EBRT for prostate cancer and increased risk for colon cancer?

A recent report by Rapiti et al. from the Geneva Cancer Registry documents a significant increase in risk for colon but not rectum cancer after earlier external beam radiotherapy (EBRT) for prostate cancer. While it is well known that radiotherapy can induce second cancers, there has been some controversy over the risk for second malignancies after irradiation for prostate cancer.

Rapiti et al. evaluated the risk of developing colon and rectum cancers after prostate cancer in irradiated and nonirradiated patients using data from the population-based Geneva cancer registry. Their retrospective analysis included all 1,134 patients with prostate cancer diagnosed between 1980 and 1998 who survived at least 5 years after diagnosis. Of these patients, 264 were treated with external beam radiotherapy. Patients were followed for occurrence of colorectal cancer up to 31 December, 2003.

The results of their study can be summarized as follows:

  • At the end of follow-up, 19 patients had developed a colorectal cancer.
  • Among irradiated patients the standardized incidence ratio (SIR) for colorectal cancer was 3.4.
  • Compared to the general population, the risk was significantly higher for colon cancer (SIR = 4.0), but not for rectal cancer (SIR = 2.0).
  • The risk of colon cancer was increased in a period between 5 and 9 years after diagnosis.
  • The overall SIR of secondary cancer in patients treated with radiotherapy was 1.35 (p = 0.056).
  • Nonirradiated patients did not have any increased risk of rectal or colon cancer.

The authors conclude that there is a significant increase of colon but not rectum cancer after external beam radiotherapy for prostate cancer, and that this risk of second cancer after irradiation, although probably small, should be carefully monitored. They are also quoted by Reuters Health as stating that “this serious long-term side effect should be discussed” with patients in weighing the pros and cons of radiation.

6 Responses

  1. What is SIR? This is too technical for me. Can you tell me in plain English how much of an increase in risk there is?


  2. Leah: A standardized incidence ratio (SIR) is a way to measure relative risks in a well-defined statistical manner. Thus, for example, if we determine that the normal risk for getting a second cancer in a patient with prostate cancer who never gets any form of treatment is x, then in the example above the SIR for getting colon cancer after radiotherapy for prostate cancer is 4x and the SIR for getting rectal cancer would be 2x.

    Thus, 19/264 prostate cancer patients treated with radiotherapy got colorectal cancer (7.2 percent) and getting a colorectal cancer had an SIR of 3.4. I haven’t seen the full paper, but at a guess I would have said about 13 of those 19 patients had colon cancer and about 6 had rectal cancer, whereas under normal circumstances among 264 prostate cancer patients who never had any treatment, only 3 would have gone on to get either colon or rectal cancer. Does that make sense to you?

  3. How can this study have any relevance today considering current EBRT technology, i.e., shaped beam, IGRT, etc.?

  4. That is a very reasonable question. Unfortunately, we don’t have an answer to it.

    It is certainly appropriate to believe that this risk has been reduced by the use of newer forms of external beam therapy. However, we would also suggest that it might be wise to follow a more recently treated group of patients to confirm this belief, rather than just assuming it to be the case.

  5. Hi. There is some good info here about the different kinds of technology that are available: http://radiotherapy.blog.co.uk/.

    It seems that as each of the different radiotherapy technologies develop, the primary purpose is to reduce toxicity and side effects for patients while making the treatment itself more effective.

    The state of things over here in the UK is much worse that you guys in the US have available — so much more investment is needed here! It’s simply sad to think that most patients here could benefit from better treatment if the health service was willing to make a greater investment.

    Chuck L.

  6. Not that this is necessarily a good thing, but investment here in the USA is also liable to be more restricted over the next few years for some obvious fiscal reasons.

    One possible benefit is that the radio-oncology community in the USA may have to learn to use currently available equipment as well as they possibly can before rushing to acquire the next available “hot” gizmo that they then need to charge more to pay for.

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