CT and Cancer Risk

Written by Clay Smith

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CT abdomen and pelvis is associated with a higher incidence of hematologic malignancies over a two year span, especially in younger patients.

Why does this matter?
Previous studies have raised concerns about CT radiation exposure and risk of malignancy. You need to read this classic NEJM review and this landmark Lancet study. Prior studies may have had confounding by indication. What if we knew that all CT scans were done only to rule out appendicitis and all patients had an appendectomy?

Scan with care…
This was a large, population-based study from S. Korea, with 825,820 people who had an appendectomy for acute appendicitis. Of these, some had CT, and some did not: CT-exposed (up to 7 days before or after appendectomy, 306,727); CT-unexposed (519,093). These were not people with known or suspected malignancy. The incidence rate ratio (IRR) for hematologic malignancies for CT-exposed vs CT-unexposed groups was 1.26 (95%CI 1.09-1.45) at 2 years, most notably leukemia. This was driven mainly by the increased IRR in patients 0-15 years old. In this subgroup, the IRR was 2.14 (95%CI 1.35-3.40). There was no increased risk for other abdominal or pelvic malignancies or for all-cause cancer at 5 years of follow up. This study has the advantage of knowing, with a high degree of certainty, the indication for the CT for included patients and has low risk of confounding by indication or reverse causation (i.e. the patient had a cancer which prompted the CT and not the other way around). Although this association is likely true, we also need to keep this in perspective. The lifetime risk of dying from cancer (not associated with radiation) is roughly 20%. A 10mSv CT abdomen/pelvis may increase that risk to 20.1%. That said, we should only use CT when it is truly necessary and should utilize the highest quality, best imaging equipment that drastically reduces scan times and lowers radiation exposure. Also, the younger the patient is, the harder you need to think about whether or not to scan.

Source
Risk of Hematologic Malignant Neoplasms From Abdominopelvic Computed Tomographic Radiation in Patients Who Underwent Appendectomy. JAMA Surg. 2021 Apr 1;156(4):343-351. doi: 10.1001/jamasurg.2020.6357.

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