Sunday, December 04, 2005

Asbestos and Colon Cancer

BrooklynDodger herewith adopts a new format for posting journal abstracts. The abstract, highlighted or excerpted will lead. The Dodger's commentary will follow.
Am J Epidemiol.
2005 Nov 1;162(9):868-78. Epub 2005 Sep 21.

Click here to read
Evidence for excess colorectal cancer incidence among asbestos-exposed men in the Beta-Carotene and Retinol Efficacy Trial.

Aliyu OA, Cullen MR, Barnett MJ, Balmes JR, Cartmel B, Redlich CA, Brodkin CA, Barnhart S, Rosenstock L, Israel L, Goodman GE, Thornquist MD, Omenn GS.

Cancer Center, Yale University School of Medicine, New Haven, CT, USA.

The relation between asbestos exposure and colorectal cancer remains controversial. The authors of this 1984-2004 US study examined the association among 3,897 occupationally exposed participants in the Beta-Carotene and Retinol Efficacy Trial (CARET) for chemoprevention of lung cancer, followed prospectively for 10-18 years. When a Cox stratified proportional hazards model was used, risks of colorectal cancer were elevated among male heavy smokers exposed to asbestos. Their relative risk was 1.36 (95% confidence interval: 0.96, 1.93) when compared with that for CARET heavy smokers not exposed to asbestos, after adjusting for age, smoking history, and intervention arm. The presence of asbestos-induced pleural plaques at baseline was associated with a relative risk of 1.54 (95% confidence interval: 0.99, 2.40); colorectal cancer risk also increased with worsening pulmonary asbestosis (p = 0.03 for trend). A dose-response trend based on years of asbestos exposure was less evident. Nonetheless, these data suggest that colorectal cancer risk is elevated among men occupationally exposed to asbestos, especially those with evidence of nonmalignant asbestos-associated radiographic changes.

BrooklynDodger's Comments: An association between asbestos and colon or other digestive cancers is still in play. Asbestos fibers were found in Lake Superior waters, polluted by taconite tailings from Reserve Mining. Did they pose a cancer risk? Did uncontroverted evidence for lung cancer by inhalation in people also support a cancer risk at any site from ingestion of fibers? Was colon cancer a consequence of asbestos at work, either by direct ingestion or recycled fibers swallowed from the respiratory tract?

There's an NAS committee at work on this subject today.

An NAS bioassay of ingested asbestos in rats and mice did not observe an association. The Dodger will go back into ancient history on that, but bottom line is that rats are very resistent to asbestos fibers by inhalation, and mice are totally resistence. These rodents are not models for colon cancer [meaning, they don't get it much, and not after treatment by carcinogens.]

The CARET trial, in the author's words, "suggests" an association. The limitation to smokers arises because only smokers were allowed in the CARET trial.

The Dodger will return to the bioassay and the present controversy soon.

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