Dosimetry: fundamentals I

good morning I want to give lessons or now courses on the symmetry fundamentals and then also details for calibration with photons and electrons and later on we will do perform an exercise these lectures do not exactly fit to the time schedule which was given to me so I take the freedom that we will stop at ten minutes before 1:00 then we have lunch but I will not still finish my lecture I will continue later on and I take the freedom to move freely with my time because I have a lot of time for electron to symmetry which is the smallest one of them so let's start so this is a content I want to to say just once more I will tell you especially for those who are working in the field for your therapy you are familiar with many of the things nevertheless I will repeat them and offer to you what I want to discuss is this term radiation dose we are always saying that those and we as physicists we should at least be aware of the precise meaning of what is those and what is the definition of those I want to give general methods for those measurements and there I want to cuss these principles of the symmetry with ionization chambers in my eyes the symmetry with ionization chambers is still by far the most important one because because why what is what may be the reason why it is so I important I think from a physical point of view reason is that we we understand the details the physical detail is much better than any others do you really understand diet have you who is a man who is a physicist who has learned solid-state physics it's it's it can be awful it's it's cramped mechanics and it's it's not easy to understand the same with a diamond I think we don't know exactly understand how a diamond works thin dosimetry how it really works that and then the developing all this physical projects are quite complex and it's easy to use but to really understand and then to describe them in mathematical formulas how the energy deposition goes up into our signal it's not always so easy with an ization chamber is much more easy still we do not understand everything it still I do not understand having having been studied ionization symmetry for thirty years principles those and air stopping power conversion into those in water Bergman air conditioning spends the ethics formulation so this is what I've used this is this famous group from the agency and this is all a very very good book I think it's it's in my eyes it's the best textbook available now for medical physicists handbook of radiotherapy and it was issued by maze Nahum and I was invited by the way I just maybe you know that but you can get the book directly from the agency website and you can also get slides which are based on that which the slides may be helpful if you are do your own teaching and if just to say that because I was involved in this pro preparation of this lighting if someone really needs a source file is I'm free to give it also as as PowerPoint files in order that if you want to use it as a teacher you can change them in new ways and you can improve them then it's better to use this power the PDF files so exact physical meaning of dose of radiation so as I already said those may be a little bit sloppy and it's the dose of religion Cardinal by the physical physical well-defined crystal cornea of absorb those d and the definition of this is given in these eyes in report number 60 which is here but now we have a new one though this is a new hi-c report number 85 and in this report and I can really strongly recommend you to get this ISO report which is summarized the mentals of what are the physical quantities required in radiology it's not so much but this is the most important source for that according to that the absorbed dose is defined by ratio of the mean energy imparted to a matter of mass divided by a small element of mass the unit is of course you know Joule per kilogram and special name is gray so we can a little bit discuss it now because they are combined with this definition there are some characteristics and I have put here for characteristic number one is the term energy imparted can be considered to be originally absorbed in a volume in such a way that the energy absorbed is that what radiation is coming into the volume minus energy going out of the volume radiation t plus some other energies which may occur if there are processes which are releasing mass for instance if you are creating a new particle so then you have to put out the energy of the radiation in order to create the mass and vice versa so there may be some changes of energy radiation and that has all been taken to account in this definition the term absorb those are first when an exactly defined volume that is so important so if you'd think and those you always think in terms of a volume or a mass with a certain volume so not a point the point in the meta meta census has no volume despite the fact it is it is defined as a differential ratio you have to think in volumes for those who are working as Montecarlo they they are quite familiar with that you you cannot calculate the dose and the point you have to calculate those in the volume you have to define the volume for that first next point is it refer to the material of the volume so absorb those always refers so it may be the dose in air or it may be those in water or whatever it may be those in tissue and I think many of them are you are familiar with this discussion in treatment planning what is important do we calc good we offer to the doctors information on the dose in T Co or in water we have always done in water we have always used the term absorbed those two water but of course it is not water it may be it bones whatever it soft tissue so maybe a better concept to to use this especially and that is all again important treatment planning systems modern treatment and system which are already commercially they are introducing monte carlo techniques and in Monte Carlo is much more directly to to calculate the dose in the volume of the tissue of the patient or some approximation they taking calculate the dose in the tissue so to get they get now that those distribution and tissue and bone so now who they had to translate it into old-fashioned war apps or what absorb those so just to see that that that those absorb those refers to a material next one that is very interesting the dough's absorb those is a quantity which is steadily in space and time that means it is a quantity you can differentiate so it is it is defined in a point it refers to a point and if you look at that that may be a contradiction to - I said it refers to volume so what about this thing we have absorbed those referring to a point you can say this is expressing this our vector and but it on the same x refers to volume so in order to understand this a little bit more closer I want to discuss now a little bit more background on what's really going on in the original especially if we have this definition this question is the sooner into the question what energy imparted is so you you know the drop dosed if is defined by the energy imparted so the question is we need a definition now for energy imparted and here's a definition the energy imparted to matter in the given volume is a sum of all energy deposit in the volume quite nice we have one definition and we substituted for with another definition with another quality energy deposit maybe that is again not really helping so we have again but let's see what what is meant with that I have made this picture so we have photons coming from the left side and there is a company fact so we have created secondary electrons and this is again we have another one which is a photo effect another one for the effect though that happens if tissue or what is irrigated so in regard to our definition now we have to introduce a volume and what is energy imparted why it's not coming oh yes let's go back one this is important step the energy imparted is all the energies just to remind you if an if an electron is traveling to material is it is giving energy continuously all the way the famous question is why electrons have a final range in contrast to photons photons are exponentially decreasing why photons and of our electrons have a final range very simple why yeah okay they elude the energy and then it's gone then have to stop that's a very nice question to medical students because we also have to teach the medical system erican students and they need some understanding of that and such a simple question they it's so simple and they really it helps to understand why or what is the process with charge panic they continually are losing energy so this is the illustration that the electrons are continuously lost in losing energy and only this part which is within the volume this is the energy imparted so this is not energy imparted here and that's all the energy imparted the volume so this is a clear illustration what is meant by energy imparted what we need for the definition of absorbed dose so and this the energy and pilot is the sum of all elemental energy deposits by those basic in the action projects which have been occurred in the volume during a time interval considered so it's the formula this R this is our energy imparted and these are the single energy deposits so we need a definition of what what is an energy deposit and again you use this formula but what is important now it refers to a single interaction process between the parting of the photons or electrons and the medium so we have single in the extra process and with each interaction process we are looking on this equation we have energy going into again we can imagine a very small volume some energy is going out on we have again change of energy now gives three examples electronic on interaction paper production post on inhalation so we have here an electron coming into in this very small volume and we have the primary electron and we have a secondary electron and this way it's a quite good question what is a primary electron and what is a secondary or electron can we can we see what is a primary can we can do we associate one is a primary one is secondary no pie this is something which is again basic physics yeah yeah but which one how we can this you see we have a knock-on process and two electrons are coming out which one is primary which one is secondary it's very simple the higher energy we call the primary very simple okay so there may be some process here fluorescence radiation may come out and if even all J electrons may come out and this now is a formula the energy the energy the possesses is energy the kinetic energy coming in with these electrons and this are now escaping so all these terms are going out so this is now a definition of an energy deposit of a single event another one is we have photon in relation creating two electrons and this is a now another formula the energy deposit is the energy of the photon then these energies of the electrons are escaping but we also have to subtract the amount of energy to create this tool and this is matter one absorb those it is positron and annulation process and again we have such a formula just to say now I think we have now a better anesthetic water and energy depositors can be very different in size and type and so on again this is a formula energy of the incident unity particle excluding the rest energy and sum of energies leaving and the change of rest energies now we can this was theory but very simple what if you do a measurement we are simply adding together all this we measurement signal is proportional to the imported energy okay now a thing which is not so well aware but I think it is important especially if you are thinking in things now in what really happens in tissue or in a cell or in an enzyme what really happens there I have offered you the illustration of different energy deposits and it's very clear that the energy deposit one single can be quite different in size so and we never knows which will come first because it's a stochastic process this again this is a the principle physics it says some probability for process but we don't do candidate this single photon will have this type of the action it will have some probability for any one okay so if if we add up stochastic quantities the result will also be stochastic so our energy imparted is stochastic and what does it mean it means that respect to repeated measurements of energy and part that what you remesh it means that it will never have the same value is that really true it's something strange if you do with those measurement expect that if you do a careful measurement you want to have an egg accuracy or uncertainty say of power of 1% and it should be not not change if you have a huge number of deposits then on average they will be almost not change but in certain cases you can you can see changes and when of course if the number of events is small and when is the number and the right conditions the number of deposits is small it's attacked a small or if the dose rate is small those rate is small in radiation protection so in radiation protection we have such problems the detector is small if we measure those that that the normal is not small so even if you go to one millimeter or say ones limit diameter but you can you can you can use a detector which has reduced pressure air pressure and there are some detectors are available you can they are called so-called Rossi counters that are filled with with with equivalent tissue quality yes and the pressure is only say 101 1 1 / hundred of normal pressure and if you do measure that you will see and it's interesting so if you are interested in study such variations you could use there's a method technique available which has been introduced 34 years ago it's called micro symmetry so Mike under symmetries asbestos better especially basically which is dealing with the thing and the other thing of course is what is happening in a Cell what is happening in in the size of DNA of course so you if you you cannot really speak of absorb those within us in a small cell it will be at some variation and the variation can be huge it can be say it can be if you if you apply one grade in a small size it could be 0.5 grades and really small so in red year in the radiation biology it may be important to take such things in the count but not for normative symmetry as normal medical physicist with another never came across with this thing but I think it's quite interesting to know that that this we have this variation and I shall show you this one picture it's showing the ratio of energy imparted by the mass and now the mass is here really a big mess and then it's it's going on smaller and smaller and you see that that the energy imparted is really strongly variation it's a famous graph which is taken from textbook 30-40 years ago and I think it's I like this picture because it underlines the the knowledge that that this is the case of energy and poverty so and this is a reason why this is not the correct definition but it's a mean value by it because telling you in just explain it's defined the absorbed dose is defined by the mean value of the energy part and that's by this definition it now a quantity which refers to a point in which is steadily in time and it can differentiate it so this mean value is quite important here so the energy imparted is Taurus the quantity and the adsorbed or season comes to has the quantity this is a conclusion from that so in in textbooks especially for a really assistant you always see this this definition its de so it's just to say days it's not quite correct and I ask you are in the role of a teacher I think you should really use the exact definition according to the high school report now we have a precise idea what is meant with those religion there there are other other to the mental quantities and one important Oh one important example is caramel and this is the definition which is again taken directly out of this report and I think we have to read it the camera is a quotient by D and this D eat which TR is a sum of the initial kinetic energy of all charged particles liberated by uncharged particles and a mass of material again it has the same the same unit it is child pack you recognize of the gain great so this is again our illustration of absorb those know absorb those is is coming photons and we are you have the energy deposits by the electrons on everything which is within these volume is taken as for the definition of absorb those and again this part of energy is not taken this subtlety so this is energy absorbed in this example also know let's show that here we have we have an ah let's go to that one here is another one into actually which is outside of the volume and the secondary electron is going through it then this part is also counted so this is now the camera and we have almost the same structure of secondary electrons created by the photons and now we are taking the kinetic energy the initial kinetic energy of each of these electrons and put this together and the camera is a sum of all these kinetic energies so and remember this is outside volume and therefore it's not counting so all this energy is not counting for the camera what is important is the difference between absorb dosa definition and carom I absorb those we want to measure and I think we can measure it not always directly but we have our annotation chamber and finally we will get the information absorb those or we can dial whatever can we measure the camera we should differentiate between the energies in the detector which are created outside we cannot it's very difficult so the typical difference is that karma can be calculated and that those can be measured on the contrast absorbed those is not easy to do really to Cal it to calculate there are some exceptions with Monte Carlo where you put where you can do but still it's some approximation there some tiny approximation but the definition of absorb dose is made to have a quantity to be measured the definition of karma is made to be calculated and here are the way how it can be calculated if we have our interaction coefficient there are a number of interaction coefficients available which describe what happens in total if a photon is interacting with material wall so we have the attenuation coefficient we have the energy transfer coefficient or we have the energy absorbed coefficient this is the most important coefficient and the calculation follows always a very simple rule if we know the fluence differential in energy if we multiply the fluence with the energy itself then it's called the energy fluent and if we calculate with this interaction coefficient and is this has to be divided by the dance by the density factor Rho then it's called the mass energy transfer coefficient then this product and you have to integrate this over all energies then this is karma this is the definition of gamma and if we want to be more realistic to be what maybe stay in the volume and not as escape from the volume you should take you should take into account only which is given by the energy absorbed coefficient though the difference between the energy transfer coefficient on one side and energy absorption coefficient it's just that the energy which is going into radiation and is escaping is not counted it's taken away so we have just to to remind you we have other Tosa metal quantities compared to observe those typically here camera and coalition camera which are these are photons and as I read told you absorbers is a quantity which is successful many by measurement and caramels adult medical quantity which cannot be measured by calculate only basic knowledge of photon fluids differential and energy so let's look now on on the absorb those which are coming from charged particles charged particles that means normally electrons for calculations of those which are created by charge politics we need to introduce the concept of stopping power and again this is our high school report which is saying what is the mass stopping power the mass stopping power of a material for charged particles so stopping power only refers to charged particles of a given type and energy is the quotient by the E and roll DL del is where e is a mean energy lost by charged particles and traversing a distance DL in the material of density Rho C again we have a definition of energy which is the mean energy lost again this is refers to to the overall process it's again a non stochastic character or scription of the energy process which goes from radiation into material the stopping mass can be expressed as a sum of independent components and we have three which are given here and this is the explanation this is called the mass electronic or collision stopping power due to interactions with atomic electrons resulting in interactions the second one is a mass radiation stopping power due to emission of ram strallan in the electric field of atomic duo or atomic electrons and we have all the a mass nuclear stopping power we have all the some interactions from the electrons with a nucleus which is a if the energy of the electrons is small it's not so important but we have three components and again this is the most important or the most interesting and why this component is small and this component if we think on our volume absorb those it's escaping from the volume we are interested in absorb those so this one is most interesting part so why stopping power is such important content to say there there are two and the energy loss at the same time is energy absorbed wonderful and there's a fundamental relationship between absorbed dose from charge parting and the mass electrons coming power for this the there is another introduction of an often does a middle quantity which is called jammer who knows that I'm afraid no one it's it does not play an important role but the people who has written the high school report there someone they are insist we have to include the chamber the chamber is given here this explanation the chamber for initiation charge particle is a quotient again it's very similar to the Kurama definition but what is interesting in is that says that the Gmail is can be calculated by again our fluence of the electrons differential energy times the electronic stopping power so if we know the fluence of electrons by some way by calculation I'm on the color calculation for instance we multiply this with a stopping power we can calculate the Jama and the Chairman to see is very similar to the absorb dose so what we can say is absorb those from electrons can be calculated with this way therefore this is important to have all this introduction so this is summary now a very summary which I that's good it fits with that time on the first part energy absorption and absorbed oh say this is a definition of absorbed dose this is the definition of energy imparted being the sum of energy deposit and the energy deposits are referring to single interactions with this balance of energy energy coming in energy coming out and some changes of resna Chi and all of which i think is interesting it's not necessary to know it's nice to know but to knows the energy absorption stochastic process so I think I will make a break now because now it's time for lunch and I will simply continue after the lunch with this course well the time is because 12:50 lunch break forty ten fourteen ten ten minutes after two I will continue [Music] you

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Length: 35min 40sec (2140 seconds)

Published: Wed Feb 07 2018