Verification imaging in prostate MR-only radiotherapy: Are fiducial markers necessary?

Purpose/objective: MR-only radiotherapy planning exploits the benefits of MRI soft-tissue delineation, whilst negating the registration inaccuracies caused by MRI CT fusion. Fiducial markers have conventionally been used in prostate radiotherapy to reduce on-treatment image matching variability. However, this is an invasive procedure for the patient, and presents technical difficulties in an MR-only pathway as fiducial markers are difficult to visualise on MRI. This study compares MR-CBCT soft-tissue matching to fiducial matching in an MR-only prostate pathway. Material/methods: Four therapeutic radiographers reviewed first fraction CBCTs for 25 patients. The CBCT was compared to the planning MRI, a T2 weighted sequence for the soft-tissue match and compared to a T1 weighted MRI sequence for the fiducial match. Inter-observer variability was quantified using the inter-observer error and 95 % limits of agreement from a modified Bland-Altman analysis. Accuracy of the soft-tissue match was quantified by calculating the difference from the fiducial match. Results: Limits of agreement on the MR soft-tissue match were 1.5 mm, 4.0 mm, 3.5 mm and fiducial match 2.5 mm, 3.6 mm, 2.5 mm (lateral, longitudinal, vertical). Inter-observer error (+/- standard deviation) on the MR soft-tissue match were 0.6(+/- 0.5) mm, 1.8(+/- 1.1) mm, 1.7(+/- 0.7) mm and fiducial match 0.7(+/- 1.1) mm, 1.1(+/- 1.5) mm, 0.8(+/- 0.7) mm (lateral, longitudinal, vertical). The difference of the soft- tissue match from the fiducial match was 0.3(+/- 1.1) mm,-0.1(+/- 2.7) mm, 0.1(+/- 1.9) mm (lateral, longitudinal, vertical). Conclusion: MR-CBCT soft-tissue matching has similar accuracy and inter-observer variability as fiducial matching. This suggests fiducial markers are not necessary in an MR-only prostate radiotherapy pathway. Implications for practice: MR-only prostate radiotherapy does not require fiducial markers since MR-CBCT soft tissue matching can be used for IGRT. (c) 2024 The Authors. Published by Elsevier Ltd on behalf of The College of Radiographers. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).