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

Abstract

To compare maximum tumour diameter (MTD) and gross tumour volume (GTV) measurements between T₂-weighted (T₂-w) and diffusion-weighted (DWI) MRI in squamous cell carcinoma of the anal canal (SCCA) and assess sequence impact on tumour (T) staging. Second, to evaluate interobserver agreement and reader delineation confidence. The staging MRI scans of 45 SCCA patients (25 females) were assessed retrospectively by two independent radiologists (0 and 5 years' experience of anal cancer MRI). MTD and GTV were delineated on both T₂-w and high-b-value DWI images and compared between sequences; T staging was derived from MTD. Interobserver agreement was assessed and delineation confidence scored (1 to 5) by each observer. GTV and MTD were significantly and systematically lower on DWI versus T₂-w sequences by 14.80%/9.98% (MTD) and 29.70%/12.25% (GTV) for each reader, respectively, causing T staging discordances in approximately a quarter of cases. Bland-Altman limits of agreement were narrower and intraclass correlation coefficients higher for DWI. Delineation confidence was greater on DWI: 40/42 cases were scored confidently (4 or 5) by each reader, respectively, versus 31/36 cases based on T₂-w images. Sequence selection affects SCCA measurements and T stage. DWI yields higher interobserver agreement and greater tumour delineation confidence. • 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.