Answer to Question #2030 Submitted to "Ask the Experts"

Q

Radiography of children requires use of lower keV x rays due to smaller mass, yet growing children are more susceptible to harmful effects. Is it correct to state that in children lower keV correlates to increased relative biological effectiveness (RBE)? What potential does the genetic revolution and the Human Genome Project hold for rewriting the book on radiobiology?

A

RBE varies with LET (linear energy transfer, the frequency of interaction along the track of a particle). X and gamma rays and electrons are sparsely ionizing (thus penetrating). Heavier particles (alphas, neutrons, heavy nuclei) are densely ionizing. There are major differences in RBE between these extremes (for example, x rays compared with alphas). There is some difference in RBE with x or gamma ray photon energy. More energetic beams have lower LETs. These differences may be significant when there are wide differences in beam energy (a few keV compared with several MeV). However, the differences within the diagnostic energy spectrum are immeasurably small. It is too early to form any strong opinions about the influence on radiobiology of learning the human genome. It is now quite certain that damage to DNA is the major molecular mechanism of radiation injury. It may be possible at some later date to provide directed repair of DNA damage (for example, mutation, carcinogenesis). Living cells, including human, possess enzymes that repair damage to DNA. There is some evidence suggesting that erroneous repair may lead to some effects. Directed repair may provide a mechanism for correcting specific defects. S. Julian Gibbs, DDS, PhD