High-energy particles and Radiation burn

[Vicol]

Hello, it is my first post created in this forum :)

I'm interested in physics of radiation burn. How does it work?

In my opinion high-energy particles (like photons, neutrons etc.) leave their energy inside human body (for example breaking bonds in DNA). This event trigger primary electrons with high kinetic energy. During their collisions with molecules/atoms they cause vibrations of the molecules/atoms. And as we know - vibrations mean high temperature.

What do you think about this explanation?

 

[gleem]

The radiation "burn" has nothing to do with temperature. If you calculate the increase in temperature due to the absorption of a given dose of radiation you will see that the rise in temperature is minuscule. A typical erythema ( inflammation) dose is about 6 gray I.e. 6 J/kg.

 

[Vicol]

Well, there are many photos which shows human body after "face-to-face" meeting with radiadiaton above 10 Gy. For example Hisahi Ouchi who lost his skin completely. I'm really instreasted in biological aspect of ionizing radiation. I'm trying to understand how organism reacts with high energy photon. If radiation burn is not connected with temeperature then question appears - why skin melts (as it is said in diary of Hiroshima survivors)?

 

[gleem]

Radiation produce copious amounts of free radicals which are more responsible for cellular trauma than the direct effect of the radiation on the cells themselves. People have varying responses to radiation. The example you gave, what was the circumstances of his/her exposure?

 

[Vicol]

16 kg of uranium - by accident Hirashi Ouchi exceed critical mass. He absorbed 14 Gy. You can google his photos after this (warning, they are not charming).

 

[gleem]

spontaneous fission accident. His absorbed dose was probably his estimated deep whole body dose, that is the dose to his internal organs. His skin dose could have been substantially higher. 6Gy skin dose is the threshold for erythema the onset which will occur in a few days. Moist desquamation is the sloughing off of the outer protective layer of the skin exposing the capillaries thus producing bleeding. A whole body 14 Gy dose is fatal and is the threshold for desquamation.

Anyway it is the free radical chemical entities characterized by an unpaired orbital electron formed from the interaction of the radiation with intercellular water. This forms radicals such as OH⋅ , H⋅, and HO₂⋅ The free radicals then interact with the other organic molecules changing their chemical behavior.

 

[mfb]

The skin burns from the Hiroshima and Nagasaki bombs were not from high-energetic radiation - they were from thermal radiation in the infrared to UV range.

An x-ray/gamma-ray dose sufficient to cause thermal burns is deadly within minutes or less because it destroys the whole cell chemistry.

 

[Vicol]

So high energy photons destroy DNA bonds not only directly. Water molecules can be "sliced" to OH (for example) and then this basic group connect with DNA.

Generally I think I should consider many "planes" of human body - not only microsopic as DNA or cell. By the way It is mysterious how microscale can trigger vomiting, headache and other effects.

Anyway - how is it possible that temeperature of body doesn't increase during/after dosing radiation? And how are these effects like desquamation connected to microsacle? Is it only due to of this chemical properties of newly created molecules (OH, H etc.)?

 

[mfb]

Anyway - how is it possible that temeperature of body doesn't increase during/after dosing radiation?

It does, but the effect is tiny even for potentially lethal doses.

5 Sv is a lethal dose, for photons this corresponds to 5 J/kg. That is sufficient to heat the body by a bit more than 1 millikelvin. The thermal effect is completely negligible.

By the way It is mysterious how microscale can trigger vomiting, headache and other effects.

The body reacts to cells that don't work properly any more (and commit suicide) because their DNA got ruined.

 

[gleem]

Generally I think I should consider many "planes" of human body - not only microsopic as DNA or cell. By the way It is mysterious how microscale can trigger vomiting, headache and other effects.

Is it?

Mere mg quantities of nature materials can kill e.g. a smear of peanut butter can elicit a life threatening anaphylactic shock in an allergic person. So it is not surprising that small quantities of free radical can cause severe reactions in the body.

While we think of DNA as a strategic target of radiation there is only a small amount compared to RNA which is necessary for the transcription of the DNA code to useful functions as protein production necessary for normal cell function.

Free radicals remain active in the cell long after the radiation as left i.e. microseconds vs nanoseconds for radiation transit.. It can be estimated that free radical are produced in apparently large amounts. There are about 3x10¹⁹eV of energy deposited by 5Gy. If each free radical contains 3eV (my guess) of excess energy that give about 10¹⁹ free radicals per Kg. That is 0.000017 mole/Kg or 300 ug of OH /Kg of exposed tissue.

 

[Ygggdrasil]

Even relatively small amounts of radiation is capable of killing cells because cells are programmed to "commit suicide" (i.e. undergo apoptosis) in response to DNA damage. This programmed cell death occurs to prevent cells with extensive DNA damage from replicating. I'm not sure the extent to which cell death in radiation burns is due to apoptosis or physical damage to the cells induced by the radiation, but both probably play some role.

 

[Choppy]

And how are these effects like desquamation connected to microsacle? Is it only due to of this chemical properties of newly created molecules (OH, H etc.)?

To add to what's been said, there's a lot that happens in response to radiation damage.

Low levels of DNA damage* can trigger differences in epigenetics - the expression of certain genes. As damage accumulates, the cell cycle will fail to progress through it's various check points - meaning that you're going to put a halt to cell division. In an organ like skin that relies on the cells continuously replacing themselves any interruption in this process is going to translate into macroscopic consequences. As Ygggdrasil pointed out, enough damage can trigger apoptosis. So now, not only does the clonogenic process stop, but the cells begin to systematically digest themselves. And the apoptotic response is not necessarily limited to cells that are directly hit either. There are "bystander effects" where heavily damaged cells can signal damage to other cells and induce apoptotic responses in them. As the radiation dose increases further, other organelles become affected too. Once you get into the 8 - 12 Gy range and above (delivered in a short period of time), there's some evidence to suggest that's where you start to see breakdown of membranes. And once this happens, you have not just the "sensitive" cells reacting, but just about all of them. The cells that make up blood vessels can be effected, leading to a breakdown in the microvasculature.

It's a complex system and a lot can happen when the equilibrium state is perturbed.


*Note that DNA is being damaged all the time, primarily by the free radicals produced in the cellular respiration process and, to a lesser extent, but background radiation. When I say "low levels of DNA damage" I am still referring to levels that are higher than this background.

 

[Vicol]

So can we say that apoptosis take a form of desquamation? Cells "know" that their DNA is damaged and in this case it is better to leave human body?

 

[gleem]

I would say no. The way I understand it apoptosis is a special type of cell death i.e. programmed cell death where at some time a cell spontaneously dies but it dies in a special way so as to let the body dispose of it, i.e., absorb it. In desquamation the cell death is due to external trauma resulting in cellular destruction the result of which does not let the body absorb the dying cells resulting in the accumulation of dead cells in that area. A apoptototic process the dead cells just disappear.