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1
Department of Radiology, University of Vienna, Waehringer Guertel 18-20,
A-1090 Vienna, Austria.
2
Department of Surgery, University of Vienna, A-1090 Vienna, Austria.
3
Department of Clinical Pathology, University of Vienna, A-1090 Vienna,
Austria.
Received December 17, 1999;
accepted after revision February 16, 2000.
Address correspondence to A. G. Maier.
Abstract
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SUBJECTS AND METHODS. Thirty-eight patients (14 women, 24 men; mean age, 65 years) with lower rectal cancer underwent transrectal sonography before surgery. Both depth of infiltration and tumor invasion into the anal canal were assessed, and results were compared with histopathology of the resected specimens.
RESULTS. Infiltration into the anal canal was found histopathologically in 12 (32%) of 38 patients. Transrectal sonography revealed a true-positive diagnosis in 11 of these 12 patients. A false-positive diagnosis of anal canal infiltration was made in two patients for a sensitivity of 91%, a specificity of 85%, and an accuracy of 92%. In the 11 patients diagnosed correctly on transrectal sonography, the depth of tumor infiltration into the anal canal corresponded with histopathology.
CONCLUSION. This study shows that transrectal sonography is an accurate method for assessment of anal canal infiltration in lower rectal cancer.
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However, for surgery of tumors with a distal edge below 8 cm from the anal verge (i.e., the distal rectal third), a special problem arises. The mesorectum that surrounds the upper and middle rectum tapers out distally, leaving the last few centimeters of rectum above the anorectal junction covered only by connective tissue. This anatomical situation provides a narrow margin of safety circumferentially. Accordingly, it is of the utmost importance to know before the operation the depth of tumor invasion into the bowel wall and beyond, especially close to the anorectal junction.
Digital examination and functional proctoscopy give only indirect evidence regarding infiltration of the levators or the anal canal [6, 7]. Tumor infiltration into the anal sphincters precludes a resection that preserves the anal sphincters, making an abdominoperineal resection with permanent colostomy necessary. Transrectal sonography is now regarded as useful for determination of the depth of transversal invasion of rectal cancer with an accuracy of 60-94% [8,9,10,11]. Only one recent study has shown that, in addition to circumferential invasion, distal submucosal spread of rectal cancer may be determined on transrectal sonography [12].
It was the aim of this study to assess the accuracy of transrectal sonography in determining invasion of distal rectal cancer into the anal sphincters and surrounding organs by of comparring results of these techniques with the histopathology of resected specimens.
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All transrectal sonography was performed routinely with commercially available sonographic scanners (Diagnostic Ultrasound Systems 1846 and 3535; B&K Medical, Naerum, Denmark). Both scanners used a mechanical rotating endoprobe (type 1850, 5-7 MHz) with a focal zone (penetration depth) of 2-4 cm, an axial resolution of 0.4 mm, and a lateral resolution of 1.0 mm. A 360° display of the rectal wall and surrounding tissue was provided. A latex balloon was attached to the end of the probe over the transducer. After a cleansing enema, the patients were placed in the lithotomy position. The endoprobe was inserted blindly, the balloon was then filled with degassed water, and the probe was slowly withdrawn while the rectal wall was being scanned. For scanning of the anal canal, the balloon was replaced with a small hard plastic cap. The upper end of the anal canal was defined as being at the level of the puborectalis muscle.
The normal sonographic appearance of the mucosa and the sphincter apparatus is shown in Figure 1A,1B.
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The transrectal sonographic classification of tumor invasion into bowel wall (uT) corresponds to the pathologic classification (pT) of the TNM system, in which uT1 represents mucosal or submucosal disease, uT2 is disease involving the hypoechoic muscularis propria, uT3 is disease extension into the hyperechoic perirectal fat, and uT4 is disease into adjacent organs or sidewall structures.
Real-time transrectal sonograms were interpreted by one observer. Hard-copy images of the investigations were also reviewed by a second radiologist. Discrepancies were resolved by consensus. Histopathologic examinations were assessed by a single observer who was unaware of the transrectal sonographic findings.
Preoperative transrectal sonographic results were compared with the histopathologic results of the resected specimens from all patients. For histopathology, all surgical specimens were dissected as outlined by Hruban et al. [13]. After overnight fixation in neutral buffered formalin (7.5%), the entire tumor was transversely dissected into 4-mm slices. All tumor slices were blocked sequentially, dehydrated in a series of graded alcohol, and embedded in paraffin wax. Two 4-µm sections of each paraffin block were stained with H and E.
The depth of local tumor spread was classified histopathologically according to the TNM classification [14] as follows: pT1 is tumor confined to the submucosa, pT2 is tumor confined to the muscularis propria, pT3 is tumor that penetrates through the rectal wall into the perirectal fat or the subserosa, and pT4 is tumor that invades adjacent organs or structures or penetrates the serosa. Infiltration of the mucosa beyond the anorectal junction and penetration into sphincter muscles were noted separately. The distal resection margin was measured on the formalin-fixed specimens. In cases of abdominoperineal excision, the distance of tumor from the perianal skin incision was recorded.
Of 38 patients, 17 underwent preoperative hyperfractionated radiotherapy with 25 Gy over 5 days [15]. Surgical procedures included 19 abdominoperineal excisions, 15 low-anterior resections, three transanal disk excisions, and one intersphincteric resection.
Differences between groups were analyzed using the two-tailed t test with Yates correction and the chi-square test as appropriate. A p value of less than 0.05 was considered significant.
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In 12 patients (32%), infiltration into the anal canal was histopathologically verified (Table 1). Eleven patients (92%) were correctly diagnosed on transrectal sonography. In two patients, a false-positive diagnosis of tumor infiltration of the anal sphincter was made on transrectal sonography for a sensitivity of 91%, a specificity of 85%, and an accuracy of 92%.
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In the 11 patients diagnosed correctly on transrectal sonography, the depth of tumor infiltration into the anal canal corresponded to histopathology. One case of tumor infiltration of the anal canal mucosa was seen (Fig. 2A,2B). Seven cases of mucosa and internal sphincter muscle infiltration were found (Fig. 3A,3B), and in three cases the mucosa and the internal and external sphincter muscles were infiltrated (Fig. 4). No isolated submucosal tumor spread was found in any of these patients.
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Discrepancies in interpretation between the two observers occurred in five patients. These discrepancies were always resolved in favor of the real-time observer, resulting in two true-positive and three true-negative diagnoses.
In all 17 patients who received preoperative radiotherapy of 25 Gy, the staging and assessment of anal canal infiltration made on transrectal sonography correlated with the histopathologic results.
All patients with infiltration and seven additional patients without infiltration into the anal canal underwent an abdominoperineal excision. The parameters of tumor grading and distal resection margin were investigated separately. The distance between the lower margin of the tumor and the anal verge was 2.4 cm in tumors that invaded the anal canal and 4.14 cm in tumors without infiltration into the anal canal (p = 0.0007). No statistically significant differences occurred between the grading of the tumors with or without anal canal infiltration (p = 0.78).
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A number of studies have established the validity of transrectal sonography as a means of tumor staging; and for the assessment of invasion depth, an accuracy of 60-94% has been reported [8,9,10,11].
In addition, longitudinal tumor spread is of importance for tumors located in the very distal rectum. Although longitudinal spread has been reported in about 26% of cases on the basis of histopathologic examination of surgical specimens [3, 16,17,18], data concerning the pre-operative assessment of longitudinal tumor spread on transrectal sonography are scarce. In a recent study addressing this particular issue, it was shown that distal longitudinal tumor spread to a maximum of 2 cm was detectable with an accuracy of 86% compared with histopathology [12]. However, infiltration into the anal sphincters was not mentioned.
Our study shows that transrectal sonography can be regarded as an appropriate method to determine both longitudinal tumor spread and infiltration of the anal sphincters with a sensitivity, specificity, and accuracy of 91%, 85%, and 92%, respectively.
Of the 38 patients investigated, the results of transrectal sonography were interpreted incorrectly in only three patients (8%). Disease of two of these patients was overstaged and of one patient was understaged. Accordingly, inadequate treatment might have resulted for only one patient (3%).
The reason for the two cases of overstaging was a massive peritumoral inflammatory reaction that was confirmed on histopathology. Although it has been shown that a peritumoral inflammatory reaction may be differentiated from the tumor by a characteristic endosonographic appearance in most cases [19], differentiation was not possible in those two cases. Both were pT3 lesions located close to the anorectal junction, where precise focusing of sonography is difficult. In addition, because of angulation between the rectum and the anal canal, other factors, such as reverberation and refraction, may have been responsible for these misinterpretations [20].
Infiltration of the sphincter muscles in rectal cancer is not classified in the current TNM system. Discussion in the TNM committee over this point is underway (Sobin LH, personal communication). Some pathologists favor staging of anal canal invasion as pT3 because involvement of the external sphincter by lower rectal carcinoma does not influence the prognosis, similar to pT3 tumors if an adequate abdominoperineal resection is done (Wittekind C, Sobin LH, personal communication). On the other hand, pathologists have reported that infiltration of the anal canal should be considered stage pT4 because the sphincters should be regarded as another anatomic structure (Wittekind C, Sobin LH, personal communication).
In recent publications, MR imaging with the help of endoanal coils has been shown to be effective in the visualization of the structures of the anal canal and its diseases [21]. Endoanal coils might also be helpful in detecting invasion of the sphincter muscles in lower rectal cancer. However, visualization of the extent of a rectal tumor may be impossible because of the limited field of view of an endoanal coil.
In conclusion, this study shows that both local tumor staging and anal sphincter infiltration can be assessed precisely on transrectal sonography, which also helps to determine the appropriate surgical strategy in lower rectal cancer.
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