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.