MRI anatomy of the rectum: key concepts important for rectal cancer staging and treatment planning

Insights into Imaging

Table 1 Tumour (T) staging in rectal cancer

T1	Tumour invades submucosa
T2	Tumour invades muscularis propria
T3	Tumour invades through muscularis propria into perirectal fat
T3	T3a: < 1 mm T3b 1–5 mm T3c > 5–15 mm T3d > 15 mm
T4a	Tumour invades peritoneum or peritoneal reflection
T4b	Tumour invades adjacent organs or structures^a
T4b	Bone, striated muscle (incl. external anal sphincter, pelvic floor, piriformis), ureters, urethra, nerves, vessels outside mesorectal compartment, any loop of small/large bowel other than loop from which the tumour originates, any fat in anatomical compartment outside the mesorectum (obturator, para-iliac, ischiorectal space)

^aDefinitions for structures to be included in the definition of T4b disease were derived from a recent publication by Lambregts et al. on controversies in TNM staging [26]
Further definitions are derived from the 8th edition of the AJCC/UICC tumour node metastases (TNM) staging manual and the 2017 ESMO guidelines on the clinical management of rectal cancer [4, 15]