Radiation damage can be either acute or chronic depending on the conditions. Acute radiation effects are due to short exposures to radiation at a relatively high dose rates and the impact is observed within days or weeks (examples include acute radiation syndromes). Whereas chronic radiation effects are typically associated with lower doses and the effects are delayed (examples include: cancer). In this post we cover acute vs chronic effects and the four high level cell damage pathways of: cell repair, first generation cell death, second generation cell death and cell mutation. Cell death is typically responsible for acute radiation effects and cell mutation is more often associated with chronic effects.
Somatic Effects (including non-carcinogenetic)
In this section we will discuss at high level the effects of radiation on somatic tissue (i.e. non germline tissue so not sperm or ova). We have discussed above the classical description for radiation damage via x-ray photons depositing energy locally in cells via energetic electrons.
In this section we will review the consequences after the damage occurs within a cell, what are the possible outcomes for that cell, and what impacts.
Cell Damage Pathways
X-Rays, when passing through human tissue, have effects on somatic sells and the effects described here are not related to cancer induction or carcinogenesis. First we will start by defining, what is a somatic cell? It is something other than a germline cells, as germline cells can be passed on to subsequent generations. Sperm and ova (i.e. human eggs) will be passed on to the next generation and somatic cells are all cells which are in your body but which are non-germline cells.
As we have described above in the diagnostic energy range x-rays primarily interact with the tissue in the human body via Compton and photoelectric effects. Compton and photoelectric effects both result in energetic electrons that are going to deposit their energy locally [link to image above on radiation biology flow chart. After the electrons have deposited their energy locally, those energetic electrons can either create free radicals (which subsequently damage DNA via indirect action), or if the electrons damage the DNA directly, that is called direct action.
After the x-rays deposit energy in a given cell there are multiple options which can occur at the cellular level
- Cell Repair
- Cell Death (First Generation)
- Cell Death (Second Generation)
- Mutation Propagated
We will first review these options at a cellular level.
The DNA damage can be repaired in some cases such a single strand breaks which can more easily be repaired either before or during cell division. Double strand breaks are more problematic but may be repaired with enzymes.
Cell Death (First Generation):
The damage is too severe to repair and the cell dies directly. One mechanism that can be responsible for this is apoptosis (programmed cell death), which helps organisms survive by not propagating mutations. Cell death is the primary cause of acute radiation sicknesses at very high levels of radiation dose.
Cell Death (Second Generation):
In some cases the damage is not significant enough to cause cell death directly but when the cell attempts to divide there is too much damage for the second generation (i.e. daughter cells) to survive.
When the cell damage is not repaired directly and the cell does not die it is possible that the mutation is passed on, or propogated. This is the mechanism for cancer indunction (i.e. carcinogensis).
Rad Take-home Point:
- There are multiple possibilities for cellular damage when x-ray radiation damages the cell.
- These include: cell repair, direct cell death, daughter cell death, and cell mutation
- Acute radiation syndrome is caused by cell death
- Cancer induction (carcinogenesis) is caused by mutations being propagated
Acute vs Chronic
These are just general terms but they are part of the ARRT x-ray content specifications, so you want to be familiar with them. We want to know that acute is just those conditions that occur over relatively short periods of time. These typically occur when higher doses are delivered a short period of time. The effects of acute radiation syndromes are relatively immediate, i.e. observed within a couple of days. And then chronic are long-term effects and can occur with a lower radiation dose. We have covered many of these possibilities on this post as well in more detail.
So, cell death again is primarily caused by high radiation doses and the short-term effects, for instance, acute radiation sickness. We have an
other video on acute radiation sickness. And then for long-term effects these include: cancer, inherited effects, and cataracts which we have a separate video on each of those as well.
Rad Take-home Point:
- Acute: high radiation dose over short period of time, immediate effects
- Chronic: typically caused by lower radiation dose, long term effects
Organ and Tissue Response:
As we discussed above the body is made up of many different tissue types and each of these tissue types will have different characteristics in terms of the radiosensitivity of the tissue. The radiosensitivity of different tissue types has been studied extensively in the context of radiation therapy treatment planning and response monitoring. In that case the desire is to expose the malignant tissue to a high radiation dose while minimizing radiation dose to the healthy tissue.
For our purposes the most important point is that the frequency of reproduction is linked to the radiosensitivity. Cells are more susceptible to damage when they are actively dividing as redundancies in the DNA cannot be used to repair the damage as easily.
The frequency of the reproduction – we talked about if there are x-rays that are incident on those cells and especially if that happens while they’re trying to reproduce, because of the frequency of reproduction and because the cells are more radio-sensitive while they are reproducing, that means that those cells that reproduce more frequently had a higher radio sensitivity.
As you can see in the figure the most radiosensitive tissues are lymphocytes, and bone marrow. On the other hand the tissue which does not reproduce at a high rate such as the muscle tissue and the nervous system tissue will be the least radiosensitive.
Rad Take-home Point: Organ and tissue response varies between different types of tissue and tissues which reproduce frequently such as blood product are more radiosentitive, whereas tissues which reproduce less frequently such as muscle and nerve tissue are less radiosensitive.