The radiation therapy department at Turku University Hospital (TYKS) has been home to a Philips Ingenia MR-RT 1.5T since January 2016. This Ingenia is a workhorse for the department, as about 60% of the MRI scans performed are to aid in planning the hospital’s 1,600 radiotherapy patients per year, with cardiac and emergency diagnostic scans making up the balance. Approximately one in five of the radiotherapy cases are prostate cancer patients and for those, MRI simulation is part of the standard clinical care.
In its traditional CT-MRI simulation and planning workflow, Ingenia has been invaluable for its superb, tunable soft-tissue contrast, particularly for planning prostate cancer cases, according to Prof. Heikki Minn, MD, Radiation Oncologist at TYKS.
“For prostate visualization, the high soft tissue contrast that MRI provides is very important,” he says. “Without it, we wouldn’t be able to distinguish the gland from the pelvic muscles and fat surrounding it. That would make it difficult to delineate the prostate and other nearby tissues. We see that our clinical target volume (CTV) for prostate is typically smaller for MRI-based planning, which can help decrease toxicity to the rectum and bladder.
The value of MRI for radiotherapy planning drove the acquisition of the Ingenia for the radiotherapy department. “We wanted to have an MRI in our own department instead of having to use the radiology department’s system, so we can assure our patients get MRI access when it’s feasible for our workflow,” Dr. Minn says.
“Using MR-only simulation and planning would give us a ‘onestop- shop’ workflow for imaging, contouring and planning of radical radiotherapy for patients with prostate cancer,” he says. “Therefore, we began the commissioning process for MR-only simulation immediately in the same month that we began using our Ingenia.”
MR-only simulation workflow
The 3D T1W FFE mDIXON sequence provides in-phase, water and fat images in one acquisition. Target and organs-at-risk are delineated on the 3D T2W TSE images. Prostate GTV is shown in orange, PTV in purple. The 3D bFFE sequence is used by the planner to mark the position of the fiducial markers (gold anchors) and contours are transferred to the digitally reconstructed radiographs (DRRs).
Based on the 3D T1W mDIXON images, MR-based density maps (MRCAT) are automatically generated. The VMAT (Volumetric Modulated Arc Therapy) plan is generated in TPS, based on MRCAT as primary image set. During the commissioning phase, dosimetric agreement between MRCAT-based and CT-based dose plans was studied and differences in the PTV dose were found to be minimal (<1% for most patients). Average difference in PTV mean values was 0.8% over the study group (n=62).
Philips Ingenia MR-RT 1.5T using the Anterior coil and the integrated Posterior coil.
The Philips MRCAT approach for an MR-only based dose planning provides CT-like density information calculated from MR images for dose calculations, as well as high contrast anatomical T2W images for target delineation. [2-3]
Electron density information is automatically generated on the MR console from a single T1W mDIXON MRI sequence in two steps:
The MRCAT images with density information are DICOM conform and can be automatically exported to treatment planning systems (TPS) as primary image dataset for dose calculations and to generate MR-based digitally reconstructed radiographs (DRRs).
Several Philips users have described their experiences with MRCAT in peer-reviewed publications. [1,4-5]
1. Kemppainen R, Suilamo S, Tuokkola T, et al. Magnetic resonance-only simulation and dose calculation in external beam radiation therapy: a feasibility study for pelvic cancers. Acta Oncologica, 56(6), 2017.
2. Köhler M, Vaara T, van Grootel M, et al. MR-only simulation for radiotherapy planning – White paper: Philips MRCAT for prostate dose calculations using only MRI data. 2015.
3. Schubert G, Vaara T, et al. Commissioning of MR-only simulation for radiotherapy planning – White paper. www.philips.com/mronlysim. 2017.
4. Christiansen RL, Jensen HR, Brink C. Magnetic resonance only workflow and validation of dose calculations for radiotherapy of prostate cancer. Acta Oncologica, 56(6), 787-791, 2017.
5. Tyagi N, Fontenla S, Zhang J, et al. Dosimetric and workflow evaluation of first commercial synthetic CT software for clinical use in pelvis. Phys Med Biol., 62(8), 2961-2975, 2017.
