We provide the first demonstration that the blood supply in the endoscopically normal rectal mucosa is altered in subjects with concurrent biologically significant colonic neoplasia. Thus, increased mucosal blood content may serve as a potential marker of field carcinogenesis and hence might be used for neoplasia screening. This blood supply augmentation tended to be localized in the capillary network underlying the epithelium (depth, ~ 100 μm). The mucosal OHb (depth, ~ 95 μm) was significantly augmented in those patients harboring either multiple nonadvanced adenomas or advanced adenomas. Using the OHb and patient age markers in a logistic regression rule gave excellent discrimination between patients with no neoplasia and those with advanced adenomas with an area under the ROC curve of 0.88, a sensitivity of 83%, and a specificity of 82%. Depth-selective measurement of hemoglobin concentration from the uninvolved rectal mucosa using the endoscopically compatible polarization-gated probe has provided in vivo human validation of our previous reports on the EIBS phenomenon.
Our group initially described EIBS as a marker of field carcinogenesis. Although there has been emerging evidence of genetic/proteomic alterations in histologically normal mucosa of patients harboring neoplasia, no previous reports had assessed the blood supply. This was largely due to the inability of conventional techniques to discriminate and accurately assess a relatively small proportion of the total colonic blood supply—mucosal microcirculation including the pericryptal plexus. Polarization-gated spectroscopy, on the other hand, allows accurate quantification of this blood content through the depth selectivity of polarization gating. Given the field effect, patients having neoplastic lesions would be expected to have hyperproliferative and hypermetabolic mucosa and would therefore require an increased blood supply. Previous studies have shown that epithelial proliferation from the rectum is elevated in subjects harboring neoplastic lesions anywhere in the colon (27
). Studies from human biopsies showed that the superficial blood supply from the midtransverse colonic mucosa was increased in patients who harbored advanced adenomas either in the rectum, sigmoid, or cecum, suggesting the presence of a diffuse field effect throughout the colon (14
). Moreover, our initial in situ
studies showed a gradient effect in which EIBS magnitude mirrored the distance to lesions (15
), suggesting that both field carcinogenesis and tumor-elaborated factors seemed to be important. Our current report showing increased blood content in the uninvolved rectal mucosa of patients possessing neoplastic lesions provides in vivo
evidence of the diffuse field effect component, which is consonant with our previous animal studies (14
). Furthermore, the current report shows how this diffuse field effect may be leveraged for screening purposes.
Attempts to use the field effect for CRC screening have yielded poor performance to date. Flexible sigmoidoscopy is commonly used, with ~ 1.2 million exams done in 2002, but the sensitivity of a sentinel distal adenoma as a marker of advanced proximal neoplasia is remarkably low (~ 33% in women; ref. 5
). Advanced technologies such as microarray and proteomic analyses have attempted to identify the genetic/epigenetic changes associated with field carcinogenesis (8
). Although these techniques have validated the approach of detecting the field effect in the uninvolved rectal mucosa, their translation to a clinical setting has been stymied by low accuracy and technological hurdles.
Methods of CRC screening not based on the field effect concept have also been implemented but they too suffer from poor diagnostic performance or difficulty being applied in the clinic. The most frequently used method, guaiac-based fecal occult blood testing, has a sensitivity of 10.8% (31
), whereas immunohistochemical fecal occult blood testing has a sensitivity of 27% (32
). CT colography (virtual colonoscopy) represents one of the most promising new-generation tests and was recently sanctioned for average-risk screening (33
). However, the reported large multicenter trial showed a sensitivity for advanced adenomas that were comparable to our rectal EIBS data (per lesion rate of 84% versus 83%). Moreover, there are many possible advantages of rectal EIBS to CT colography, including lack of need for colon purge, less pain and expense, and no radiation exposure.
Our results show that rectal mucosal blood content measurement is sensitive to advanced adenomas and multiple nonadvanced adenomas, but not to single nonadvanced adenomas. From a clinical perspective, this is acceptable as the advanced adenoma has been suggested by Winawer and Zauber to be the primary target of screening because it is associated with a relatively high risk of progressing to colon cancer (34
). The insensitivity of our technique to single nonadvanced adenomas supports the notion that the magnitude of field carcinogenesis alterations mirrors the neoplastic outcomes. This may be clinically adequate given studies in which the vast majority of nonadvanced adenomas never progress to CRC. On the other hand, the presence of multiple nonadvanced adenomas has been clearly shown to be a marker of significant long-term risk for advanced neoplasia (35
). Indeed, clinical guidelines for polyp surveillance frequently consider advanced adenomas and multiple nonadvanced adenomas equivalently (36
). A patient with multiple adenomas may have a more robust field effect, which is consonant with our finding of augmented microvascular blood content. Thus, rectal OHb seems to be sensitive to clinically significant neoplasia (advanced adenomas and multiple nonadvanced lesions), supporting the potential viability of rectal EIBS measurement for screening purposes.
There are several limitations to this study that need to be acknowledged. In our study, patients were classified into diagnostic categories based on colonoscopic findings. Colonoscopy, however, is not perfect. Tandem colonoscopy studies reveal that approximately one quarter of adenomas may be missed (37
). Advanced adenoma detection rates can vary 4-fold among endoscopists, indicating a sizable miss rate (38
). Given these findings, it is plausible to assume that some of our controls actually harbored adenomatous lesions. If this were true, then actual baseline blood content levels from controls would likely be lower than observed in our study. This would actually improve our technique’s discrimination between patients harboring adenomas and those who do not. The estimation of adenoma size by endoscopy may be somewhat inaccurate, leading to the possibility of misclassification of some size-borderline advanced adenomas but this should not bias our results (39
). Given patients were enrolled for screening, the cancer rate was very low as expected. Thus, it is not possible to comment on whether rectal EIBS engendered by carcinomas would be of greater magnitude. However, as and previous reports show (15
), larger adenomas give rise to more EIBS than smaller lesions, supporting the “dose dependence” of rectal EIBS. This will need to be confirmed in separate studies focusing on patients diagnosed with cancer.
The small number of “events” (advanced adenomas) always raises concerns about overfitting with regard to performance characteristics. However, given that only one variable was assessed (OHb concentration and patient age), this seems quite unlikely. This concern is further mitigated by the LOOCV, which shows the stability of our statistical methods. After validation, the area under the ROC curve remained high at 0.84. Our results will still need to be replicated in larger studies testing the prediction rule prospectively.
In conclusion, we show for the first time that the microvascular blood content in the histologically normal rectal mucosa is elevated in patients with multiple and advanced lesions. Our study is a proof-of-concept that the quantification of rectal blood content may offer a new method for colon cancer screening. For clinical application, we have developed a fiber-optic polarization-gated probe that allows in vivo interrogation of the rectal mucosa. If validated in future larger clinical studies, this thin and flexible probe could be potentially used by primary care physicians during the annual rectal exam to determine the need for colonoscopy.