The investigators will assess the feasibility of intraoperative use of C-arm cone-beam CT imaging for intraoperative dosimetric evaluation in prostate low-dose-rate (LDR) brachytherapy. Patients recruited are those who will receive standard LDR brachytherapy for prostate cancer treatment. Consenting patients will undergo additional 3D C-arm cone-beam CT imaging (3D C-arm) and motorized trans-rectal ultrasound (TRUS). Volumetric and dosimetric measures will be compared to those using the standard Day-0 CT images. The investigators hope that the intraoperative 3D C-arm provides quality assurance measures sufficient to eliminate the need for Day-0 CT LDR brachytherapy and provide the opportunity for intraoperative modification of seed delivery plan.

1. Objectives

   • The primary objective of this study is to determine the feasibility of replacing Day-0 CT with intra-operative 3D C-arm imaging. The main benefit of this change is to receive immediate feedback on the quality of treatment. This can lead to the possibility of performing "live" adjustments to the treatment plan based on feedback of actual seed positions achieved after implantation.
   • Secondly, by replacing Day-0 CT, a separate scan performed in a different site will be eliminated from the treatment procedure, freeing up the CT scanner for other clinical uses

2. Hypotheses

   • Primary hypothesis: Intraoperative use of 3D C-arm cone-beam CT can eliminate the need for Day-0 CT for post-implant dosimetry in prostate LDR brachytherapy by providing similar or improved dosimetric accuracy
   • Secondary hypothesis 1: Dosimetric values obtained with the TRUS probe inside the rectum are not significantly different from those measured when the probe is outside the rectum
   • Secondary hypothesis 2: Sagittal TRUS imaging provides better seed and boudnary detection for dosimetric purposes compared to transverse imaging

3. Justification

   Standard post-implant quality assessment at the BC Cancer Agency involves a pelvic CT 2-3 hours after treatment. This scan is performed outside the operating room, in a different location. As a result, it is not possible to receive immediate feedback on the quality of treatment. Furthermore, soft tissue visualization is poor in CT imaging and contouring variability is large. The use of other imaging modalities such as ultrasound would be beneficial. However, since the scan is performed at a different time, and with the patient in a different position, registering the images would be challenging.

   Intra-operative C-arm CT imaging would provide immediate feedback on the quality of treatment, provide the opportunity of using information from intra-operative ultrasound imaging for soft tissue visualization, and eliminate the need for a separate scan in a different location.

4. Research method

   Patients who are receiving low-dose-rate brachytherapy and consent to participate will undergo the additional imaging interventions described in section 5.7 of the application.

   Patients will also undergo a pelvic CT as the standard of care.

   To limit patient exposure to additional radiation dose, the investigators will divide the participants into 3 groups, each of which will receive a portion of the abovementioned scans. Details are described in the attached protocol.

   Seed locations will be identified on the 3D C-arm volume. The prostate volume on the TRUS images will be delineated and the contours will be mapped onto the 3D C-arm volume to compute standard dosimetric parameters (V100, D90). Seeds and contours will be outlined on the corresponding Day-0 CT and standard dosimetric parameters will be computed. Day-0 CT will be contoured by multiple observers to compute variability in the resulting dosimetric parameters. Dosimetric values from the 3D C-arm will be compared to those of Day-0 CT and their intra-observer variability.

   The seeds will be identified on 3D C-arm volumes with the TRUS probe inserted and retracted to create two seed clouds. Corresponding seeds will be matched. The difference between the seed locations in the two seed clouds will be used to model the prostate deformation due to the presence and absence of the TRUS probe.

5. Statistical analysis This is mainly a descriptive one-arm study. No comparative statistical tests will be applied

Inclusion Criteria:

• Must be 18 years of age or older
• Must be able to give informed consent
• Must have been diagnosed with prostate cancer, as determined by a prostate biopsy
• Must not have had any prior history of cancer. Participants that have had some types of skins cancer are still able to participate
• Must be eligible for and receiving low-dose-rate prostate brachytherapy

Exclusion Criteria:

• They are unable to undergo general or spinal anesthesia
• They are on anticoagulation therapy (blood thinners)
• They have had previous radiotherapy to the pelvis