17TH ANNUAL SURGICAL UPDATE - RECTAL CANCER
VANCOUVER DECEMBER 2, 2000
W Douglas Wong, MD New York; Paul Bernick, MD
Preoperative staging is important in the management of rectal cancer as treatment is determined by the preoperative stage. Furthermore, reliable preoperative
tumor staging may be useful to determine prognosis. There are many therapeutic options available to the surgeon in the management of patients with operable rectal
cancer. The most important factors determining the choice of operation for rectal cancer are the level of the tumor and the preoperative tumor stage. Endorectal
ultrasound is currently the best modality to assess local tumor stage. -It can select patients appropriate for local therapy in addition to selecting appropriate
candidates for preoperative adjuvant chemoradiation therapy. Careful patient selection is required to determine the most appropriate therapeutic option for the
treatment of the rectal cancer .
The clinical evaluation of rectal cancer has relied mainly on the findings of barium enema, endoscopy and digital rectal examination. Other imaging modalities include
conventional computed tomography ( CT) and magnetic resonance imaging (MRI) .Although these imaging studies continue to be important in the evaluation of rectal cancer,
they have limitations in determining the depth of tumor invasion and presence of lymph node metastases. In this regard, endorectal ultrasound is currently the best
modality to assess local tumor stage of a rectal cancer. Endorectal coil MRI has shown equivalent accuracy, however it is more expensive and impractical for widespread
use. Clinical assessment
Clinical evaluation by digital rectal examination and proctoscopy had been the mainstay of preoperative staging of rectal lesions. Correlation between digital rectal
examination and final pathologic stage was first established by Mason. Digital rectal examination by an experienced surgeon may be quite accurate in predicting pathological
stage, particularly for advanced and fixed tumors. However, digital rectal examination is subjective and not reproducible and cannot reliably predict pathological stage
for early lesions. Moreover, the accuracy of digital examination is related to clinical experience, and digital examination is unable to assess tumors in the proximal
third of the rectum.
The accuracy of digital rectal examination versus trans rectal ultrasonography for the staging of rectal cancer was compared in a prospective, blind study by Rafaelsen
et al. They found that transrectal ultrasound was significantly more accurate in staging rectal tumors, particularly for less advanced tumors without penetration of the
rectal wall. Furthermore, in the detection of regional lymph node metastases, ERUS detected 11 of 19 lymph node metastases compared to digital examination that detected
none. Similarly, Beynon showed superior accuracy with endorectal ultrasound compared to digital rectal examination for the preoperative staging of rectal tumors. CT and
MRI Imaging
Other imaging modalities (CT, MRI) have been assessed and compared to endorectal ultrasound for the evaluation 0[ rectal tumors. Most studies have shown comparable or
superior results with the use of endorectal ultrasound for staging of rectal tumors. Computed tomography ( CT) has been useful in assessing tumor extent beyond the
rectal wall and distant metastatic disease; however, CT is unable to accurately delineate the depth of rectal wall invasion (T stage) or the presence ofmetastatic lymph
nodes. fu most comparative studies, ERUS has been shown to be more accurate than digital examination or CT . Overall accuracy of computed tomography is approximately
50-75%. Goldman et al compared CT to endorectal ultrasound and found accuracy rates of 52% and 81 %, respectively for perirectal fat invasion and 64% and 68%, respectively
for lymph node involvement. When Beynon compared the accuracy of ERUS compared to CT for both depth of tissue invasion and lymph node involvement, ERUS was significantly
more accurate than CT. In fact, accuracy rates were 68%, 82%, and 91% for 44 patients evaluated with digital examination, CT, and ERUS respectively. Waizer et al.
similarly showed that ERUS was accurate in 76% of cases compared with CT, which was accurate in only 65% of cases for assessment of rectal wall invasion. Overall
detection of depth of invasion is more accurate and reliable with endorectal ultrasound than with CT. Furthermore, endorectal ultrasound is minin1ally invasive and is
less expensive than CT .
However, ERUS should not be considered a replacement for computed tomography in the evaluation of rectal cancer. Rather, ERUS should be considered complementary to both
digital rectal examination and computed tomography. Digital rectal exan1ination is very important and is quite accurate when performed by experienced observers; however,
rectal examination is subjective, limited to the length of the examining finger, and often cannot define the upper border of the tumor. Though ERUS is more accurate than
CT in determining rectal tumor stage, CT remains an important imaging modality used to evaluate contiguous organ involvement and distant metastatic disease.
Another modality that has been evaluated for the staging of rectal tumors is magnetic resonance imaging (MRI) .Magnetic resonance imaging with endorectal coils has been
studied in a number of small studies for the evaluation and staging of rectal tumors. With the addition of endorectal surface coils to conventional MRI, spatial resolution
is increased and anatomic definition is improved. Endorectal MRI and endorectal ultrasound demonstrate equivalent efficacy in the preoperative T -staging of rectal tumors.
Overall accuracy rates of 70-90% have been reported for staging of rectal tumors using MRI with endorectal coils. In the evaluation of lymph nodes, MRI does not offer
significant improvement in accuracy rates compared to endorectal ultrasound. Though high-resolution endorectal MRI can detect pararectal lymph nodes as small as 2-3 mm
in diameter, differentiation between inflammatory and malignant nodes is imprecise Overall, endorectal MRI appears to be equivalent to endorectal ultrasound in terms of
the ac~uracy of staging of rectal tumors, particularly in regard to depth of wall invasion; however, it is unlikely that MRI will gain widespread usage because of lack
of availability and significantly increased financial costs.
Endorectal Ultrasound
Ultrasound classification of tumor stage has been proposed by Hildebrandt et al. and described according to a modification of the TNM classification. Ultrasound staging classification (uTNM) is depicted in Table I. The prefix t'u 11 denotes ultrasound staging, as opposed to the prefix 'p 11 which denotes pathological staging. Rectal carcinomas appear as hypoechoic masses with tumor extension causing disruption of subsequent layers.
Depth of tumor invasion is classified as follows: uTO lesions are benign, non-invasive lesions that are confined to the mucosa. uTI lesions are invasive tumors that are confined to the mucosa and submucosa. A uT2lesionpenetrates the muscularis propria but remains confined to the rectal wall. A uT3 lesion penetrates through the entire thickness of the bowel wall and invades the perirectal tissues. A uT4 lesion penetrates a contiguous adjacent organ (ie. uterus, vagina, cervix, bladder, prostate, seminal vesicles) or the pelvic sidewall or sacrum.
The major advantage of endorectal ultrasound over other imaging modalities is its -ability to determine the depth of tumor invasion within the bowel wall. However, endorectal ultrasound is only moderately accurate in the assessment of lymph node involvement. Accurate and reliable preoperative staging is critical to the management of rectal cancer as therapeutic options vary according to stage. The accuracy of endoluminal ultrasound for the staging of rectal cancer has been established from studies comparing preoperative ultrasound staging with the pathological staging from surgical specimens. Accuracy of endoluminal ultrasound has been reported in the range of 75 -94% for tumor depth of invasion. Overstaging by endorectal sonography has been reported in approximately 10% of patients and is thought
to result from peritumoral inflammation at the leading edge of the tumor. Understaging has been reported in approximately 5% of patients for depth of wall invasion. Understaging is considerably more serious than overstaging since inadequate management may result. With overstaging, potentially more aggressive management is advised than might be required.
Detection of lymph node involvement has been less accurate with accuracy rates of 58-83% in reported series. Solomon and McLeod pooled the raw data from eight published cross-sectional surveys to determine the accuracy of lymph node detection with endorectal ultrasound. They found a moderate correlation between endorectal ultrasound and histopathology for lymph node detection (unweighted kappa 0.58). Furthermore, positive predictive value was 74% and negative predictive value was 84%, demonstrating only a moderate accuracy for lymph node detection among combined series.
Accuracy rates have been shown to significantly improve with experience. A significant learning curve can be seen when accuracy rates are reviewed over various periods of time. Endorectal ultrasonograpy is highly operator-dependent; thus, reproducibility and reliability are dependent on the experience and expertise of the examiner. Overall staging accuracy improves with adequate training and experience, optimal technique, and high-quality equipment.
It is particularly important to recognize factors that affect the accuracy of endorectal ultrasound for the staging of rectal t1imors. There are several factors that can lead to misinterpretation of ultrasound images For example, distortion of ultrasound imaging can occur if the ultrasound probe is not at a 90- degree angle with the region of interest. Balloon-wall separation can occur and can mimic a non-existent rectal lesion. Poor bowel preparation and retained feces and air can affect the accuracy of ultrasound imaging. Furthermore, significant cautery burns from endoscopic biopsies or excisions can cause changes in the image that can significantly affect the accuracy of sonographic assessment.
Errors in the interpretation ofboth depth of invasion and nodal status may lead to both
understaging and overstaging of rectal tumors by endorectal ultrasound. Stenotic lesions may prevent the full evaluation of a rectal tumor with ERUS. As the entire lesion may not be fully imaged, understaging of the lesion may occur due to an incomplete evaluation. Three-dimensional endorectal ultrasonography is a novel technique described that may be useful to stage patients with obstructing, stenotic rectal cancers.
The use of endorectal ultrasound is unreliable following radiation therapy. As many centers currently recommend preoperative radiation therapy based on ERUS staging, some experience has been obtained with postradiation therapy ultrasound re-evaluation. Radiation therapy has been shown to downstage tumors pathologically. In fact, 10-20% of patients treated with preoperative radiation therapy will have no evidence of residual tumor in the final pathology .Radiation therapy causes edema and tissue fibrosis that makes ultrasound evaluation difficult to interpret. F ollowing radiation therapy, tissue planes are altered and one cannot accurately distinguish residual tumor from radiation therapy induced changes sonographically. Re-evaluation of rectal tumors with endorectal ultrasound following radiation therapy is inaccurate, unreliable and not recommended. In fact, all conventional imaging modalities (ERUS, CT , MRI) are unreliable at detecting complete response following radiation therapy treatment. Benefits of Pre-operative vs. Post-operative Staging -
Pre-operative staging can accurately predict rectal cancer stage so that treatment can be based on this determination. Thus, early lesions that are confined to the rectal wall with no evidence of regional lymph node metastases can be identified and selected for consideration of conservative therapy. F or those early lesions that are locally excised, final post-operative T stage as identified by pathologic assessment is the final determinant and may influence if additional therapy is necessary .Pre-operative staging is also advantageous in the identification of locally advanced rectal cancers which might benefit from pre- Wong, page 4
operative adjuvant therapy which has been demonstrated to be safer and less toxic than when administered post-operatively. The sole potential disadvantage of relying on pre-operative staging to direct pre-op adjuvant therapy is that overstaging can occur that might direct a small percentage of rectal cancer patients to have radiation +lor chemotherapy unnecessarily. Overall in my opinion the advantages of pre-operative staging for directing rectal cancer treatment outweigh the potential disadvantage and the overall accuracy as demonstrated in a number of studies supports its routine use.
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