MR-only prostate radiotherapy – how transferable are CT based image matching skills?

Keywords: MR-only radiotherapy planning, cone beam computed tomography (CBCT) Introduction: MR-only planning has dosimetric and clinical benefits including greater geometrical accuracy, one less patient visit and smaller GTVs, potentially reducing side effects. At our centre we have delivered an MR-only radiotherapy pathway for 10 patients using MRI for target delineation, generation of synthetic CT to enable dose calculation and online verification. Research to date has used online fiducial matching with an MR-only pathway (1), yet in the UK, 66% of centres use CBCT soft-tissue matching for verification purposes (2). As the first department in the world to use online soft-tissue matching to an MRI reference image set, this study aimed to assess the transferability of radiographer skills and associated training needs for MR-CBCT soft-tissue matching. Methods: A prospective cohort study of 23 participants between February 2019 and July 2019 matched anonymised prostate images offline. Inclusion criteria were therapeutic radiographers that were prostate soft-tissue image analysis trained and excluded if had MRI acquisition experience. Each participant completed 10 CT-CBCT matches to establish a baseline, followed by 10 MR-CBCT matches. Each radiographer then received an in-house training package which included MRI principles, MRI anatomy training and considerations when matching MRI images. Finally, each participant repeated the 10 MR-CBCT matches with the images re-labelled and re-ordered. Inter-observer error was calculated as the standard deviation of the matches across all observers per patient. Results:Tabled 1Table 1. Mean (± standard deviation) of the inter-observer error across modalities.CBCT-CT (mm)MR-CBCT (mm)MR-CBCT (mm)(post training intervention)Vertical1.4 (±0.8)1.5 (±0.7)0.9 (±0.3)Longitudinal1.2 (±0.7)1.5 (±0.4)1.2 (±0.4)Lateral0.4 (±0.1)0.6 (±0.4)0.5 (±0.1) Open table in a new tab Results showed similar inter-user variability across the modalities, although variation reduced after training intervention. Conclusion and Discussion: Therapeutic radiographers require minimal training to use MRI as a reference data set for online soft-tissue image matching for prostate patients. This prepares for other treatment sites using an MR-only pathway. Numerical References (1) Maspero, M., Seevinck, P.R., Willems, N.J., Sikkes, G.G., de Kogel, G.J., de Boer, H.C. and van den Berg, C.A., 2018. Evaluation of gold fiducial marker manual localisation for magnetic resonance-only prostate radiotherapy. Radiation Oncology, 13(1), p.105. (2) Ariyaratne, H., Chesham, H. and Alonzi, R., 2017. Image-guided radiotherapy for prostate cancer in the United Kingdom: a national survey. The British journal of radiology, 90(1070), p.20160059.