Why is radiation an effective treatment for some types of cancers?
To explain this concept, the following is an excerpt from: Basic Radiation Protection Technology, 3rd Edition, Daniel A. Gollnick, 1994
Cell Radiosensitivity Theories
Ideally we would like to have a law analogous to Newton's Laws of Motion or Ohm's Law which would predict exactly the kind and amount of injury produced in a given cell following an exposure to radiation. Unfortunately, such a law remains yet to be discovered. The field of radiobiology is thus dependent on "rules of thumb" to estimate radiation effects. Simplistically, this is the underlying reason for the adoption of the ALARA philosophy. ALARA is the acronym for "As Low As Reasonable Achievable." This attitude is applied to radiation doses received by nuclear technicians because we have no accurate theory to predict what really happens at the low dose rates of 0.01 to 0.05 Sv (1 to 5 rem) per year encountered occupationally.
The oldest, and perhaps best, rule of thumb was developed by two French radiobiologists, Bergonie and Tribondeau, in 1906. It offers a prediction about the relative sensitivity of two different types of cells or tissues to radiation. The so-called Law of Bergonie and Tribondeau concluded that cells tend to be radiosensitive if they have three properties:
- Cells have a high division rate.
- Cells have a long dividing future.
- Cells are of an unspecialized type.
The first condition can be determined by measuring the cell cycle time, i.e., the time between divisions. The second property refers to the fact that many cells go through phases in an overall life cycle. They begin by undergoing many cell divisions (childhood). They then enter a phase in which they stop active division and instead put together the internal structures to function in some usable capacity (adolescence). Finally they enter the last phase where they function fully in the job assigned (adulthood). Cells with a long dividing future would be those in the early immature phases where they are still dividing.
The last criterion, unspecialized, needs further comment. In the biological sense, this means a cell which is capable of specialization, at some future time, into one of several different "adult" cell types. An example might be one of the immature blood cells. Many types of blood cells are "born" unspecialized. Depending on the feedback signal received long after they are formed, they can choose to "grow up" and mature into lymphocytes, or different types of granulocytes. Probably the most unspecialized human cells is a fertilized ovum. From the single cell, daughter cells develop into such widely different mature cells as bone, brain, blood, and fingernails.
The generalization of the Law of Bergonie and Tribondeau is that tissues which are young and rapidly growing are most likely radiosensitive. A very practical application of the Law is given by NRC Regulatory Guide 8.13 which is titled "Instruction Concerning Prenatal Radiation Exposure." This Guide requires that woman of reproductive age be informed of the increased risk of injury of the human fetus from radiation exposure because such a tissue meets all the criteria of the Law of Bergonie and Tribondeau. The human fetus is particularly sensitive in the first few weeks of pregnancy when organs are forming. This is the time period when the woman may not be aware of her pregnancy. Most radiation protection standards recommend that the dose to a developing embryo and fetus be kept below 0.5 rem during the entire nine months of gestation.
Another rule of thumb is the Target Theory. It was developed in the 1920s and assumed that cells had a single, absolutely essential-to-life key structure (the target) which determined if a given cell survived or died after radiation exposure. If the target were hit, the cell would die -- a miss meant survival. Unfortunately, later work showed that animal cells don't have such a target. The theory applies strictly only to certain yeasts and viruses.
A final theory of radiosensitivity is worth mentioning. This is the ICV Theory developed by Sparrow at Brookhaven National Lab in 1965. It is able to accurately predict the lethal dose of radiation to any type of plant based on knowledge of the size of the nucleus and the number of chromosomes. Unfortunately it fails miserably when applied to animal or human tissues.
