Flat, non-polypoid colorectal lesions, which may be difficult to detect, appear to be relatively common and may have a greater association with cancer than colorectal polyps, according to an article in the March 5 issue of the Journal of the American Medical Association.

Colorectal cancer is the second leading cause of cancer death in the United States. Prevention has focused on detection and removal of polypoid neoplasms. Recent studies, however, have demonstrated that colorectal cancer can also arise from non-polypoid colorectal neoplasms.

"Nonpolypoid colorectal neoplasms (NP-CRNs) are more difficult to detect by colonoscopy or computed tomography colonography because the subtle findings can be difficult to distinguish from those of normal mucosa. As compared with surrounding normal mucosa, NP-CRNs appear to be slightly elevated, completely flat, or slightly depressed," the authors wrote. Data are limited on the significance of this class of lesion.

Roy M. Soetikno, MD, MS, and colleagues with the Veterans Affairs Palo Alto Health Care System, Palo Alto, California, examined data from a group of 1,819 patients undergoing elective colonoscopy to estimate the prevalence of non-polypoid lesions and to characterize any association with colorectal cancer.

The overall prevalence of non-polypoid lesions was 9.35 percent (170 patients). Prevalence rates in the subpopulations for screening, surveillance, and symptoms were 5.84 percent, 15.44 percent, and 6.01 percent, respectively. The overall prevalence of non-polypoid lesions with cancer that was in situ or had invaded beneath the mucous membrane was 0.82 percent; in the screening population, prevalence was 0.32 percent. Overall, non-polypoid lesions were nearly 10 times more likely to contain cancerous tissue than polypoid lesions, irrespective of size.

The positive size-adjusted association of the lesions with cancer in situ or invasive beyond the mucosa was also observed in subpopulations for screening and surveillance. The depressed type of lesion had the highest risk (33 percent). Nonpolypoid colorectal neoplasms containing cancer were smaller in diameter than polypoid ones.

"In conclusion, in this population of patients at a single Veterans Affairs hospital, NP-CRNs were a relatively common finding during colonoscopy. They were more likely to contain carcinoma compared with polypoid neoplasms, independent of lesion size. Recent studies have pointed out differences in the genetic mechanisms underlying nonpolypoid and polypoid colorectal neoplasms. Future studies on NP-CRNs should further evaluate whether the diagnosis and removal of NP-CRNs has any effect on the prevention and mortality of colorectal cancer and particularly focus on their genetic and protein abnormalities," the authors wrote.

In an accompanying editorial, David Lieberman, MD, of Oregon Health & Science University, Portland VA Medical Center, Portland, Oregon, commented on the findings of Soetikno and colleagues.

"[Nonpolypoid colorectal neoplasms] may be biologically distinct from polypoid lesions and appear to be more likely to harbor malignant features. Detection and complete removal at colonoscopy may be challenging. The current study emphasizes the importance of quality in the performance of colonoscopy," he wrote.

"The optimal methods for enhancing colonoscopic imaging of NP-CRNs are uncertain. … Additional studies are needed to determine whether imaging modalities such as computed tomography colonography will be able to detect NP-CRNs. Finally, longitudinal studies are needed to determine whether patients with NP-CRNs require more intensive colonoscopic surveillance compared with patients with polypoid lesions of similar size and histology."