Clinical workflow for MR-only simulation and planning in prostate

Abstract

Purpose: To describe the details and experience of implementing a MR-only workflow in the clinic for simulation and planning of prostate cancer patients.

Methods: Forty-eight prostate cancer patients from June 2016 - Dec 2016 receiving external beam radiotherapy were scheduled to undergo MR-only simulation. MR images were acquired for contouring (T2w axial, coronal, sagittal), synthetic-CT generation (3D FFE-based) and fiducial identification (3D bFFE-based). The total acquisition time was 25 min. Syn-CT was generated at the console using commercial software called MRCAT. As part of acceptance testing of the MRCAT package, external laser positioning system QA (< 2 mm) and geometric fidelity QA (< 2 mm within 50 cm LR and 30 cm AP) were performed and baseline values were set. Our current combined CT + MR simulation process was modified to accommodate a MRCAT-based MR-only simulation workflow. An automated step-by-step process using a MIM™ workflow was created for contouring on the MR images. Patient setup for treatment was achieved by matching the MRCAT DRRs with the orthogonal KV radiographs based on either fiducial ROIs or bones. 3-D CBCTs were acquired and compared with the MR/syn-CT to assess the rectum and bladder filling compared to simulation conditions.

Results: Forty-two patients successfully underwent MR-only simulation and met all of our institutional dosimetric objectives that were developed based on a CT + MR-based workflow. The remaining six patients either had a hip prosthesis or their large body size fell outside of the geometric fidelity QA criteria and thus they were not candidates for MR-only simulation. A total time saving of ~15 min was achieved with MR-based simulation as compared to CT + MR-based simulation. An automated and organized MIM workflow made contouring on MR much easier, quicker and more accurate compared with combined CT + MR images because the temporal variations in normal structure was minimal. 2D and 3D treatment setup localization based on bones/fiducials using a MRCAT reference image was successfully achieved for all cases.

Conclusions: MR-only simulation and planning with equivalent or superior target delineation, planning and treatment setup localization accuracy is feasible in a clinical setting. Future work will focus on implementing a robust 3D isotropic acquisition for contouring.

Keywords: Clinical workflow; MRCAT; Prostate cancer; Synthetic CT.

Fig. 1

Prostate MR simulation setup (immobilization, knee roll and coil positioning) on a 3 T Philips Ingenia scanner. The inset shows the Philips flat table-top that was modified to match the CT pelvis board and include markings/ruler for reproducible setup as well as removable pegs for the Aquaplast mold

Fig. 2

Flowchart explaining MR-only simulation workflow for the prostate. The dashed blocks represent the modification made to our existing CT + MR simulation workflow to accommodate a MR only workflow

Fig. 3

Contouring session in MIM displaying multiple MRs simultaneously. The display shows ROIs for PTV (pink) as well as rectal spacer (green)

Fig. 4

3D-2D registration between the MRCAT syn-CT and CT orthogonal scouts

Fig. 5

Dosimetric summary of all 42 cases planned with MR only workflow. Red horizontal lines represent our institution’s clinical objectives

Fig. 6

Treatment localization in Offline Review performed using: a 2-D kV DRRs from OBI and MRCAT syn-CT (b) 3-D match between CBCT and MRCAT syn-CT, (c) 3-D match between CBCT and MRCAT source MR using the MRCAT in-phase MR as a reference image