Fundamental principles of radiobiology: Rad Tech

Fundamental principles of radiobiology are used to explain the physical and biological factors that affect the radiobiologic response of tissue. As a radiologic technologist, knowledge of these radiobiologic factors is essential for understanding the potentially harmful effects of exposure to digital imaging and the positive effects of radiation oncology.

The law of Bergonié and Tribondeau explains why some tissues are more radiosensitive than others. In general, radiosensitivity is greater in cells that divide rapidly, have high metabolic activity, or are less differentiated, meaning they have not yet become specialized. Examples include blood-forming stem cells in bone marrow, intestinal cells that continually replace the intestinal lining, and the germ cells that give rise to sperm cells. Developing tissues, such as those in an embryo or fetus and in growing children, also contain many immature, rapidly dividing cells, making them generally more radiosensitive than mature tissues.

In radiation oncology, the law helps explain how tumors and nearby normal tissues will respond to radiation, and guides treatment planning so an effective dose can be delivered to the tumor while limiting the effects on surrounding healthy tissue.

The biologic effect of an absorbed radiologic dose depends on more than the dose itself; it also depends on physical factors that involve the type of radiation and how the dose is delivered. Key physical factors include linear energy transfer, or LET, relative biologic effectiveness, or RBE, protraction, and fractionation.

Linear energy transfer describes how densely radiation transfers energy along its path through tissue. Low-LET diagnostic x-rays produce relatively sparse ionization of tissue, while high-LET alpha particles used in radiation therapy produce dense ionizations along a shorter path. In general, higher LET produces more biologic damage.

Relative biologic effectiveness, or RBE, compares the dose of a standard x-ray beam with the dose of another type of radiation, called the test radiation. These types of radiation compare the dose needed to produce the same biologic effect.

If a smaller dose of the test radiation is needed, its RBE is higher. Diagnostic X-rays are assigned an RBE of 1, while fast neutrons and alpha particles, which can be used in specialized forms of radiation therapy, generally have higher RBE values.

So, in general, radiation with a higher LET also has a higher RBE, meaning that less absorbed dose is needed to produce the same effect.

Protraction delivers the dose continuously at a lower dose rate over a longer time. Fractionation divides the total dose into smaller doses separated in time. For the same total dose, both protraction and fractionation can reduce the biologic effect by allowing intracellular repair and tissue recovery.

Radiation therapy uses fractionation to help limit the response of nearby normal tissue.

Physical delivery is only part of the picture. Conditions within the tissue also modify its response. One important factor is the oxygen effect, which means oxygenated tissue is more radiosensitive than tissue with little or no oxygen, especially when exposed to low-LET radiation.

Age also affects radiosensitivity. Developing tissues are most radiosensitive before birth and during childhood. Sensitivity generally decreases toward maturity and then sensitivity tends to rise somewhat later in life.

Sometimes, cells can recover from radiation damage. At a cellular level, if a radiation dose doesn't kill a cell before its next division, the cell may have enough time for intracellular repair of the damage. At the tissue or organ level, surviving cells can divide and replace cells lost after irradiation, a process called repopulation. Together, intracellular repair and repopulation contribute to recovery from radiation damage.

Moving on to radiation hormesis. This is the hypothesis that very low doses of radiotherapy may stimulate hormonal and immune responses and reduce some of its effects. There's limited evidence supporting this theory, so hormesis does not justify additional radiation exposure. Radiation protection continues to follow ALARA, meaning As Low as Reasonably Achievable, while obtaining the images needed for patient care.