How safe are CT, X-Rays as first-line tools to screen for Covid-19?

Recently, Randeep Guleria, director, All India Institute of Medical Sciences New Delhi, warned against CT scans in mild Covid cases linking CT scan radiation to cancer risk.

He pointed out that one CT scan is equivalent to 300 to 400 chest X-Rays and frequent CT scans in younger age groups increases the risk of cancer in later life. This caused some anxiety among patients who have undergone CT scans and who require CT scans. Thus, it is worthwhile putting the radiation risk in proper perspective.

In the early days of Covid pandemic, several hospitals across the world did not have a perfected laboratory test to diagnose Covid-19. So they resorted to the use of imaging tests, usually CT scan or chest X-Ray.

While patients with Covid-19 can show an abnormality on either a chest X-Ray or CT scan, many other lung problems and respiratory infections can look similar. Also the absence of an abnormality on either a chest X-Ray or CT scan does not necessarily exclude Covid-19. It is for this reason, most experts advise against the use of an imaging test alone to diagnose or rule out Covid-19. However lab testing for Covid-19 has increased greatly since the outbreak of the pandemic.

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Hence, even the American College of Radiology has recommended against CTs and X-Rays as a first-line tool to screen for Covid-19 citing three major reasons.

It states that unlike the swab tests which can diagnose Covid-19 accurately, imaging findings are not specific enough to confirm Covid-19.

Also, a significant percentage of patients with Covid-19 have normal chest CTs or X-Rays. For them, a normal imaging result could falsely convince them that they are healthy. If such persons do not isolate themselves, they can spread the virus to others.

Also, using CT scan is a huge challenge for the facility as Covid-19 is highly contagious. CT scanners are large, complex and expensive equipment. To decontaminate the equipment and the room for every potential Covid-19 patient is a huge effort. This would also mean shortage of CT scanners for non-Covid patients.

Radiation risk

A CT scan is a combination of multiple X-Rays and a computer process to create a picture of the body (any part) that provides more details than the regular X-Ray. Its detrimental radiation effect is also known to be stochastic or probabilistic in nature and can cause cancer. The radiation risk has been assessed by the International Commission of Radiological Protection (ICRP) based on the data of past human exposure to radiation including the one from survivors of atomic bombings at Hiroshima and Nagasaki.

What is the risk of cancer due to CT scan? One may get a dose of about 20 mSv in a lung CT scan. Incidentally, this is the number prescribed as annual limit for the radiation workers by the regulatory bodies all over the world. Radiation is only one of several agents, like tobacco, environmental chemical pollutants, certain types of foods etc that can cause cancer. Also, the risk probability is so low that even if the person gets cancer, there is no way one can attribute it to the exposure from CT scan.

Therefore, however low the radiation risks are, application of radiation for any purpose is governed by three fundamental principles on which ICRP regulations are formulated.

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Justification of the practice: Use CT scan only if needed.

Optimisation: Exposure given should be as low as possible. CT scan machine parameters settings such as tube current, scan time etc should be such as to achieve the desired quality of the image with minimum exposure.

Limit the risk: Exposure should not exceed the guidance values recommended for the practice.

Covid management

Nevertheless, CT scans do have a role to play in the management of the pandemic. In cases with no lab tests performed earlier but with severe symptoms, doctors resort to CT scan for immediate diagnosis of Covid-19 as lab test results take more than 24 hours and any delay can cost the patient dearly. Also, CT scans for patients with severe symptoms help in assessing the seriousness of the disease and to determine the treatment plan.

Therefore to ensure the radiation generating equipment like X-Ray machines and CT scanners in India are used in a safe manner, their users are licensed by the Atomic Energy Regulatory Board.

The Board ensures only equipment meeting the radiological safety requirements are purchased and the operators of these equipment are qualified by accredited bodies. As part of the licensing requirement, each institution using radiation generating equipment has a full-time Radiological Safety Officer (RSO) approved by the Board. Duties of the RSO include periodic tests for quality assurance of the equipment, monitoring the radiation exposures of the patient and the operating staff and submission of periodic safety reports to the Board, among other things.

(The writer is Former Director, Radiological Safety of Atomic Energy Regulatory Board)