The role of endoscopic ultrasound in the evaluation of rectal cancer
© Siddiqui et al; licensee BioMed Central Ltd. 2006
Received: 10 October 2006
Accepted: 18 October 2006
Published: 18 October 2006
Accurate staging of rectal cancer is essential for selecting patients who can undergo sphincter-preserving surgery. It may also identify patients who could benefit from neoadjuvant therapy. Clinical staging is usually accomplished using a combination of physical examination, CT scanning, MRI and endoscopic ultrasound (EUS). Transrectal EUS is increasingly being used for locoregional staging of rectal cancer. The accuracy of EUS for the T staging of rectal carcinoma ranges from 80-95% compared with CT (65-75%) and MR imaging (75-85%). In comparison to CT, EUS can potentially upstage patients, making them eligible for neoadjuvant treatment. The accuracy to determine metastatic nodal involvement by EUS is approximately 70-75% compared with CT (55-65%) and MR imaging (60-70%). EUS guided FNA may be beneficial in patients who appear to have early T stage disease and suspicious peri-iliac lymphadenopathy to exclude metastatic disease.
Approximately 41,000 new cases of rectal cancer will be diagnosed in the year 2006 with an estimated 8,500 deaths . The prognosis and management of this malignancy is dependent upon its stage at the time of initial presentation. Previously unrecognized lymph node metastasis may present in up to 10% of T1 lesions and 17% of T2 lesions . In contrast to colon cancer, clinical preoperative tumor staging is essential since it allows selection of patients in need of neoadjuvant chemoradiation and those who may benefit from tumor load reduction to facilitate resection and potentially result in sphincter-preserving resections. Neoadjuvant chemoradiation is currently recommended for patients with advanced locoregional rectal cancer, i.e. those with tumor extension into the perirectal fat and/or involvement of the mesorectal or pelvic lymph nodes (T3, T4 N0, or Tx N1, N2) . In these patients, neoadjuvant therapy followed by surgery results in better local control , and similar or reduced toxicity when compared with standard postoperative adjuvant regimens . The Swedish Rectal Cancer Trials showed that a short-term regimen of high-dose preoperative radiotherapy reduced rates of local recurrence and improved survival among patients with resectable rectal cancer .
Currently available methods for assessment of rectal tumors include digital rectal examination, rigid proctoscopy, computer tomography (CT) scan, magnetic resonance imaging (MRI), and endorectal ultrasound (EUS). Digital examination allows for assessment of size and degree of fixation of rectal tumors. It has limited value because of its subjective nature and dependence on the examiner's experience . Rigid proctoscopy (which is usually performed in conjunction with rectal digital examination) provides the most precise assessment of tumor location and distance from the anal verge. CT scan is an excellent modality to evaluate for distant metastasis and adjacent organ involvement, but lacks specificity for loco-regional staging due to its inability to distinguish between mural layers. The staging accuracy for CT scan for rectal cancer is approximately 75% . The rigid transrectal ultrasound (TRUS) is a diagnostic modality for pre-operative staging of mid and distal rectal cancers (i.e. tumors within 10 cm from the anal verge). TRUS makes it possible for assessment of bowel thickness involvement and lymph node status. MRI has a similar accuracy to TRUS and appears to be superior in more advanced lesions [9–11]. For technical reasons a rigid transrectal US (TRUS) is less feasible for evaluation for more proximal rectal cancers.
Transrectal endoscopic ultrasound has emerged as the diagnostic modality of choice for clinical staging of rectal tumors. Because EUS can delineate the layers of the rectal wall [12–14], it is superior to CT in staging accuracy. EUS and MRI can be used as complementary methods in the preoperative staging of rectal cancer. EUS is more accurate in determining bowel wall penetration of the tumor, while MRI is comparable to EUS in the evaluation of lymph node involvement [15, 16]. In the present report, we review the current status of EUS in the diagnosis and staging for rectal cancer.
Technique of rectal EUS
Staging of rectal cancer
AJCC TNM Rectal Cancer Staging
Primary tumor can not be assessed.
No primary tumor identified.
Carcinoma in situ (tumor limited to mucosa).
Involvement of submucosa, but no penetration through muscularis propria.
Invasion into, but not penetration through, muscularis propria.
Penetration through muscularis propria into subserosa (if present), or pericolic fat, but not into peritoneal cavity or other organs.
Invasion of other organs or involvement of free peritoneal cavity.
Nodal metastasis can not be assessed.
No nodal metastasis.
1–3 pericolic/perirectal nodes involved.
4 or more pericolic/perirectal nodes involved.
Distant metastasis can not be assessed.
No distant metastases.
EUS-guided FNA does not improve the preoperative staging of rectal cancer in most patients. Harewood et al evaluated the role of EUS-guided FNA of perirectal lymph nodes in the preoperative assessment of a cohort of 80 patients with newly diagnosed, nonmetastatic rectal cancer . The overall accuracy for N staging was similar (80%) for CT, EUS, and EUS FNA with no benefit in accuracy from performing FNA. FNA may not significantly improve nodal involvement accuracy compared to EUS alone because perirectal lymph nodes are usually too small to be visualized by EUS unless they contain metastatic disease.
Accuracy of rectal US in staging rectal cancer compared with surgical pathology
The impact of rectal EUS on management
Correlation of staging with recommended treatment for rectal cancer
Invasion of mucosa and submucosa
Invasion of tumor into the muscularis propia
Invasion of tumor through the serosa
Pre-op chemoradiotherapy → resection of tumor
Invasion of tumor into adjacent organs
Pre-op chemoradiotherapy → resection of tumor → post-op chemotherapy
Rectal EUS staging after radiation therapy
Accuracy of EUS for staging rectal cancer after radiation therapy is decreased markedly due to post-radiation edema, inflammation, necrosis, and fibrosis. Studies suggest that the T-stage accuracy after radiation is 50%, with a 40% overstaging rate [27, 34]. Lymph node staging accuracy is also decreased. Thus, at this juncture, restaging tumors after neoadjuvant therapy is limited and clinical correlation is most important to dictate operative and postoperative management modalities.
Recurrent rectal cancer
Local recurrence rate after surgery alone for advanced rectal cancer is approximately 25% and decreases to 10% after radiation . The risk of recurrence is greatest in the first 2 years after surgery. Early detection of recurrent local tumor might result in earlier treatment and improved survival. The ability of CT scan is limited because if may not be able to distinguishing recurrence from postoperative change due to fibrosis or inflammation . CT images can also be obscured by artifacts from surgically placed metal clips. Two prospective studies demonstrated that EUS was superior to CT scan for local recurrence detection in rectal cancer [35, 36]. The sensitivity of EUS for detecting recurrence was higher (100 percent) in both studies compared to CT (82 to 85 percent). EUS was also more sensitive than digital rectal examination, CT and CEA levels to detect local recurrence of rectal cancer in patient who were asymptomatic from their disease . EUS specificity is limited by its inability to distinguish between mucosal inflammation and recurrence. Although the optimal interval for repeating EUS after surgical treatment of rectal cancer is unknown, performing rectal EUS every 6 months for the first 2 years after low anterior resection or transanal excision may be a reasonable screening for recurrent rectal cancer [38, 39].
The use of preoperative EUS is an accurate modality for clinical staging of rectal cancer to guide neoadjuvant treatment. EUS guided-FNA may be beneficial in patients who appear to have early T stage disease and suspicious peri-iliac lymphadenopathy. Whether the accurate staging ability of EUS and EUS guided-FNA translates into improved outcomes in terms of reduced recurrence rates and ultimately prolonged survival remains uncertain. At this juncture, the utility of FNA-guided needle aspiration for evaluation of metastatic disease remains unclear.
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