TOPIC 4: Variables in Dose Effects

Let’s look at the different variables that can explain dose effects.

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The Law of Bergonie and Tribondeau, 1906 was developed by French researchers’ work on rodent testes.  The result was a widely promulgated “law” regarding the cell sensitivity to radiation.  This law states:  Sensitivity to radiation varies directly with reproductive capacity and inversely with differentiation or maturity of the cell.  That is the basic concept radiation therapy of cancer cells.  Cancer is a very rapidly dividing and not a very differentiated (mature) cell.  Thus, the cancer cell is more sensitive to radiation.

Differentiated or mature cells are not as sensitive to radiation.  The most differentiated cell is the nerve cell.  So, since the brain cells are the more differentiated and not rapidly dividing, these cells are doubly resistant to radiation.   Thus, using radiation treatments on cancers in the brain can be effective.

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Some tissues in our bodies are more resilient to radiation than others.  This concept is used to assign a weighting value, or factor, for risk to the tissue from radiation.  Sensitivity to radiation varies directly with reproductive capacity and inversely with differentiation (maturity of the cell). The more sensitive tissues or organs have the highest weighting factor.  These values are called Tissue Weighting Factors (WT).

As you can see from the table, scientists believe that the gonads, which are rapidly dividing cells involved in reproduction, are the most sensitive tissues. You can scan down the list to see which organs or tissues are sensitive enough to be singled out. Some organs and tissues are not very sensitive at all, such as muscle tissue and even the brain. The brain has highly specialized, or differentiated, cells that can withstand higher radiation doses.

The main point of this section is to show you that some organs and tissues can withstand radiation damage better than others. The more sensitive organs and tissues are to radiation the higher the risk to negative health effects like cancer.

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We have looked at the sensitivities of different organs, but what about the radiation sensitivity of the entire person as it changes with maturity? As you may have suspected, the more developed and mature a person is, the less sensitive that person is to radiation.

As you can see, healthy adults are the least radiosensitive people on average.

How about older folks?  Their overall body health is less than a younger person.  Their immune system is not as robust as when they were younger.  Thus they are unable to withstand as much dose as a healthier, younger person.

Older age is an important factor when doses are extremely high, because they don’t heal as well.  But for low doses, where the concern is development of some form of cancer, older adults may not have enough lifespan remaining for any cancer to develop.  It sounds morbid, but it is a fact of life.