X-rays induce many types of lesions (tiny biological damage) in the tissues through which they pass. They include breaks of one strand or both strands of the cellular DNA. Cells repair most of the former lesions promptly. Double Strand Break (DSB) is probably the most important effect. Scientists have developed extremely sensitive methods to assay DSBs caused by a single computed tomography (CT) test.
Cell repair
Cells can either repair DSBs properly restoring overall integrity of the genome, or mis-repair, resulting in drastic consequences such as cancer.
Kufner, Schwab and colleagues, researchers from Germany, described a method to measure biological dose in computed tomography (CT) scanning procedures (European Congress of Radiology, 2009). They demonstrated Double Strand Breaks (DSBs) of DNA in the white blood cells of patients undergoing cardiac CT and angiography examinations by sampling blood from the patients before and after the tests.
Physical energy absorbed by tissue is an imperfect indicator of what goes on in the tissue. A true estimate of radiation risk requires an accurate, reproducible biological measurement of radiation-induced damage, (auntminnie.com, 2009). By counting the DSBs, the researchers measured such a biological quantity.
Assay method
Radiation exposure causes DSBs in mammalian cells. When cells are exposed to radiation, certain types of molecules called histone H2AX, adjacent to the nascent breaks, undergo “phosphorylation” (a process involving specific biochemical changes).
The response is highly amplified and rapid; hundreds to thousands of H2AX molecules join within minutes. This newly phosphorylated protein called gamma-H2AX mobilises DNA repair proteins and form discrete foci, one per DSB. Specialists count these biomarkers by using immuno-fluorescence microscopy.
Counting of DSBs is thousand times more sensitive than other biological methods such as counting of chromosome aberrations.
A dose of one mGy (typical skin dose in medical x-ray examinations) generates on average one track per nucleus and thus is considered the lowest dose that can affect a whole cell. With one mGy, nearly 3 per cent of irradiated cells can sustain a DNA DSB. (Bonner, Proceedings of the National Academy of Sciences, April 23, 2003)
Medical X–ray procedures
Researchers examined 32 patients, 16 on each of two CT scanners using various protocols. They collected blood samples before and 30 minutes after the CT scan and counted the DSBs formed. They found that the number of DSBs increased linearly with dose length product, a normally used physical parameter
They observed that sequential CT scans are less biologically damaging than spiral CT. Radiation dose delivered over time was less damaging than the same dose delivered all at once. They did not observe any abnormal DNA damage in the blood cells of interventional radiologists when measured repeatedly throughout the normal working day.
The researchers demonstrated the presence of DSBs in the blood cells of all 37 patients undergoing conventional angiography. The number of DSBs at the end of the procedure varied widely, declining rapidly after the examinations due to DSB repair processes.
The sensitive methods evolved by the researchers may be very useful in understanding the mechanisms of cellular damage. It may help to evolve better cancer treatment strategies. Cancer cells are rogue cells with no rules. If they have any rules they are their own rules!
CT, a unique tool
The findings referred to in this article indicate the unparalleled technological advances in the study of cells and do not in any way diminish the potential use of computed tomography or conventional angiography in clinically indicated instances. CT scan unit is a unique tool to diagnose disease, trauma or abnormality and to plan, guide and monitor treatment.
It will cause harm to the patient if he refuses CT examinations prescribed by a qualified physician. For such tests, the benefit far outweighs the possible harm. Technological revolutions should not skew perception of risks.
( The author is a Former Secretary, Atomic Energy Regulatory Board, India. He can be contacted at: >ksparth@yahoo.co.uk )
