The impact of MRI sequence on tumour staging and gross tumour volume delineation in squamous cell carcinoma of the anal canal.

• MTD and GTV measurements are significantly lower on DWI than on T 2 -w MRI. • Such differences cause T staging discordances in up to a quarter of cases. • DWI results in higher agreement between inexperienced and experienced observers. • DWI offers greater tumour delineation confidence to inexperienced readers.