Evaluation of the Changes in the Shape and Location of the Prostate and Pelvic Organs Due to Bladder Filling and Rectal Distension

Prostate brachytherapy has become an increasingly popular treatment for localized prostate cancer. A steep dose gradient between the prostate and organs at risk (rectum and bladder) is ideal in this treatment modality, so prostate displacement and deformation due to bladder filling and rectal distension play an important role in critical organs post-implant dose. The purpose of this study was to evaluate the interrelationship between normal rectal distension, bladder filling, and their movements. The study also aimed to quantify total prostate displacement and deformation due to physiologic organ filling measured by using MRI-based shifts and estimate the precision with which the shifts were made in supine and left decubitus positions.Patients and 3 patients who were referred for transrectal prostatic biopsy (Shahid Faghihi hospital, Shiraz, Iran) with different prostate sizes were selected for this study. A 1.5-Tesla MRI system (Avanto, Siemens, Germany) and an ultrasound system (Logiq 500, GE medical systems, USA) were used to collect images of the patients’ prostates at different stages of bladder and rectum fullness. The mean displacement of the prostate after bladder filling in the supine and left decubitus positions along the anterior-posterior (AP) axis was posterior by 3.5 mm (range = 0.7 mm to 6.3 mm) and along the superior-inferior (SI) axis was inferior by 3.4 mm (range = 1.4 mm to 5 mm). Prostate displacement in the left-right (LR) axis was negligible. The mean prostate displacement after rectal distension was anterior by 7.1 mm in the supine position, 5.1 mm anterior in the left decubitus position, and along the SI axis was inferior by 2.9 mm in the supine and left decubitus positions. The maximum prostate deformation due to rectal distension and bladder filling in the supine position was as large as 3.2 mm, 1.9 mm, and 1.2 mm in the AP, SI, and LR directions, respectively. While in the left decubitus position, maximum prostate deformation was 2.6 mm, 1.2 mm, and 1.3 mm in the AP, LR, and SI directions, respectively. It is probably important to evaluate the influence of the changes in the shape and location of the prostate due to bladder filling, rectal distension, and patient position in post-implant brachytherapy dosimetry. Using images of patients in the left decubitus position with a full bladder and distended rectum is suggested in planning for treatment.