4–7]. IMRT is delivered either with a number of static beams or as volumetric modulated arc therapy (VMAT). Also, proton beam therapy is used either as boost in combination with EBRT using photons or as the only treatment method [8–10]. Various techniques to decrease the rectal dose have been introduced, such as spacer gels and endorectal balloons [11–15]. The spacer gel is injected between the prostate gland and the anterior rectal wall, which increases the distance in between, resulting in significantly decreased dose to the rectal wall [11–13]. The endorectal balloon is placed in the rectum and inflated with air to expand the rectum [13, 14]. The anterior rectal wall moves toward the prostate and displaces it frontally, while the posterior wall remains in its position resulting in an increased distance between the
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