Current controversy, confusion, and imprecision in the use and interpretation of rectal MRI

Abstract

There has been a rapid increase in the utilization of MRI in rectal cancer staging in the USA essentially replacing endorectal ultrasound and mimicking the trend in Europe seen in the 1990s and 2000s. Accompanying this trend, there is a demand, and recognized need, for greater precision and clarification of confusing, misunderstood and poorly understood concepts, facts, statements and nomenclature regarding rectal cancer and the use of pelvic MRI for diagnosis. As such, this Review, part evidence-based and part expert opinion, will attempt to elucidate and clarify several concepts the authors have encountered in 25 years of imaging rectal cancer, focusing on MRI.

Keywords: Opinion; Rectal Cancer; Restaging; Staging.

Figure 1.

Sagittal T2 weighted MRI of the pelvis in 42-year-old male with a polypoid rectal adenocarcinoma. Measurement of height of tumor from lower border of external anal sphincter [EAS] (corresponding to histologic anal verge), yellow lines. Measurement of height of tumor from bottom of internal anal sphincter [IAS] (often corresponding to surgical anal verge due to anesthesia induced skeletal muscle relaxation or forcible manual separation of EAS at time of sigmoidoscopy. Difference in these distances is only about 1-cm and represents the intersphincteric notch (asterisk).

FIGURE 2.

Axial T2 weighted elvic MRI at multiple levels. Anatomical classification of lateral lymph nodes in 2 different middle-aged male patients with rectal cancer (primary lesions not shown). A (proximal nodes) and B (distal nodes): Red-shaded areas are external iliac nodes, blue shaded areas are obturator nodes and green shaded areas are internal iliac nodes. Division between obturator and internal iliac represented by vertical plane through axis of internal iliac vessel. Lateral to this are obturator nodes .EIA = external iliac artery, EIV = external iliac vein, ob a/v/n = obturator artery//nerve, IOM = internal obturator muscle, int pud a/v = internal pudendal artery/vein, IIV = internal iliac vein, IIA = internal iliac artery. C and D, malignant pelvic sidewall nodes: Enlarged heterogeneous obturator nodes on the left in C, (solid arrow). Enlarged minimally heterogeneous left obturator node in D not removed initially leading to pelvic sidewall recurrence 9 months following surgery (solid arrow). Clinically unimportant posterior external iliac elongated lymph nodes (dashed arrow). IOM = internal obturator muscle. Int pud a/v = internal pudendal artery and vein 9enclosed in dotted circle), EIV = external iliac vein. IIA/V = internal iliac artery and vein.

FIGURE 3.

Pelvic MRI in middle-aged female with recurrent mucinous rectal adenocarcinoma. A: Axial T2 weighted image. High T2 signal intensity mass in mid pelvis and asymmetric appearance of piriformis muscles. More normal appearing on the left. High T2 weighted soft tissue along right sciatic notch seems to represent fat due to its iso-intensity. B: Fat -saturated T1 weighted contrast-enhanced image. Note enhancement around central mass and tumor extending out of sciatic notch, now more conspicuous.