Does Radiation Therapy for Prostate Cancer Cause Rectal Cancer?

By: Dan Sperling, MD

Perhaps because I am a radiologist, I have been asked by prostate cancer patients whose urologists are recommending proton beam or IMRT if radiation can cause rectal cancer. In a sense, this is a tricky question because there is a difference between “causality” and “correlation.”

Radiation is everywhere around us. Here are some of the ways each of us is commonly exposed to low-dose radiation of different types: natural background radiation such as cosmic rays and radon in the air; going through some types of scanning equipment at the airport; even our food and water contain very small amounts of radioactive substances that we ingest. Thus, we all live harmlessly with negligible radiation.

The most common exposure to higher levels of radiation comes from medical diagnosis and intervention, such as xrays, CT scans, and radiation therapy. The dosage is carefully calibrated to do only what is intended with as little harm as possible. However, we can’t ignore it when a significant number of people exposed to certain scans or procedures begin to have a side effect at a higher rate than commonly observed among everyone else. This is called a “correlation” because a relationship between one thing occurring and something else occurring afterward  has been observed.

When a correlation is noticed, scientists and researchers begin to explore all possible connections and explanations. Coincidence and chance must be ruled out, and the best way to do this is to look for other groups in which this same correlation occurs. If other researchers find the same phenomenon with at least one other completely different group, the correlation gets attention.

Ever since the detonation of the first two atomic bombs in Japan in 1945, and the rise in certain cancers among the survivors, we have known that exposure to radiation at higher-than-normal levels puts people at risk. However, when radiation therapy is indicated, the benefits outweigh the risks, especially with more advanced equipment that can better target the radiation (less scatter).

A better question for patients to ask would be: is there an observed correlation between beam radiation for prostate cancer and increased incidence of rectal cancer? The simple answer is yes. Perhaps the first published article on this came out almost a decade ago, out of the University of Minnesota.[i] The authors looked back at 21 years of data from the SEER database, covering the years 1973-94. They found records on over 55,000 men who had prostatectomy and over 30,000 men who were treated with beam radiation, and all patients had survived at least five years. For the radiation group, they calculated the anatomical areas that had definitely been exposed, potentially exposed, and not exposed at all. Compared with the surgery group, the authors noted a “noted a significant increase in development of rectal cancer after radiation for prostate cancer.” As you can see, they did not state that the radiation caused cancer, but rather that they saw a correlation.

That’s the simple answer, and the published study was valuable in establishing the relationship. However, there’s a more complex answer that can be somewhat reassuring. The radiation equipment and methods that were used between 1973 and 1994 are by now outdated. Today’s radiotherapy methods to treat prostate cancer are more targeted in an effort to control the scatter effect. The dosages are delivered more efficiently, and follow-up can be done using powerful MRI imaging, which has no radiation. While there will still be a risk of urinary, sexual and bowel side effects, including collateral cancers of the rectum and bladder, long term follow up should hopefully reveal a diminished risk of all of these, especially cancer. This is what medical evolution is all about—improving the benefits and minimizing the risks—and it’s thanks to studies like the UMN article that our attention is brought to bear on this challenge.

Finally, it’s important to remember that because of the location of the prostate gland, and the problems with conventional diagnostic methods, all traditional prostate cancer treatments carry some risks. The search for minimal-to-noninvasive treatments that focus on the significant tumor(s) is what our mission is all about.