Rectal Cancer Diagnostic Workup

Adenocarcinoma is the most common rectal malignancy. The diagnosis is usually histologically established prior to imaging. Cross-sectional imaging is used to stage the tumor. The treatment of rectal cancer depends on the extent of disease and has changed substantially over the last decade. EUS and MRI are important in the management of these patients. T2-weighted sequences are pivotal in MRI of rectal cancer. These provide optimal contrast between the rectal wall and surrounding fat and organs. T1-weighted sequences after intravenous contrast medium administration do not provide much additional information. No bowel preparation is necessary.

Rectal tumors are identified as lesions with intermediate signal intensity, while stool often has an inhomoge-neous low signal intensity at T2-weighted sequences. Tumors present as a mass, but may also have a more superficial extent. Mucinous tumors have a high signal intensity.

Tumor limited to (sub)mucosa (T1 disease) is often treated by transanal endoscopic microsurgery. EUS is preferable for local staging of these tumors. The higher spatial resolution of EUS as compared to MRI results in a more detailed demonstration of the rectal wall layers. EUS depicts five interfaces, while MRI shows a maximum of two interfaces. This leads to a significantly higher (p = 0.02) specificity of EUS (86%) as compared to MR imaging (69%), indicating overstaging of T1 (or lower) tumors with MR imaging [21]. Endoluminal US and MR imaging have similar sensitivity estimates of 94% for these tumors.

The tumor often invades deeper than the (sub)mucosa and may extend into the lamina propria (T2), beyond the muscularis propria (T3) (Fig. 8) or even involve surrounding structures (T4). For muscularis propria invasion, EUS and MR imaging have similar sensitivities; specificity of EUS (86%) is significantly higher than that of MR imaging (69%) [21]. For perirectal tissue invasion, sensitivity of EUS (90%) is significantly higher than that of CT (79%) and MR imaging (82%); specificities are comparable (75%, 78%, and 76%). For adjacent organ invasion and lymph node involvement (Fig. 9), estimates for EUS, CT, and MR imaging are comparable (70%, 72%, 74%, respectively). Specificity estimates are also comparable (97%, 96%, 96%). Stenotic tumors may hamper endosonography.

The importance of T-staging has decreased with the widespread use of total mesorectal excision (TME). In this procedure, the rectal cancer and the surrounding envelope of mesorectal fat bordered by the mesorectal fascia is removed en bloc. This operation has been shown to

Mesorectal Fat

Fig. 8. Coronal oblique T2-weighted turbo spin-echo of a rectal tumor (T) extending through the rectal wall into the mesorectal fat. A spiculated margin (arrowhead) is seen, where imaging cannot differentiate between desmoplastic tumor reaction without or with tumor. There is several centimeters distance between the tumor margin (including spiculations) and the mesorectal fascia (arrow) and therefore total mesorectal excision is possible. There are not any enlarged lymph nodes

Fig. 8. Coronal oblique T2-weighted turbo spin-echo of a rectal tumor (T) extending through the rectal wall into the mesorectal fat. A spiculated margin (arrowhead) is seen, where imaging cannot differentiate between desmoplastic tumor reaction without or with tumor. There is several centimeters distance between the tumor margin (including spiculations) and the mesorectal fascia (arrow) and therefore total mesorectal excision is possible. There are not any enlarged lymph nodes

Total Mesorectal Excision

Fig. 9. Axial T2-weighted turbo spin-echo of a rectal tumor (T) with multiple mesorectal lymph nodes. In general, imaging is not an accurate technique for differentiating between malignant and benign lymph nodes. However, in this patient one lymph node (L) extends through the mesorectal fascia (curved arrow) and one presacral lymph node (S) has invaded the sacrum, indicating malignancy loaded lymph nodes. Normal mesorectal fascia at the left (arrowhead)

Fig. 9. Axial T2-weighted turbo spin-echo of a rectal tumor (T) with multiple mesorectal lymph nodes. In general, imaging is not an accurate technique for differentiating between malignant and benign lymph nodes. However, in this patient one lymph node (L) extends through the mesorectal fascia (curved arrow) and one presacral lymph node (S) has invaded the sacrum, indicating malignancy loaded lymph nodes. Normal mesorectal fascia at the left (arrowhead)

reduce significantly tumor recurrence rate. For this treatment, determining the distance between tumor and mesorectal fascia (circumferential resection margin; CRM) has become crucial. TME was introduced due to the high initial local tumor recurrence (approximately 28%). With TME the recurrence rate has decreased to 1015%. In Europe, mobile rectal cancers are treated with preoperative radiotherapy before TME and this has led to a further decrease of recurrence, especially for cancers in the mid-rectum (3.9% vs 14.9%; p = < 0.001).

MRI can accurately determine the distance between tumor and mesorectal fascia and differentiate between patients with a wide circumferential resection margin, close margin and involved margin, with accuracy rates of 92-100% and high reproducibility (Fig. 8 and 9) [22, 23]. Low rectal cancers and anterior cancers are more difficult to assess than other rectal cancers, as the mesorectal fascia is less readily identified.

Detection of nodal disease is important since it is a predictor for distant metastases and local recurrence. In rectal cancer, lymph drainage is upward along the superior rectal vessels to the inferior mesenteric vessels, while the lower rectum drains along the middle rectal vessels to the internal iliac vessels. In patients treated with TME, perirectal nodes are removed, but lateral nodes may lead to recurrent disease. Lateral lymphatic spread is more prevalent in low rectal cancer. To reduce the local recurrence rate, radiotherapy and dissection of the lateral nodes is used, although there has been little interest shown in the latter treatment. Nevertheless, improved preoperative identification of lymphatic spread is a major step forward in the treatment of rectal cancer.

The accuracy of MRI, EUS, and CT with respect to lymph node staging is disappointing. The size criterion used to differentiate between benign and malignant nodes has serious limitations [24]. In a systematic review, sensitivity estimates for EUS, CT, and MR imaging were comparably low: 67%, 55%, and 66%, respectively [19]. Specificity values were also comparable: 78% for EUS, 74% for CT, and 76% for MR imaging. Improved lymph node criteria, such as presence of a spiculated border, indistinct border and mottled heterogenic pattern strongly correlate with tumor invasion (p < 0.001), but still have limitations [25]. The use of ultra small iron oxide particles (USPIO) has shown promising results in differentiating normal sized nodes that contain tumors [26].

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Responses

  • Facondo
    What is mesorectal fascia?
    8 years ago
  • Giordano
    What is perirectal extending through mesorectal mean?
    6 years ago

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