hi and welcome to radonk talks a lecture series designed for students and residents of radiation oncology my name is Ria mulherker I'm currently a fourth year radiation oncology resident at the University of Pittsburgh and today I will be taking you through our second installment in our introduction to radiation oncology series talking about the basics of radiation biology if you had a chance to listen to part one of our lecture series introduction to clinical radiation oncology you probably got a decent overview of what we do as radiation oncologists and the different radiation therapy modalities that we use in this particular talk I'd like to get more into detail about how exactly radiation works we'll talk about what it is what it does at the cellular level and then lastly touch upon why radiation treatment is usually given in multiple fractions the first question we have to ask ourselves is what even is radiation and the answer to that is that radiation is simply energy which has an effect in our bodies at the atomic level if you recall an atom consists of a nucleus made of protons and neutrons that are tightly bound together and electrons that orbit around the nucleus in various shells the radiation that we talk about is categorized as ionizing versus non-ionizing based on what effect it has on those electrons now I have to understand things in as simple terms as possible so I will show you pictures of electron orbitals but just bear with me for a second and listen to my dog analogy so imagine the house is an atom and a dog is an electron let's say that at rest the dog is in its ground state or in its lowest shell which is the couch lowest energy when Mom rings the doorbell and comes into the house the dog gets excited so he goes from that low energy shell of the couch to a higher energy shell which is the window so the dog is excited but he hasn't left the house so similarly when non-ionizing radiation hits an atom it can cause an electron to move from its resting shell to a higher energy shell so the electron doesn't leave the orbit but the atom is said to be excited ionizing radiation on the other hand I analogized to the mailman coming home so unlike mom who just causes excitement the mailman is actually aggravating right so the dog is already at the window in that outer shell and then whenever the mailman rings the doorbell and the doors open to let the packages in that dog is exiting the the house right that electron is exiting the orbital and that type of radiation is considered ionizing radiation when the electron actually leaves the outer shell and that atom is said to be ionized now there are two types of radiation that you should be familiar with the first is electromagnetic radiation and you might be familiar with the electromagnetic spectrum the same spectrum that has microwaves and visible light rays at kind of the lower energy end at the high energy end of that Spectrum exists X-rays and gamma rays everything on the spectrum is considered to be photons which are just massless packets of energy um the lower energy end of the spectrum so the microwaves the visible right light rays and things that we talked about they have the ability to cause excitation of the electron so the dog goes from the couch to the window um however at the high energy end the x-rays and the gamma rays these are the types of photons that can actually cause ionization so the dog goes from the window to actually leaving the house these electrons get ionized and then they are free to go and interact with other molecules most of the time when we talk about radiation therapy we are using photons so most commonly it is x-rays you might also hear of a gamma knife machine that uses gamma rays the only difference between X-rays and gamma rays is how they are produced so x-rays are produced extra nuclearly and gamma rays are produced intranucerally by the decay of very heavy nuclei the other type of radiation to be aware of is particulate radiation so like I said most commonly when we're talking about using x-ray therapy or when we're talking about radiation therapy we are referring to x-ray therapy which is a type of photon however sometimes you will hear about different types of particles being used now particulate radiation all of it is ionizing and very commonly you might hear about electron therapy electrons are beta particles and you know electrons have a fairly low mass so we said photons are massless electrons have a mass but they have a fairly low mass and so they don't travel very deep into the tissue so that's why usually we use electrons to treat superficial targets such as Skin tumors the other very common particulate radiation that you might hear about is proton therapy protons are very heavy actually their mass is about 2 000 times that of an electron and so in order to accelerate them to energies that are high enough to actually be therapeutic they require very intense Machinery such as a cyclotron there's a lot of new proton centers that have been installed in the United States but they are still not quite ubiquitous but proton therapy is another type of radiation and just because of the heavier Mass whereas photons can travel completely through the body and they usually have an entry dose and an exit dose protons have more of a stopping point and so they are better in some cases at sparing critical structures that might be Beyond a particular Target so I don't want to get into too much detail about that right now but you should just be aware that you know electrons and protons are some types of particulate radiation you might see used in treatment other types you know we have alpha particles and an alpha particle is basically a helium nucleus which consists of two protons and two neutrons so as you can imagine alpha particles are quite heavy the other particulate radiation that you might hear about in your radial biology lectures is neutrons so that was a lot of information but the point is that radiation is energy and it can exist on an electromagnetic spectrum or it can exist as particulate radiation the main type of radiation that we are talking about when we talk about radiation therapy or even the radiation that's used in Diagnostic Radiology is ionizing radiation so ionizing radiation comes from photons at the high energy end of the electromagnetic spectrum these are X-rays and gamma rays as well as different types of particulate radiation such as electrons protons neutrons and alpha particles now the next question you're probably wondering is well how does radiation actually work and why is it an effective treatment against cancer so let's get into that we understand now that we are talking about ionizing events and ionizing radiation is what we use in Diagnostic and therapeutic radiation however there's a difference between directly ionizing and indirectly ionizing radiation so let's talk about that charged particles such as protons and electrons are directly ionizing so as long as they have enough energy they can directly pass through the tissue and they themselves can produce chemical and biologic changes that lead to the DNA damage which is how cancer cells die electromagnetic waves and neutrons on the other hand are not charged themselves and so they are indirectly ionizing they don't produce the chemical and biologic changes themselves as they get absorbed in our tissue they give up some of their energy to produce fast-moving charged particles such as electrons that we talked about that in turn can produce fast damage so the the dog analogy that I described earlier it really applies to X-rays and gamma rays which are photons right on the electromagnetic spectrum they interact with the atoms inside of our tissue and they produce fast electrons neutrons on the other hand have again a mass about around 2000 times that of an electron and they're much heavier they actually can produce spaliation products so they can directly interact with the nucleus and produce you know multiple alpha particles neutrons and things like that and so the point here is that if a particle itself is charged it can be directly ionizing so these are protons and electrons X-rays and gamma rays as well as neutrons are indirectly ionizing most of the time when we talk about radiation we're going to be talking about photons so X-rays and for this the indirectly ionizing component I really want you to think about that dog analogy now photons can actually have slightly different interactions with atoms depending on the energy of these photons so there's different effects and this is all you know covered in great detail if you are to pursue radiation oncology and take a radiation Physics course but just to give you an overview one such interaction that usually happens at the energies that we use for Diagnostic Radiology is the photoelectric effect so what happens in the photoelectric effect a photon hits an electron usually at a lower energy shell and that electron ends up getting ejected from the atom so the electron leaves the atom because that lower energy electron spot is now empty an electron from the higher energy shell will release some energy and actually move down in energy to fill the empty spot left by that lower energy electron right so kind of think about it like seats in an auditorium if the you know if the front row is you know you you want to fill up from the front row back so if the front row is empty and a spot opens up then somebody from a back row will go ahead and fill in that empty spot in the front row so the same thing is happening here think of the front row as that inner shell low energy orbital so when that electron leaves an electron from a higher shell goes in and fills that space in the process however that electron is losing some energy because it's going from a higher energy state to a lower energy State and so in that process something known as a characteristic x-ray is released that x-ray is a photon that just represents the energy difference between those two shells so this is called the photoelectric effect and this is the main effect that predominates at energy levels that we use for Diagnostic x-rays now if we have photons of higher energy interacting with the atom then they are also going to interact with the higher energy outer shell electrons so photons at high enough energy can interact with these outer shell electrons causing something known as the Compton effect and when we think about what is happening at the atomic level in therapeutic radiation it is really the Compton effect that predominates so a high enough energy Photon hits an outer shell electron of an atom causing that electron to be expelled the photon has high enough energy that it's actually um you know the photon simply gets scattered in this process and now the photon has somewhat lower energy but it's still high enough that it can go and interact with another electron having that similar effect so when the competent effect occurs at these therapeutic levels of radiation what we end up with is an ionized atom as well as an electron that has been ejected from its orbital now keep in mind that both of these interactions are probably happening simultaneously it's just that the probability of a given interaction happening is different based on the energy so again the photoelectric effect is higher probability at lower energies whereas the Compton effect has higher probability at higher energies which we use in therapeutic radiation just to kind of sum things up for you I think the major takeaways thus far we talked about ionizing versus excitation when it comes to radiation and we know that ionizing radiation is really what we're talking about and then there's electromagnetic radiation and there's particulate radiation electromagnetic radiation which is ionizing refers to X-rays and gamma rays which are called photons and then particulate radiation is divided into charged and uncharged particles and then we kind of talked about those interactions that are actually happening when the radiation hits the body we talked about the photoelectric effect and the Compton effect but without getting into too much detail about it the the biggest thing to remember is that all these ionizing radiations are leading to the production of electrons and ionized atoms so thus far we have really focused on what radiation is and what directly what immediately happens once that radiation hits the body now we figured out that once radiation hits the body most commonly what's going to happen is we're going to end up with a bunch of electrons getting released from their orbitals and we're going to have all these ionized atoms The Next Step then is to actually cause DNA damage and DNA damage is really the mechanism by which radiation is working to kill cancer cells so when we talk about how that DNA damage occurs we can categorize it as a direct versus indirect action of radiation now this is not to be confused with direct versus indirect ionization which we talked about earlier remember we talked about how charged particles are directly ionizing whereas electromagnetic Rays as well as neutrons are indirectly ionizing we've already talked about that the ionization has happened now we have a bunch of loose electrons so the action of these electrons to go and damage the DNA can also be categorized as direct versus indirect the direct action can happen when the electron that is ejected directly goes and interacts with the DNA and causes DNA damage so that is a direct action of radiation the indirect action which is actually much more common when that ionized atom or electron goes and interacts with a molecule such as water causing a generation of a free radical and then the free radical that is generated actually goes and interacts with the DNA causing DNA damage so the you know it can happen both ways direct or indirect but the indirect action is much more common because water is so prevalent in the body it's very likely that once you have that electron that's dispelled and interaction will take place with water generating a free radical and then it's that free radical that goes and actually damages the DNA and so essentially you know all of this lecture so far has been to explain that the main mechanism by which radiation works is damage to the DNA of the cancer cells and when radiation hits the body we first have some interactions that take place that result in the creation of electrons those might result in the generation of free radicals and then it's either the electrons or the free radicals that go and interact with the DNA and cause DNA damage of course when radiation causes DNA damage there are different types of DNA damage that can occur single stranded breaks are probably the most common of um you know radiation induced damage that occur but single standard stranded breaks are generally not lethal and they can easily be repaired because our cells do obviously have a lot of repair mechanisms it's really the double-stranded breaks when both copies of the DNA are damaged that are lethal however these are less common as you can imagine another type of damage that could occur is chromatid aberrations which are also lethal so just to be aware of you know different types of DNA damage may or may not be repaired it's really the damage that is lethal such as the double-stranded breaks that we are interested in causing because this is what will lead to cancer cell death another thing to kind of keep in mind is the time frame of all this right so when the radiation hits the body the electrons are produced the electrons create free radicals all of that happens on the order of somewhere between 10 to the negative Fifth and 10 to the negative 15th seconds so it's all happening very very very rapidly the end result is DNA damage and for the cell itself to recognize the DNA damage to attempt and fail at repairing it and then to induce either apoptotic cell death or mitotic catastrophe that process can take you know days and so when I always tell patients is that after they're done with radiation treatments they're definitely not Radioactive however the effects of the radiation are going to continue to affect their cells in the days and weeks to come so patients are not radioactive when the X-ray machine is turned off however the effects of the radiation continue to last for days to weeks and that's why it can actually take time for patients to notice Improvement of symptoms or to notice the onset of side effects from radiation now one thing to keep in mind is that the cell has a different susceptibility to radiation depending on where in the cell cycle it is so if you recall the cell cycle has the G1 interphase stage then we have the S phase where DNA synthesis happens and then we have the G2 phase where we're getting ready for cell division and then we have mitosis and cytokinesis so in the S phase and the late G2 phase our DNA has already divided and we have two copies of the DNA and if you remember anything about DNA repair from medical school you'll recall that there is homologous recombination and non-homologous recombination and homologous recombination is more accurate because we've already divided the DNA we have two copies to work with if one copy gets damaged we can just look at the other copy and fix the damage so when the DNA has already divided in late S phase and in G2 phase when radiation does happen our cells are better able to fix it and so in these stages the cell is more resistant to radiation now in the mitotic phase you know we have kind of minimized the contents of our cell and we've kind of slimmed down everything and we're just trying to divide the DNA at that point you know it's either you go all the way or you don't go at all and so at that point if the DNA gets damaged then the cell will almost certainly die via mitotic catastrophe and so it's in M phase when those cells are dividing that they're actually most sensitive to radiation and it's really important for us to realize you know that mechanism because that affects which types of cells are going to be more sensitive to radiation just as chemotherapy affects the cells that are rapidly dividing so that includes cancer cells but also the normal cells in our body including those of the hematopoietic system the reproductive system the gastrointestinal system and the skin cells and hair follicles radiation therapy also preferentially affects those cells which are rapidly dividing and again it goes back to that cell cycle and the fact that cells which are in the S phase and G2 phase are most resistant to radiation and the cells in the M phase are most sensitive to radiation so the cells that are you know in the active mitosis or the ones that are most likely to die and all these cells that I just mentioned that are actively dividing are most likely to be in mitosis now another parallel to chemotherapy just as chemotherapy is given in multiple Cycles either one week or a few weeks apart radiation therapy is typically divided into fractions so the full dose of radiation is divided into multiple fractions and most commonly these are given every day Monday through Friday for a period of weeks but sometimes we can do what is called hypo fractionation or Ultra hypofractionation and kind of reduce the number of fractions so really briefly in this last section I just want to review kind of why this is the case and why we think about giving radiation in multiple fractions the rationale for fractionation really falls back on the four r's of radiobiology which are repair repopulation redistribution and re-oxygenation so let's go through them one one at a time and kind of understand the first two r's of repair and repopulation kind of came about after some animal experiments that showed if we give one giant dose of radiation all at once it causes a desired effect and in these experiments the desired effect was sterility but it also causes a lot of toxicity to the skin however if we give that same dose of radiation split up into multiple small fractions we still cause sterility however we cause less skin toxicity and the reason for this is that cancer cells and normal tissue cells have different capability of repairing after radiation so fractionating radiation and splitting up the dose into multiple small doses actually takes advantage of this and it allows for our normal tissues to heal in between treatments while gradually building up to the dose that will cause cancer cells to die the idea is that the cancer cells are much more mutated and they're less capable of repairing damage whereas um are normal cells actually have the ability to repair and repopulate thus causing less toxicity the third R is redistribution so we already talked about why cells are more sensitive to radiation during the mitotic phase and they're more resistant to radiation during late s and G2 phase well you can't expect that a particular cancer at any given time is going to have all the cells inside of it being in mitotic Phase right they're going to be in different phases of the cell cycle so another advantage of fractionating radiation is that we can allow for some redistribution so you know initially when you give a dose of radiation the cells that are in the mitotic phase will die and then in between treatments you assume that the surviving cells are going to continue to go through the cell cycle and get redistributed to different phases of the cell cycle and so that just allows time for more cells to reach that M phase so redistribution is another rationale for fractionating radiation the final R of radiobiology is re-oxygenation so we know and we talked about how cells the damage of DNA is mediated by the creation of these free radicals that can go on to cause DNA damage and oxygen is required to create these free radicals so especially for sparsely ionizing radiation such as X-rays and gamma rays the presence of oxygen is going to be really important to help create these free radicals and help mediate that DNA damage but why is re-oxygenation something that we even worry about I think this diagram helps us to picture it a little bit better so if you think about how cancer cells grow you know they they can divide very rapidly and in the process of that rapid division the growth of blood vessels may not be able to keep up with the cancer cells dividing so you can end up with you know kind of a sphere where the cells on the outside of that sphere are well supplied with blood and they are aerobic however the cells that are more Central are hypoxic and if they become hypoxic enough they can even get necrotic so when the radiation first hits it's most likely going to kill off these aerobic cells because they are the cells where the free radicals can be created leading to DNA damage and eventually as those aerobic cells die off and the cancer starts to shrink the blood vessel can better reach those hypoxic viable cells and then that oxygen can help enhance the effect of radiation and more effectively kill off those cells and so that's kind of what we mean when we talk about re-oxygenation so all these four r's of radiobiology really help us to explain the rationale for fractionating radiation you might wonder though what about hypofractionation or even stereotactic radiation where we can deliver very high doses of radiation and as few as one to five fractions why does that work and the answer is that dose is dose and if you're able to give a high enough dose of radiation to kill the cancer then even one fraction is effective for example in a lot of brain tumors we just treat these with one fraction however um in the time that radiation was first starting to be delivered we did not have these sophisticated techniques so any dose that was given to our Target organ was very likely to be received by all the surrounding critical structures and so the treatments were very toxic and so fractionating the radiation helped us to kind of spare the toxicities allowing our normal tissues to heal while building up to a dose that would effectively kill the cancer Which is less able to repair now in the modern era with hypofractionation and stereotactic radiation a lot of the times we're still taking advantage of some of these principles but at the same time we're able to give much higher doses of radiation effectively killing the cancer cells while now being able to much better spare all those surrounding critical organs so sharper dose falloffs more conformal treatment plans and so on and so that's kind of why hypofractionation and stereotactic radiation are are better for us in the modern era in some cases so that is all I have for now in terms of a basic introduction to radio biology these are some questions that I had when I was a medical student rotating through the field and even as a resident it took me a little bit of time to understand exactly what is radiation how is it working and why do we give it infractions and so hopefully you found this talk useful and if you didn't enjoy my dog analogy then hopefully you at least enjoyed the cute pictures and thank you so much for your time