X rays, gamma rays, neutrons declared carcinogens

Photo: K. Pichumani

Date:31/03/2005 URL: http://www.thehindu.com/thehindu/seta/2005/03/31/stories/2005033100041500.htm Sci Tech



X-rays, gamma rays, neutrons declared carcinogens



REDUCING EXPOSURE: Medical physicists should ensure that all radiographic equipment are properly calibrated and maintained so as to provide the highest quality images at the lowest possible radiation dose.

NOW IT is official. The 11th edition of the Report on Carcinogens (ROC) released by the U.S. Department of Health and Human Services on January 31, 2005, included X and gamma-radiation and neutrons in the list of cancer-causing agents. ROC is a scientific and public health document.

It identifies potential cancer hazards. The listing does not by itself establish that a substance presents a cancer risk to an individual in daily life.

The listing for X-radiation and gamma radiation is based on sufficient evidence of carcinogenicity in humans and that for neutrons is based on the studies of their mechanisms of carcinogenesis. "X-radiation and gamma radiation are most strongly associated with leukaemia and cancers of the thyroid, breast and lung" the report asserted.

Childhood exposure

"The risk of developing these cancers, however, depends to some extent on age at the time of exposure with childhood exposure mainly responsible for increased leukaemia and thyroid cancer risks" the report cautioned.

"Exposure during reproductive years increases the risk for breast cancer, and exposure later in life increases risk for lung cancer, a press release from the National Institute of Environmental Health Sciences, National Institutes of Health noted.

Associations between radiation exposure and cancer of the salivary glands, stomach, colon, bladder, ovaries, central nervous system and skin also have been reported.

The National Toxicology Program (NTP), which prepared the report relied on the findings of sufficient evidence of carcinogenicity in humans by the International Agency for Research on Cancer as the basis for nominating X - and gamma radiation as cancer-causing agents.

Rigorous review

NTP evaluates agents of public health concern by using tools of modern toxicology and molecular biology. The rigorous review process to nominate an agent takes two and a half years.

The American College of Radiology (ACR) criticised the listing. ACR felt that many patients who desperately need X-ray examinations may avoid them.

They may mistakenly believe that they are being placed at undue risk while undergoing a medical X-ray procedure. ACR will petition NTP to have ionising radiation removed from the list.

Dr. Donald Frey, Chairman of the Board of the American Association of Physicists in Medicine (AAPM), an organisation of over 5000 medical physicists echoed similar fears. "It would be a tragedy if patients did not have needed exams because of fears raised by the report", he said.

Key role

"One of the key roles that medical physicists provide is to ensure that all radiographic equipment is properly calibrated and maintained so as to provide the highest quality images at the lowest possible radiation dose." Dr. Howard Amols, president of the AAPM noted.

Patients must ask radiologists and medical physicists about the risks and benefits of medical radiation procedures.ROC does not try to balance potential benefits of exposures to certain `carcinogens' in special situations. In a clinically indicated medical radiation procedure when carried out by qualified professionals using optimally adjusted equipment, the benefits far outweigh the risks.

Physicians routinely order many X-ray examinations. The indications are often not clinical. Repeat examinations due to poor techniques lead to unwanted X-ray dose to unsuspecting patients.

At times, even qualified professionals order exams more frequently because they cannot allow the costly equipment to remain idle! High levels of ionising radiation cause cancer. But the cancer- inducing potential of low doses of radiation is controversial.

Till we settle the issue, the radiation doses to patients should be as low as reasonably achievable (ALARA) after taking into account the clinical requirements. The listing once again highlights the importance of enforcing the ALARA principle.

K.S. Parthasarathy

ksparth@vsnl.com

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