Ultrasound staging classification (uTNM) is presented in Table 7.1.

uT0 lesions are benign, noninvasive lesions confined to the rectal mucosa. Sonographically, the mucosal layer (inner black band) is expanded with an intact submucosa (middle white, hyperechoic line) (Fig. 7.3).

Benign rectal villous adenomas are classified as uT0 lesions and may be treated with local excision with excellent results. Important in this decision is to accurately exclude any focus of invasion.

The accuracy of ERUS is probably highest for T0 lesions.

uT1 lesions are early invasive cancers. uT1 lesions have invaded the mucosa and submucosa without penetrating into the muscularis propria. Sonographically this is characterized by an irregular middle white line (submucosa) without alteration of the outer black line (muscularis propria) (Fig. 7.4). Irregularities are indicated by a thickening or stippling of the submucosal layer, but there must not be a distinct break in the submucosal layer. A distinct break in the submucosal (middle white line) layer indicates invasion of the muscularis propria, hence a T2 lesion.

uT2 lesions penetrate into the muscularis propria (second hypoechoic, black line) but are confined to the rectal wall. Sonographically the hallmark finding is a distinct break in the submucosal layer. Characteristically, there is an expansion of the muscularis propria (outer black line), but the interface between the muscularis propria and the perirectal fat (the outermost white line) remains intact. The expansion of the muscularis propria may be variable depending on the degree of invasion.

“Early” uT2 lesions may just invade the muscularis propria with minimal expansion of the layer. “Deep” uT2 lesions have significant expansion of the muscularis propria (outer black line) and may appear to scallop the outer aspect of the muscularis propria but preserve the interface with the perirectal fat.

uT3 lesions penetrate the full thickness of the muscularis propria and into the perirectal fat. Contiguous structures are not involved. The sonographic appearance reveals disruption of the submucosa, thickening of the muscularis propria, and disruption of the outer hyperechoic, white line indicating penetration into the perirectal fat (Fig. 7.5). The recognition of perirectal fat invasion is an important determinant in the preoperative evaluation of the rectal cancer patient.

uT4 lesions are locally invasive into contiguous structures such as the uterus, vagina, cervix, bladder, prostate and seminal vesicles, or involve the pelvic sidewall or sacrum. They are clinically fixed and tethered. Sonographically, there is loss of the normal hyperechoic interface between the tumor and adjacent organ (Fig. 7.6).

Unfortunately, the accuracy of detecting involved lymph nodes is less than the accuracy in determining the depth of invasion. The accuracy of ERUS in detecting lymph node metastases ranges from 50 to 83 %. ERUS determination of metastatic lymph nodes is certainly more accurate than clinical (digital) evaluation as well as other imaging modalities including computed tomography (CT).

As indicated in Table 7.1, lymph node staging parallels pathologic TNM staging classifying tumors with (uN1) or without (uN0) lymph node involvement. Undetectable or benign-appearing lymph nodes are classified as uN0. Malignant-appearing lymph nodes are classified as uN1.

Normal, nonenlarged lymph nodes are usually not detectable by ERUS. Inflamed, enlarged lymph nodes appear hyperechoic with irregular borders. Lymph nodes suspicious for malignancy include larger, round, hypoechoic lymph nodes with an irregular contour.

Hypoechoic lymph nodes greater than 5 mm are highly suspicious for metastases. Involved lymph nodes are usually found adjacent to the primary tumor or within the proximal mesorectum.

ERUS differentiates two main groups of lymph nodes: hypoechoic and hyperechoic lymph nodes.

Compared with pathologic findings, hypoechoic lymph nodes represent metastases, whereas hyperechoic lymph nodes are visualized because of nonspecific inflammation.

There is no definitive size threshold to determine if an identified lymph node is malignant.

Overall, four nodal patterns are seen with differing probabilities of being involved with metastatic disease. Nonvisible lymph nodes on ultrasound have a low probability of harboring lymph node metastases.

Hyperechoic lymph nodes with nonsharply delineated boundaries are more often benign resulting from inflammatory changes. Hypoechoic lymph nodes larger than 5 mm are highly suggestive of lymph node metastases.

Mixed echogenic lymph nodes larger than 5 mm are difficult to classify but should be considered malignant.

Accurate lymph node staging of rectal cancers by ERUS relies on the experience of the examiner. False-positive results may occur because of inflammatory lymph nodes or confusing the cross-sectional appearance of perirectal blood vessels for metastatic lymph nodes.

Scanning longitudinally will distinguish between blood vessels and lymph nodes because blood vessels will extend longitudinally, change direction, and/or branch. The sonographic continuity of the hypoechoic vessel over a distance greater than the cross-sectional area is the criterion used to distinguish the two. Three-dimensional imaging can help in making this distinction.

False-negative results are also problematic in interpreting nodal involvement on ERUS. Lymph nodes harboring micrometastases are difficult to detect. Grossly malignant lymph nodes may be present outside the range of the ultrasound probe and remain undetectable. This may be the case of lateral pelvic lymph nodes such as the obturator nodes as well as those within the mesorectum beyond the proximal extent of the rigid probe.