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

Background: A steep dose gradient between prostate and organs at risk (rectum and bladder) is ideal in treatment modality, so prostate displacement and deformation due to bladder filling and rectal distension play an important role in critical organs dose. This study aims to evaluate the changes in the shape and location of the prostate and pelvic organs due to bladder filling and rectal distension. Methods: Three patients who referred for transrectal prostatic biopsy (Shahid Faghihi Hospital, Shiraz, Iran) with different prostate sizes were enrolled. 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 patients prostate at different stages of bladder and rectum fullness. Results: 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 4.9 mm (range: 0.7-6.3 mm) and along the Superior- Inferior (SI) axis was inferior by 3.4 mm (range: 1.4-5 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 2.5 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, it was 2.6 mm, 1.2 mm and 1.3 mm in the AP, LR and SI axis respectively. Conclusion: It is probably of importance 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 the patients in the left decubitus position with full bladder and distended rectum for planning a treatment are suggested.