This very interesting question raises several important issues. We’re beginning to understand the genetic basis of many cancers; while some patterns are emerging between different types, there’s presently no obvious relationship between bowel and prostate cancer. Even so, these are common cancers for middle-aged and older men, and we frequently see men diagnosed with both over the course of their lives. Screening tests for bowel cancer exist — men with prostate cancer should take advantage of them in the same way as anyone who doesn’t have this condition.
Intermittent painless rectal bleeding is a well-known and relatively harmless side effect of radiotherapy (usually due to mild radiation rectal injury). But it’s also one of the signs of bowel cancer. Men should bring all rectal bleeding to the attention of a physician because, occasionally, a rectal or colon cancer will be detected as the cause.
Finally, there’s the question of whether radiotherapy increases one’s risk of developing another cancer. Radiation exposure can cause cancer; the risk is highest when the exposure happens in childhood or early adulthood and decreases with age. A recent study of 243,000 men treated for prostate cancer across the US found more rectal cancers in men who had radiotherapy compared to those who didn’t. Although the number of extra cancers was small, and other factors (e.g. better screening or follow-up among those who received radiotherapy) may have contributed to the difference, some of these cancers were likely caused by radiotherapy.
Any man diagnosed with prostate cancer must weigh the risk of this and other radiation-related side effects against those of surgery, or of no treatment.
