A new workflow of the on-line 1.5-T MR-guided adaptive radiation therapy

Abstract

Purpose: The aim of this study was to develop a new workflow for 1.5-T magnetic resonance (MR)-guided on-line adaptive radiation therapy (MRgART) and assess its feasibility in achieving dose constraints.

Materials and methods: We retrospectively evaluated the clinical data of patients who underwent on-line adaptive radiation therapy using a 1.5-T MR linear accelerator (MR-Linac). The workflow in MRgART was established by reviewing the disease site, number of fractions, and re-planning procedures. Five cases of prostate cancer were selected to evaluate the feasibility of the new workflow with respect to achieving dose constraints.

Results: Between December 2021 and September 2022, 50 consecutive patients underwent MRgART using a 1.5-T MR-Linac. Of these, 20 had prostate cancer, 10 had hepatocellular carcinoma, 6 had pancreatic cancer, 5 had lymph node oligo-metastasis, 3 had renal cancer, 3 had bone metastasis, 2 had liver metastasis from colon cancer, and 1 had a mediastinal tumor. Among a total of 247 fractions, 235 (95%) were adapt-to-shape (ATS)-based re-planning. The median ATS re-planning time in all 50 cases was 17 min. In the feasibility study, all dose constraint sets were met in all 5 patients by ATS re-planning. Conversely, a total of 14 dose constraints in 5 patients could not be achieved by virtual plan without using adaptive re-planning. These dose constraints included the minimum dose received by the highest irradiated volume of 1 cc in the planning target volume and the maximum dose of the rectal/bladder wall.

Conclusion: A new workflow of 1.5-T MRgART was established and found to be feasible. Our evaluation of the dose constraint achievement demonstrated the effectiveness of the workflow.

Keywords: Adapt to shape; Adaptive radiation therapy; MR-Linac; MRgART; Workflow.

Fig. 1

Elekta Unity 1.5-T MR-Linac system. The internal structure consists of a linear accelerator rotating gantry placed outside of the MRI magnets

Fig. 2

Workflow of the MR-guided on-line adaptive radiation therapy (MRgART). The left side of the figure shows the off-line procedure of creating an MR-Linac reference plan prior to the treatment start day. On-line workflow of the adaptive planning was depicted on the right half. Some of the key steps of the on-line MRgART are circled in red. a Pre-treatment MR imaging with the MR-Linac system. b Re-planning with modification of targets, risk organs, or body surface contours (ATS: adapt to shape) and re-planning without any contour modification (ATP: adapt to position). c Evaluation of dose distribution and DVH and assessing achievement status of dose constraints. d Reconfirm the location of the tumor and risk organs with MRI just prior to irradiation. e Irradiation while confirming beam-on imaging with real-time cine MR. f MR images are also taken immediately after the end of irradiation for confirmation

Fig. 3

Comparison between the dose distribution and the dose-volume histogram in actual ATS plan and a virtual plan in patient with prostate cancer. All dose constraints are met in the actual ATS (Plan 1), but neither the minimum dose received by the highest irradiated volume of 1 cc (D1cc) in the planning target (PTV) volume nor the maximum dose (Dmax) in the rectal/bladder wall are achieved in the virtual plan (Plan 2). In the virtual plan, both Dmax of the rectal and bladder wall are nearly 110% of the prescribed dose (yellow dotted circles)