Physics Lecture 7
1Gy
100cGy=100rad=
1rad
1cGy=10^-2Gy=
1R
2.58 x 10^-4 C/kg
KERMA
kinetic energy released per unit mass (Gray or J/Kg) or amount of KE transferred from photons to electron Sum of all kinetic energies of all ion pairs liberated in volume of matter/mass of mass =Utr/p
Extrapolation/parallel plate chambers
•At times, especially for electrons, you want to measure as close to the surface of the detector as possible. (electrons don't penetrate as much as photons) •One electrode consists of a very thin foil where the electrons enter
Pelec Correction factor
•Electrometer applies voltage to create an electric field in the ionization chamber •It measures the charge (or current) produced resulting from the ionization of the mass of material in the cavity •Electrometer also needs calibration •If it is calibrated separately from the chamber, then Pelec is the electrometer calibration factor which corrects the electrometer reading to true coulombs. •Usually this is sent off for calibration
Ptp correction factor
•Makes the charge or measured current correspond to the standard environmental conditions for which the calibration factor applies •Many chambers are open to the air in the room (not vacuum sealed) •Since the measurement depends on the collecting volume mass and the volume's mass depends on density which depends on temperature and pressure we will need a correction factor
mass stopping power
expected rate of energy loss per distance by the charged particle in the medium
*Only valid in air and only with photons *It is difficult to tell how many secondary IP are produced
*Below 3MeV *Above 3MeV exposure is:
Measuring absorbed dose using an ionization chamber
*Estimates of absorbed dose in soft tissue can be measured using an ion chamber *Use the Bragg-Gray theory to convert ionization to absorbed dose *This is how we do it in Radiation Oncology
density
*The amount of ionization collected in a small volume of air is not influenced by the ________ of the medium surrounding the volume in air *The large distances in air required for electron equilibrium can be replaced by a smaller thickness of more dense air equivalent (Zeff close to 7.64 the Zeff of air) material
Measuring radiation dose
*Using dosimeters *Materials absorb dose like they're tissue (tissue-equivalent) *Few materials are exactly tissue equivalent, so correction factors are used
Absorbed dose
Amount of energy deposited by ionizing radiation in a material (Gy or J/kg) D=dEavg/dm
amount of energy and type of radiaiton
Chemical/biological effects of radiation depend on
Free-air Ionization Chambers (FAIC)
Found mostly in national calibration labs *Are the "gold" standard that we compare other ion chambers with *Not used clinically due to size
g
Fraction of the energy of secondary charged particles that is lost to bremsstrahlung in the material (radiative losses)
2
How many measuring concepts are there?
exceeds
If electronic equilibrium isn't held, then Kerma ________ absorbed dose The dose initially builds up to a maximum value and then decreased at the same rate (slope) as Kerma
F factor
If we include the dose to exposure relationship we get Dmed=X(R) x Ffactor Ffactor=(0.876cGy/R)x[(Uen/P)med/(Uen/P)air]
fluence and exposure
If you set both equations of Ea equal to another, we find a relationship between:
Minimal energy variation Minimal directional variation Linear response Minimal stem correlation Minimal ion recombinaiton
Important properties of thimble chambers
interaction of photons in the air-equivalent wall of the chamber
In a thimble chamber, most of the ionization collected in the air volume originates during
range of electrons
In thimble chambers, ionization increases with wall thickness until the thickness equals the ______ __ _________ liberated by the incident photons. At this thickness, the electrons from the outer wall just reach the cavity and the ionization inside the cavity is a maximum A thinner wall would not provide equilibrium, but a thicker wall would attenuate photons unnecessarily
secondary electrons
In electronic equilibrium when energy is deposited in a specified volume in air, some ________ ________ are produced outside of the collecting volume by primary ion pair that escape the collecting volume
screw
In extrapolation chambers, how is the distance between the electrodes adjusted?
independent of beam direction Used for electron, photons, protons heavy ions Most popular design Cross calibrated against FAIC Can be used up to 20 MV (Can be used for any clinical energy
Cylindrical thimble ion chamber
Quality Factor
Describes the lethality of the radiation Need to know for dose and exposure estimates
same
Different types of radiation (photon, electron, neutron, heavy ion) will have different effects even if the amount of radiation absorbed is the _____
Ortho- 0 Co60- 0.5 4MV- 1 6MV- 1.6 10MV- 2.5 15MV- 3.0 18MV- 3.3 20MV- 3.5
Dmax for different energies
Dose
Energy absorbed in the medium per unit mass (Gray or J/kg)
Ea
Energy absorbed per unit mass of air during exposure X (C/kg) is 33.97 * X [J/kg]= or W[J/m^2]*(Uen)m[m^2/kg]
Electronic equilibrium
Energy deposited by charged particles produced inside a volume and deposited outside the volume is equal to energy deposited by charged particles produced outside the volume and deposited inside the volume.
collisional Kerma in air
Exposure is the ionization of equivalent of __________ ________ ___ _____ X=(Kcol)air(e/W)air=(Kcol)air/33.97 X=Wair(Uen/p)air(e/w)air
*Beam is collimated and enters the chamber *Radiation ionizes the air *Two electrodes collect the free ions (with a biased voltage between them) *The chamber reads out a charge (Q) that is corrected to exposure
FAIC operation X=Q/ALp
33.97 J/C
For (dry) air, the average energy expended per ion pair formed: W/e=
No
Is KERMA always equal to dose?
Collisional Kerma
Is equal to the photon energy fluence times the mass energy absorption coefficient (Uen/p) Kcol=w(Uen/p)
Estimation of dose to a medium from a calibration in air
It is possible to relate the exposure in air to the dose in air Dair=X(R) x 0.876cGy/R *This relationship is very important in radiation therapy
Collisional Kerma and Radiative Kerma
KERMA can be divided into two parts
Electron equilibrium
Kerma can be directly related to fluence, but dose can be calculated only in the assumption of _________ ___________: in any volume, as many electrons are stopped as set in motion
Dmax
Kerma is greatest at the surface due to high photon intensity (or more interactions) *Electrons set into motion from the interactions at the surface travel several mm before all energy has been given to the medium *Dose builds up to the maximum at dmax *Beyond dmax: Dose decreases gradually as photons are attenuated This concept relates the skin sparing effect of x-ray and gamma ray
Radiative Kerma
Krad=w(Uen/p)(g/1-g) w is the photon energy fluence Uen/p is the average mass energy absorption coefficient g is the average fraction of an electron energy lost to radiative (bremsstrahlung processes)
Calibration of MV beams
Refers to the determination of dose delivered at a reference point in a beam of radiation *Must be performed before a radiation beam can be used for treatment *Repeated on a consistent bases to make sure it doesn't change (monthly, annually)
Thimble chambers
Most commonly used for radiation therapy measurements The active volume is housed within a thimble shaped cavity with an inner conductive surface (cathode) and a central anode A bias voltage applied across the cavity collects ions and produces a current which can be measured with an electrometer
Normal Temperature and Pressure
NIST uses a temp of 22 degrees c (293.15K, 68F) and an absolute pressure of 101.325kPa (14.696psi, 1atm, 760torr, 760 mmHg) This is called: NTP
rad
Old unit of dose is:
1.6x10^-19 C
One ion pair produces
depth
Parallel plate chambers are used to measure ionization as a function of
TG-51
Physicists take care of calibrating linacs to make sure that when therapists deliver the treatments, the dose delivered is what the oncologist prescribed. The protocol they follow is outlined by a report called:
Linear energy transfer
RBE varies with _______ ______ _______ which varies with type of radiation, so we calculate a dose equivalent
biological effects
Some radiation is much more effective than others at producing _______ _______
dEtr
Sum of initial kinetic energies of all charged particles (electrons and positrons) liberated by uncharged particles (photons) in a material of mass (dm)
relative biological effectiveness
Tells us how much radiation it takes to achieve some biological endpoint (cell death) Dose (Sv)=absorbed dose(Gy)*Q
ionization
The calibration of MV beams involves a measurement of _________ followed by calculations to estimate the dose at the location of measurement
Dose and Kerma
The deposition of energy from radiation into tissue can be described in two ways
deposition of energy from the radiation into the tissue
Tissue changes due to radiation are caused by
Exposure (X)
Total charge (Q, + or -) released from ionizing radiation per unit mass (m) of air (C/kg or Roentgen, R) X=Q/m
KERMA is maximum at the surface and decreases with depth linearly
Transfer of energy to charged particles (KERMA) does not take place at the same location as the absorption of energy deposited by charged particles (dose)
Energy Transfer Coefficient
Utr Fraction of photon energy transferred into kinetic energy of charged particles per unit thickness of absorber Defined as the product of energy transfer coefficient and (1-g): Uen=Utr(1-g) Utr=(Etr,avg/hv)u Etr=average energy transfered into kinetic energy of charged particles per interaction
Mass energy transfer coefficient
Utr/p where p=density of medium Relates to dose where quantity of energy absorbed per unit mass of the medium
Bragg-Gray Cavity Theory
We can figure out the dose to a cavity if we assume two things: 1. The cavity is small enough not to disturb the charged particle field 2. The energy deposited is only from charged particles completely crossing the cavity
tissue
We can relate the dose in air to the dose in ________ Dmed=Dair[(Uen/p)med (Uen/P)air]
principle of free-aur ionization chamber
We can't collect and measure all of these secondary ionizations but we can specifically choose a volume where the ionization produced outside the volume (by ion pairs originating inside the collecting volume) is balanced by ionization produced inside the volume (by IP that originate outside) This is the
•Pelec - corrects for inaccuracy of the electrometer if calibrated separately •Pion - ion recombination correction •Ppol - corrects for chamber polarity effects •PTP - temperature & pressure variation correction
We must correct that reading on the electrometer with the following correction factors
Correction factors
We use an electrometer connected to the well-guarded (very low stem leakage) thimble chamber to figure out the dose
If those assumptions are true then we can say that the dose to the medium, is related to the dose in the cavity by, Dmed/Dcav=mSmed/mScav Where mS is called that mass stopping power which is the expected rate of energy loss per distance by the charged particle in the medium
What does the Bragg-Gray theory tell us?
Energy Transfer Coefficient
When a photon interacts with electrons in material, part or all of energy is converted into KE of electrons *If part of energy is given to electron, the photon is scattered with reduced energy (it may interact again with a partial or complete transfer of energy to the electron) *A photon may experience one or multiple interactions in which the energy lost by the photon is converted into KE of electrons
Ionization
When photons interact in a medium, they can ionize the atoms in the medium or create free ion pairs (IP) *# of IP formed is proportional to energy absorbed in medium *If medium is air, it is called Exposure
*Photon energy knocks electron out of the atoms and they travel away with kinetic energy (KE). Higher velocity= higher KE *Electrons are also called secondary particles and are what deposits energy in the matter
When photons interact with matter/atoms in material, they become ionized....
dEavg
average eneergy imparted by ionizing radiation to a material of mass (dm)
Ppol Correction Factor
•Takes into account any polarity effect in the response of the ion chamber •Sometimes the ion chamber will collect more efficiently when the voltage is negative and sometimes it will be the other way (depending on the polarity) •To correct for this we do both and take the average reading •The difference between the two readings is required to be to be <0.5% •The polarity effect is more significant for electron beams than photon beams
Pion correction factor
•Takes into account the incomplete collection of charge from an ion chamber •Some of the ions formed by the radiation recombine with ions of the opposite sign before reaching the collecting electrode, and are not measured •Several ways exist to measure, but AAPM recommends measurements be made at the normal collecting voltage and one at half that voltage
Final Corrected Measurement
•The fully corrected charge reading from an ion chamber is given by: M=PionPtpPelecPpolMraw •Where Mraw is the raw ion chamber reading in Coulombs
