Cigarette smoking is more strongly associated with an increased risk of invasive bladder cancer than with low-grade superficial bladder cancer, according to California researchers.

Xuejuan Jiang, MD, of the Keck School of Medicine at the University of Southern California in Los Angeles, and colleagues studied 1,586 bladder cancer patients and individually matched controls without cancer. Subjects ranged in age from 25 to 64 years.

Compared with controls, regular smokers had a 2.2 times increased risk of low-grade superficial, a 2.7 times increased risk for high-grade superficial tumors, and a 3.7 times increased risk for invasive tumors, the researchers reported online in the International Journal of Cancer.

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The pattern was consistently observed regardless of the smoking exposure index under examination (smoking status, number of cigarettes smoked daily, or number of years smoking), the researchers noted. Women had a higher risk of invasive bladder cancer even when they smoked comparable amounts of cigarettes as men. 

However, Dr. Jiang’s group observed no gender difference in the association between smoking the risk for low-grade superficial bladder cancer and no differences in racial or ethnic distribution of the malignancies. The researchers noted that the risks for all subtypes of bladder cancer were significantly higher among smokers and increased with increasing number of cigarettes smoked per day and the number of years of regular smoking.

Permanently quitting smoking reduced the risk of all subtypes of bladder cancer, “and the reduction in risk was proportional to the duration of smoking cessation,” according to Dr. Jiang’s team.

Although the study has a number of strengths, including is population-based design, relatively large sample size, and well-characterized risk/protective factors, a potential weakness is the fact that bladder cancer histology was not determined by a single reference laboratory, but recorded by trained personnel from pathology reports, the authors pointed out. “The various grading schemes used by pathologists who produced the original pathology reports can be a source for misclassification,” they noted.