Rectal volume variations and estimated rectal dose during 8 weeks of image-guided radical 3D conformal external beam radiotherapy for prostate cancer

Abstract

The introduction of modern image-guided radiotherapy (IGRT) has given new insight regarding organ motion in radiotherapy (RT), both in general and in treatment for prostate cancer (PC) [1]. IGRT with daily Cone-beam computed tomography (CBCT) is now considered as part of standard external beam radiotherapy (EBRT) in an increasingly number of cancer patients. Interfraction displacement of the prostate gland during RT is often observed in response to the variations in rectum and bladder filling, and can range from 0 to 20 mm [1], [2], [3], [4], [5], [6], [7]. Accordingly, rectal volume variation (RVV) during RT may increase the risk of biochemical and local failure [8], [9], [10], [11], [12], [13]. Moreover, the rectal volume (RV) receiving ≥ 60 Gy is associated with increased risk of grade ≥ 2 late rectal toxicity or rectal bleeding and can be a limiting factor for dose escalation [14], [15], [16], [17], [18], [19]. Some small sample-size studies have reported variable rectal dose distribution due to RVV during RT [2], [20], [21]. In order to minimize RVV, some authors advocate rectum emptying using an enema eventually combined with laxatives and dietary measures at the time of the initial planning computed tomography (CT) and during the treatment period, especially if daily IGRT is not applied [22], [23]. Consequently, studies on radiation dose distribution and variations in Organs at Risk during the total treatment period are essential to gain knowledge about the accuracy of dose delivery to the tumor and the surrounding normal tissue. The aim of this study was to answer the following research question: Are rectal volumes reduced or increased, and are rectal doses consequently reduced or increased during eight weeks of radical 3D conformal CBCT-IGRT in patients treated for PC?