CT: cancer risks for the elderly

Online edition of India's National Newspaper
Thursday, Dec 02, 2010

The study showed cancer incidence from CT scans was less threatening

— photo: K. Murali Kumar

The focus: The study estimates cancer risk to persons above 65.

In a paper presented at the 96 {+t} {+h} Scientific Assembly and Annual Meeting of the Radiological Society of North America, Dr Aabed Meer and co-workers at the Stanford University at Palo Alto, California, have claimed that the risk of developing radiation-induced cancer from computer tomography (CT) may be lower than previously thought.
It is the first time in many years that a scientific study showed that the impact of CT on the incidence of cancer was less threatening! But the RSNA paper does not state that CT scans are risk free. The paper has not undergone any peer-reviewing as it was presented at the RSNA meeting, and not published in a peer-reviewed journal.
The study has certain other limitations. It estimates cancer risk to persons above 65 years. The views of learned bodies and professional associations on the study are yet to be published.
This study is statistically respectable as it included 10 million records of patients from 1998 to 2005. Based on Medicare database, they analyzed the distribution of CT scans, determined the radiation doses associated with them and estimated the associated cancer risk in a population of older adults. Medicare is a social insurance program administered by U.S.government, providing health insurance coverage to people who are aged 65 and over, or who meet other special criteria (Wikipedia).
The researchers included the data from two study groups; 5, 2767,230 records from 1998 through 2001 and 5,555,345 records from 2002 through 2005. They analyzed the number and types of CT scans that each patient received to find out the percentage of patients exposed to “low” radiation doses of 50 mSv to 100 mSv and “high” radiation doses in excess of 100 mSv (Sv is a unit of biologically significant dose and it involves the absorption of one joule per kg of radiation energy; mSv is a thousandth of a Sv).
They calculated the number of cancers that may be induced by using standard cancer risk models.
CT scans of the head numbered 25 per cent of the examinations in the first group and 30 per cent in the second. They found out that abdominal CT exposed patients to the greatest proportion (nearly 40 per cent) of doses in each group. The second and third largest sources of radiation were imaging of pelvis and chest
From 1998 to 2001, 42 per cent of the patients underwent CT scans; the corresponding percentage for 2002 to 2005 was 49. The researchers also found that the percentage of patients exposed to radiation doses in both the low and high ranges nearly doubled from the first group to the second. This was consistent with the increasing use of high speed CT in patient diagnosis and management.
They estimated the cancer incidence associated with exposure to radiation from CT to be 0.02 percent and 0.04 percent of the two groups respectively. The authors found a significantly lower risk of developing cancer from CT than the previous estimates of 1.5 per cent to 2 per cent of the population.
The study which gave higher numbers refers to a paper published by Drs David Brenner and Eric Hall, researchers in the Columbia University Medical Centre, New York in the New England Journal of Medicine (November 29, 2007)
This paper became very controversial. The American College of Radiology, the Radiological Society of North America and the Association of Physicists in Medicine reacted to the paper with predictable alacrity. These associations argued that the conclusions of the paper may scare away patients from clinically needed CTs.
The NEJM study got extensive media coverage as it focused attention on the overuse of CT. It was then estimated that one million children and 20 million adults in the USA undergo unnecessary CT scan procedures annually.
Medical radiation procedures must be carried out only if they are justified. Physicians must ensure that radiation doses are As Low As Reasonably Achievable (ALARA) without compromising clinical needs.

K.S. PARTHASARATHY
RAJA RAMANNA FELLOW DEPARTMENT OF ATOMIC ENERGY

( ksparth@yahoo.co.uk)