Low LET High LET
Figure 1. Linear energy transfer (LET). Individual ionization events (small circles) occur along paths followed by the radiation (lines). The large circles contain equal numbers of ionizations for low LET and high LET radiation. In the case of high LET radiation, all energy deposition occurs along a single short ionization track With low LET radiation it takes a larger number of radiation emissions as each one has a relatively small chance of tissue interaction.
Dose equivalent was developed in an effort to incorporate biology into the physics of radiation exposure. Not all forms of radiation (alpha, beta, neutron, gamma, X-ray) produce the same biological effect for equal absorbed doses. For example, neutron irradiation has five times the biologic effect of X-rays while alpha radiation has twenty times the biologic effect of X-rays. This is denoted by the Quality Factor (1 for X-ray, gamma and beta irradiation; 5 for neutron and 20 for alpha irradiation). The dose equivalent is the absorbed dose multiplied by the radiation's Quality Factor. For instance, 0.01 Gy (1 rad) of beta radiation produces a dose equivalent of 0.01 Sv (1 rem) while 0.01 Gy (1 rad) of alpha radiation produces a dose equivalent of 0.2 Sv (20 rem).
In order to be able to compare different radiation doses with differing forms of radiation affecting different parts of the body it is necessary to express each radiation dose as if it had been distributed evenly throughout the body. This uses information on the radionuclide's biodistribution, tissue weighting factors which adjust for the varying susceptibility of different tissues and organs to radiation damage, and quality factors which reflect the RBE of different forms of ionizing radiation. A radiation dose expressed in this way is termed an effective dose equivalent (or simply an "effective dose"). These principles are illustrated in the effective doses shown in Table 3 which compare a thyroid radioiodine uptake (assuming a 24-hour uptake of 25%) with a bone scan (assuming normal renal function). Despite considerable differences in the radiation emissions, activities and biodistributions (critical organs) it is possible to see that the bone scan should result in about twice the biologic effect of the uptake
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