Comparison of prostate delineation on multimodality imaging for MR-guided radiotherapy.

Objective:: With increasing incorporation of MRI in radiotherapy, we investigate two MRI sequences for prostate delineation in radiographer-led image guidance.

Methods:: Five therapeutic radiographers contoured the prostate individually on CT, T weighted (TW) and T* weighted (T*W) imaging for 10 patients. Contours were analysed with Monaco ADMIRE (research v. 2.0) to assess interobserver variability and accuracy by comparison with a gold standard clinician contour. Observers recorded time taken for contouring and scored image quality and confidence in contouring.

Results:: There is good agreement when comparing radiographer contours to the gold-standard for all three imaging types with Dice similarity co-efficient 0.91-0.94, Cohen's κ 0.85-0.91, Hausdorff distance 4.6-7.6 mm and mean distance between contours 0.9-1.2 mm. In addition, there is good concordance between radiographers across all imaging modalities. Both TW and T*W MRI show reduced interobserver variability and improved accuracy compared to CT, this was statistically significant for T*W imaging compared to CT across all four comparison metrics. Comparing MRI sequences reveals significantly reduced interobserver variability and significantly improved accuracy on T*W compared to TW MRI for DSC and Cohen's κ. Both MRI sequences scored significantly higher compared to CT for image quality and confidence in contouring, particularly T*W. This was also reflected in the shorter time for contouring, measuring 15.4, 9.6 and 9.8 min for CT, TW and T*W MRI respectively. Conclusion: Therapeutic radiographer prostate contours are more accurate, show less interobserver variability and are more confidently and quickly outlined on MRI compared to CT, particularly using T*W MRI. Advances in knowledge: Our work is relevant for MRI sequence choice and development of the roles of the interprofessional team in the advancement of MRI-guided radiotherapy.