Session Item

For pelvic radiotherapy the imaging standard for treatment planning is CT-based delineation supported by co-registered MR scans for soft tissue contrast. In MR-only treatment planning the CT scan is replaced by using the MR images to simulate a CT (sCT) with correct HU for dose calculation. In June 2020 we introduced MR-only treatment planning as our clinical standard for curative pelvic radiotherapy. Here we present our experiences with the MR-only workflow for scanning, delineation, treatment planning and daily IGRT based on nearly 400 treated patients with rectal, anal or prostate cancer.

We use the Philips MRCAT pelvis suite (Philips Healthcare) to generate the sCT. We present the experiences of radiographers, clinicians, RTTs and physicist in the department collected during daily clinical use.

MR-only reduces the CT workload at the cost of a little more time spent per MR scan. Initially gains in staff- and scanner time were reduced by sCT reconstruction failing e.g. in case of algorithm errors or patients unfit for the MR-only workflow, requiring unplanned CT scans. Staff training and better patient selection, along with improved software, has improved the success rate to ~95%. Patients spend significantly less time being scanned, especially for patients in bladder filling protocols this improved the comfort.

We currently perform delineations on sCT using MR to discriminate between tumour and surrounding soft tissue. This benefit from the minimal organ motion between sCT and MR compared to a standard CT and MR workflow. However, it causes two challenges: First, bones are not always perfectly reconstructed in the sCT, leading to risk of incorrect delineation. Second, the lack of contrast enhancement on sCT, makes it more difficult to separate bowel loops, muscle and vessels. Both problems are alleviated by using the MR images as a reference, but is a little more time consuming compared to regular, contrast enhanced CT.

The sCT is a drop-in replacement for real CT for treatment planning and dose calculation. The differences in calculated dose between sCT and real CT are clinically insignificant at <0.5 Gy in high dose (60-80 Gy) regions. The deviations in bone segmentations are too small to affect photon dose calculation.

For daily imaging during treatment the sCT again is a drop-in replacement for CT. Our departments Elekta linacs accept the sCT and all automated match algorithms work as well as for real CT. Only minimal training of RTTs was necessary to use the sCT for daily IGRT. The main challenge is errors in bone segmentation, which will show up strongly when compared to CBCT. This previously caused some false alarms on changed anatomy, but are now quickly resolved based on experience.

MR-only treatment planning has increased the number of available timeslots on CT, reduced the time patients spend being scanned, but for the department to get full benefits requires effort to optimize the scanning and delineation workflows and training.