Figure 24:

66-year-old man with primary rectal cancer 2.2 cm from the anal verge. (a–c) External facility baseline MRI included axial oblique 3-mm slice T2WI showing a nodular tumor at the anorectal junction (a; arrow), b1000 3-mm slice DWI showing restriction (b; arrow), and ADC mapping showing findings corresponding to that of DWI (c; arrow). (d–g) Post-CRT axial T2WI shows a scar (d), but with a bright signal in the tumor bed (e; arrow) that is from the mucosa, not the wall as shown by the bright signal on the ADC map, indicating a T2 effects (f; arrow). Endoscopy indicates clinical complete response (g). (h–k) 3 months later, axial T2WI shows no change in the scar (h; arrow), but on b800 DWI/ADC, a new/different pattern emerged with signal at the periphery, not in mucosa, indicating restriction suspicious for tumor regrowth (i, j; arrows). However, endoscopy (k) reveals only “scarring and radiation proctitis”. This represents MRI/endoscopy discordance which usually means MRI is falsely positive (80% of time in our experience). (l–o) Further follow-up imaging at 2 months shows a slightly bulkier scar with some intermediate T2 signal (l; arrow) and a further increase in DWI signal in SAME area (m; arrow), with greater dark signal in the corresponding area on the ADC map (n; arrow). Now endoscopy reveals an obvious tumor (arrows), concordant with MRI (o). The patient underwent abdominal perineal resection, pT3N0, and now is with no evidence of disease.

TEACHING POINT: Tumor regrowth may be seen beneath the mucosal surface by MRI and not by endoscopy in up to 20% of cases of positive DWI signal but negative simultaneous endoscopy. This means that in most discrepant pairings, endoscopy is correct.