Penile bulb sparing in prostate cancer radiotherapy: Dose analysis of an in-house MRI system to improve contouring

Objective This study aimed to assess the reduction in dose to the penile bulb (PB) achieved by MRI-based contouring following drinking and endorectal balloon (ERB) instructions. Patients and methods A total of 17 prostate cancer patients were treated with intensity-modulated radiation therapy (IMRT) and interstitial brachytherapy (IBT). CT and MRI datasets were acquired back-to-back based on a 65 cm 3 air-filled ERB and drinking instructions. After rigid co-registration of the imaging data, the CT-based planning target volume (PTV) used for treatment planning was retrospectively compared to an MRI-based adaptive PTV and the dose to the PB was determined in each case. The adapted PTV encompassed a caudally cropped CT-based PTV which was defined on the basis of the MRI-based prostate contour plus an additional 5 mm safety margin. Results In the seven-field IMRT treatment plans, the MRI-based adapted PTV achieved mean (D mean ) and maximum (D max ) doses to the PB which were significantly lower (by 7.6 Gy and 10.9 Gy, respectively; p