Ever since physicians started using computed tomography (CT) for medical imaging, its use grew rapidly. The benefits of CT imaging in clinically needed cases are well known. However, its potential for increased cancer risks and relatively high cumulative doses from multiple scans have raised concerns among the medical and scientific community. Sensitive sections such as children, adolescents and young adults are vulnerable and technologists must use appropriate protocols for them while they undergo CT scans.
Radiation doses at moderate (over 100 mGy) to high (over 1 Gy) values are known to cause haematological malignancies (blood cancers) in both children and adults and other cancers. However, there has been uncertainty about risk at low doses (less than 100 mGy) that are typically associated with diagnostic CT examinations. A recent study (Nature Medicine, 9 November 2023) suggests that even low doses of radiation have a small probability to cause blood cancer.
Also read: How accurate are India’s cancer registries? | Explained
“Our results strengthen the body of evidence of increased cancer risk at low radiation doses and highlight the need for continued justification of paediatric CT examinations and optimisation of doses,” the researchers asserted. “This is especially concerning given that CT scanning is the largest contributor to the world’s average annual effective dose per person from medical radiation sources, in both children and adults.”
Researchers from the International Agency for Research on Cancer (IARC) and its partners followed up a multinational cohort of 9,48,174 individuals who underwent CT examinations before age 22 years in 276 hospitals from nine European countries. The final study covered about 1.3 million CT scans in nearly 9,00,000 patients. They estimated the radiation doses to the active bone marrow of each subject based on the body part scanned, patient characteristics, time period and inferred CT technical parameters. Fifty-one percent of the cases were younger than 20 years at diagnosis, whereas 88.5% were younger than 30 years.
Findings
Researchers followed up the group for at least two years following their first CT scan. They identified 790 haematological malignancies including 578 cases of lymphoid malignancies and 203 cases of myeloid malignancies and acute leukaemia (AL). Mean follow-up period was 7.8 years.
The researchers found a clear association between cumulative dose and risk of all haematological malignancies, with an excess relative risk of 1.96 per 100 mGy. Gray (Gy) is a unit of absorbed dose of radiation. The energy absorbed in one Gy of radiation is one Joule per kg of tissue. Since Gy is a large unit, milli (one-thousandth) or micro (one-millionth) of a Gy are commonly used.
Researchers estimated that “for every 10,000 children examined today (mean dose 8 mGy), one-two persons are expected to develop a haematological malignancy attributable to radiation exposure in the subsequent 12 years.”
Dose-effect studies
Many earlier studies that estimated the blood cancer risk associated with CT scan radiation in children and young adults have low statistical power, inadequacies in individual dosimetry and potential bias from confounding by indication (when those who undergo CT examinations are at higher risk of cancer than those who do not, due to underlying conditions)
Researchers based the earlier studies mainly on linear extrapolation of risk from the higher doses of the Japanese atomic bomb survivor studies. This extrapolation remains controversial
In the present study, the distribution of dose to the active bone marrow (referred to as ABM dose) was such that most individuals received low doses. The mean and median cumulative ABM dose at the end of the follow-up was 15.6 mGy and 10.7 mGy, respectively, in the cohort and 20 mGy and 13.0 mGy among the overall cases.
Notable features
First, it is a large-scale multi-centre study designed to estimate directly the risk of blood cancers associated with ionizing radiation exposure from CT examinations during childhood and young adulthood; it aims to address criticisms of previous studies related to dosimetry, statistical power and potential biases.
Second, the size of the study (nearly one million patients) has considerably increased the statistical power compared with previous national studies.
The results showed a clear dose-response between cumulative ABM dose and risk of haematological malignancies, both lymphoid and myeloid, with increased risk at doses as low as 10-15 mGy.
It is a curtain down on the dose-effect controversy; low dose ionising radiation causes a finite risk not greater than what was estimated thus far from various studies. This small risk is acceptable in view of clinical benefits to the patient. Patients must realize that denying to undergo CT scans needed to diagnose disease is also harmful. The study highlights the need for diligently following the basic principles of radiation protection.
(K.S. Parthasarathy was formerly Secretary of the Atomic Energy Regulatory Board. ksparth@yahoo.co.uk)
