The split scar sign has been described as a feature on rectal cancer MRI studies acquired following chemoradiotherapy and having a high specificity and positive predictive value for a complete response 1. In a recent meta-analysis that observed an average complete response rate of 38%, the presence of the split scar sign would indicate a positive predictive value of approximately 83% for a complete response ².
It is identified on high resolution T2 weighted imaging and refers to the presence of low signal on the inner wall of the rectum at the site of the tumor, corresponding to fibrosed submucosa, with a layer of intermediate signal intensity immediately external to this, representing the muscularis propria. The outermost layer is low signal perirectal fibrosis, although this may be absent with tumors that have not breached the muscularis propria, i.e. T2 or less. The scar, therefore, refers to the low signal fibrotic tissue in the submucosa and the immediate perirectal fat, with the split representing intermediate signal tissue of the muscularis propria.
There may be intermediate signal tissue on the surface of the inner scar, representing mucosa. In the case of mucinous tumors, the central layer may be high signal on T2 weighted imaging.
The sign forms one of the tools used to assess whether patients have achieved a complete response following neoadjuvant chemoradiotherapy, alongside the volume of intermediate signal on T2 weighted imaging, and restriction of diffusion.
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