Most colorectal cancers are thought to arise from polypoid adenomas - growths that protrude from the mucous membrane in the colon or rectum. A study from the Veterans Affairs (VA) Health Care System in Palo Alto, CA, published in the March 5 Journal of the American Medical Association adds to a growing body of evidence that nonpolypoid colorectal neoplasms (NP-CRNs) - abnormalities that can appear either flat or depressed relative to the surrounding membrane - can also contain precancerous or cancerous cells. Previous studies established the existence of NP-CRNs in Japan, but their prevalence and importance in other parts of the world has remained unclear.

In the VA study, gastroenterologists and pathologists who were trained in a special exchange program with Japanese cancer centers examined 1,819 patients at the Palo Alto VA hospital undergoing colonoscopy for screening, surveillance (for people at high risk of colorectal cancer), or symptoms of colorectal neoplasms. Patients underwent biopsy, removal of polyps or NP-CRNs, or surgery as needed.

The investigators found NP-CRNs in 170 patients (9.35 percent of those examined). Although these lesions were less common than colorectal polyps, they were more likely to contain precancerous or cancerous cells, accounting for only about 15 percent of identified neoplasms but 54 percent of superficial carcinomas. "Nonpolypoid morphology was strongly associated with findings of in situ or submucosal invasive carcinoma," stated the authors.

NP-CRNs missed during colonoscopy may help explain the occurrence of interval colorectal cancers - cancers that arise between scheduled screening colonoscopies in patients who appear not to have any precancerous polyps - explains Dr. David Lieberman from Oregon Health and Science University in an accompanying editorial.

The existence of NP-CRNs will have important implications for colorectal cancer screening in the United States, continued Dr. Lieberman. "The finding that NP-CRNs have high rates of serious pathology suggests that effective screening programs will need to accurately identify patients who harbor these lesions."