• Magnetic Resonance Imaging
• Endorectal Ultrasound
• Rectal cancer

Purpose: To evaluate the agreement between endorectal ultrasound (ERUS) and magnetic resonance imaging (MRI) in preoperative staging of rectal cancer.

Materials and Methods: Fifty patients with rectal cancer underwent ERUS and 3-Tesla MRI for preoperative staging. With both imaging techniques were evaluated the following features: lesion site, tumour longitudinal extent, distance between lesion and puborectalis muscle, levator ani muscles infiltration, depth of extramural spread, mesorectal lymph nodes involvement and pelvic organs infiltration.
MRI evaluated also the following features: maximum thickness of the lesion, distance between lesion and mesorectal fascia and overcoming of the peritoneal reflection. All MR examinations were evaluated by two experienced radiologists.
The correlation between MRI and ultrasound data was calculated for each parameter using the Spearman rank test and considering statistically significant p-values <0.05.
The interobserver agreement for MR imaging was assessed by using the Cohen’s kappa statistics.
Eleven patients underwent directly to surgical resection without neoadjuvant therapy and the surgical resection specimen was used as standard of reference for determination of depth of invasion (T stage) and perirectal nodal involvement (N stage).

Results: ERUS and MRI showed a statistically significant correlation for the lesion site (MRI observer A vs ERUS: rs=0.873, p<0.000001/ MRI observer B vs ERUS: rs=0.8485, p<0.000001), the tumour longitudinal extent (MRI observer A vs ERUS: rs=0.378, p=0.010393/ MRI observer B vs ERUS: rs=0.3794, p=0.010131), the distance between lesion and puborectalis muscle (MRI observer A vs ERUS: rs=0.7954, p<0.000001/ MRI observer B vs ERUS: rs=0.7989, p<0.000001) and the depth of extramural spread (MRI observer A vs ERUS: rs=0.5107, p=0.000149/ MRI observer B vs ERUS: rs=0.5046, p=0.000186).
Moreover, TRUS and MRI were able to demonstrate the levator ani muscles infiltration with an overall agreement of 82% for MRI reader A and 80% for MRI reader B, the lymph nodes involvement with an agreement of 68% for MRI reader A and 76% for MRI reader B and the pelvic organs infiltration with an agreement of 80% for both MRI reader.
MRI allowed, however, the evaluation of other very important features for the correct staging of rectal cancer, in particular the distance between lesion and mesorectal fascia.
The interobserver agreement between MRI reader A and B was almost perfect for the lesion site and the distance between lesion and puborectalis muscle, there was a substantial agreement for the tumour longitudinal extent, the depth of extramural spread and the maximum thickness of the lesion; instead there was only a moderate agreement for the distance between lesion and mesorectal fascia.
There was also an agreement between the two observers of 100% for the pelvic organ involvement, of 96% for the overcoming of the anterior peritoneal reflection, of 88% for the mesorectal lymph nodes involvement and of 82% for the levator ani muscles infiltration.

Conclusion: The good agreement between MRI and TRUS in preoperative staging of rectal cancer argues in favor of the use of MRI, because it also allows a more comprehensive local assessment.