TG51 related Flashcards
what does TG51 addendum include
Addendum includes MC calcs that better simulate the chambers
addendum includes kq for 30 vs 18 ion chambers
-includes guidance on FFF linacs
why does quality have to be determined at 100 cm SSD, 10x10?
kq is calculated using Monte Carlo calculations which are done at 100 cm SSD, 10X10
TG51 addendum Pion
Cinit: the component of the ion recombination correction
factor, Pion, to take account of initial
recombination.
Cgen: the coefficient of general (volume) recombination.
The product of Cgen and the dose per pulse, Dpp, is the component of the ion recombination
correction factor, Pion, to take account
of general recombination. Cinit and Cgen are defined
such that the ion-recombination correction
factor, Pion = 1 + Cinit + CgenDpp.
Note that Cinit is inversely proportional to the polarizing
voltage: e.g., Cinit = 0.002 at 300 V becomes Cinit
= 0.004 at 150 V. A higher polarizing voltage pushes the charges apart so that they can’t recombine initially?
Pleak in addendum
correction factor to take account of leakage
If the leakage current is at or below the 0.1% level then it is
reasonable to set Pleak = 1.000 (no correction for leakage)
Prp in addendum
the correction factor to take account of the
variation of the radial dose distribution that is
averaged by the detector
To determine Prp in the clinic, one calculates the average of
the radial dose profile over the dimensions of the active part
of the chamber
issue with PP chambers and photon beams
chamber-to-chamber variations and long term stability
what did addendum do regarding shifts?
-determined more accurate shifts for each ion chamber than 0.6 rcav, but noted that the effect on the dose is less than 0.1% for the chambers included
what did addendum say regarding Pb foil
-Lead foil has led to confusion- instead use the interim measure in TG-51 to convert from %dd(10) to %dd(10)x -introduces error of no more than 0.2 %- take into account as increased uncertainty and also only use for FF beams (not FFF)
what does addendum say about FFF beams?
the significant radial nonuniformity of the beam can have an effect on volume averaging within the chamber volume. Use a chamber with a short collection volume. Use lead foil for FFF beams, even if below 10 MV, to eliminate potential effect of accelerator-produced electron contamination
why are microchambers not recommended for reference dosimetry?
The kQ values presented in Table I are based on calculations
for beams with flattening filters only.20, 28 In
a study of central-electrode effects,37 it was shown
that these same values apply for FFF beams within
0.1% or so for chambers with low-Z or aluminum
electrodes. However, for chambers with high-Z electrodes,
values of kQ can vary by more than 1% in
FFF beams for a given %dd(10)X. This is another reason
these microchambers are not recommended for
reference dosimetry.
factors with 0.5% uncertainty
-SSD, depth, field size, charge, Ptp, kq, assignment of kq, reference chamber stability, Ppol, Pion, Ptp
how much does kq vary?
0.95 TO 1
explain Ptp correction
-ionization depends on mass of air which depends on air density- air density is proportional to P and inversely proportional to T
what measurement do you use if there is a large polarity effect
true reading is taken to be the mean of the absolute values of readings taken at the two polarities
where does Pfluence come from?
Ionization chamber introduces a low density hetero-geneity (gas cavity) into a medium and this causes a perturbation of the electron fluence
a low density cavity will scatter out fewer electrons than are scattered in. This results in an increase in the electron fluence toward the downstream end of the cavity in comparison with the fluence in a uniform medium at same dept
2 effects: in-scatter (increases fluence in cavity because electrons are not scattered out by the gas) and obliquity effect (decreases fluence in cavity because electrons go straight instead of scattering)
Pfl < 1 which means the in-scatter dominates, making the observed fluence too large. Note that the correction is very large at low-energies or for large diameter chambers and in this case it is best to use plane-parallel chambers with large guard rings.
Factors with 0.1-0.3% uncertainty
humidity, leakage current, linac stability, Pelec
Factors with ~ 1 % uncertainty
- Calibration factor, pre-irradiation history
where does uncertainty in Mraw come from?
chamber, extension cable, and electrometer
resolution of measuring devices for Ptp
0.1 degree and 0.1 kPa resolutions
change in kq versus change in beam quality data
a 1% change in %dd(10)X leads to a
∼0.15% change in kQ
relative uncertainty
in determining %dd(10)X is at most 2%, which corresponds
to a relative uncertainty in kQ of about 0.25%.
What are voltage-dependent polarity effects caused by?
• Distortion of electric field by potential difference between the guard and the collecting electrode. • Space charge distortion of electric field lines defining the gas sensitive volume. • Difference in mobility of positive and negative ions causing differences in space charge distribution around the central electrode.
Compton current
causes an increased reading for positive chamber polarity and a decreased reading for negative chamber polarity.
In the dose buildup region of the electrode, these interactions cause a loss of electrons from the measuring electrode that is not fully compensated by the arrival of electrons from the upper layers of the phantom.
For depths beyond zmax, both positive and negative chamber polarities yield the same reading, because electronic equilibrium exists on the measuring electrode (as many electrons land on the measuring electrode as are ejected by photon interactions from the measuring electrode).
why do we sometimes use build-up caps?
The wall thickness of an ion chamber must be greater than or equal to the range of secondary charged particles that are produced in the wall to maintain electronic equilibrium. In high energy beams, a build-up cap is often needed.
explain spencer attix vs bragg gray
-chamber diameter is 4-6 mm
-The energy of an electron with a continuous slowing down approximation
(csda) range in air of 5 mm is ≈15 keV
-delta rays escaping would have to be perfectly balanced by those entering- this woudn’t be the case unless the gas is perfectly matched to the medium and the wall
-Spencer andAttix introduced a cutoff energy such that the incoming electrons
all have energies greater than and all energy losses less than are treated as
“local,” and are assumed to remain in the cavity or the medium where created.
- 2 terms:
- the 2nd term represents the final deposition of delta energy (phi * energy delta) as those electrons with energy less than delta deposit all their energy in the cavity. The first term represents electrons depositing energy as they slow down (but still cross the cavity). Ldelta is stopping power restricted to losses less than delta, representing that not ALL of the energy is deposited in the cavity.
Explain why the water-air mass stopping-power ratio increases
steadily with increasing depth in water in a megavoltage electron beam, whereas
it is almost constant in a megavoltage photon beam.
-for electrons, they lose energy as they slow down and thus the stopping power increases (for water)- this effect is not as significant for air since polarization is not significant in air at moderate energies. Polarization is responsible for decreasing stopping power with increasing energy. The photons release electrons of around the same energy as the beam is attenuated through the depth thus the effect is not seen since electrons are of around the same energy .
what do guard rings prevent aganst?
in-scattering effects
This is why we don’t have Pfluence for parallel plate chambers. For low energy beams, Pfl is significant, thus why we have to use PP chambers
the two types of detectors (based on size vs electron range)
- Detectors that are large compared to the electron ranges, and in which, therefore,
CPE is approximately established (photon radiation only) - Detectors that are small compared to the electron ranges and which therefore
act as “sensers” of the electron fluence existing in the uniform medium
(Bragg-Gray cavities)
burlin cavity theory
don’t fall into ether large detector (photon only) or small detector (electron stopping power only)
Burlin uses a factor which is a weighted mean of the stopping power ratio and mass energy absorption coefficient ratio
-factor includes chamber dimensions
Predict and explain the behavior of in the build-up region of (i) a
monoenergetic megavoltage photon beam (e.g., 10 MeV photons) and (ii) a
megavoltage bremsstrahlung beam (e.g., 10 MV x rays). N.B.: you can ignore
any “contamination” due to electrons generated in the air.
-I would expect the 10 MV photons to release around the same energy electrons as they are attenuated- thus stopping power ratio should stay stable. For the poly-energetic beam, lower energy photons will be attenuated first. These would produce lower energy electrons which would have higher stopping power ratios. Thus, the stopping power ratios would decrease as the beam hardens. Again, it is the polarization effect that significantly decreases stopping power for electrons in water for increasing energy, but not in air.
Why is Pfluence not required for photon dose determinations made at or beyond dmax in a broad beam
because transient electron equilibrium exists
The Fano theorem tells us that under conditions of charged particle equilibrium the electron spectrum is independent of the density in the medium. To the extent that the cavity gas is just low-density medium material, this theorem tells us that the electron fluence spectrum is not affected by the cavity except in the sense of the gradient correction, which in essence accounts for there being transient rather than complete charged particle equilibrium. Hence no fluence correction factor is needed in regions of transient CPE.
-Pfluence is not needed in regions of transient CPE. Thus it is only needed in build-up
region or near the boundaries of a photon beam or anywhere in an electron beam
where do the standard corrections for Ptp break down?
for low energy radiation for some chambers with non air equivalent components. This breakdown occurs when the
ranges of the electrons entering the chamber cavity are short compared to the dimensions
of the cavity, and when the photon cross sections are different between the chamber
wall and air. For low-energy brachytherapy sources in well-type ionization
chambers, there is a greater deposition of the ionization products than expected. This
can amount to as much as an 18% effect at higher altitudes
Does Ppol depend on measuring depth?
yes
The polarity effect depends on the energy and the angular
distribution of the incident radiation and both the depth of the measurement and the
field size. The effect can even reverse sign as a function of depth since forward ejected
electrons near the surface create a region with a net loss of electrons,
whereas a higher negative charge is accumulated at deeper depths where electrons
stop in the medium. The charge deposition in the collecting electrode may either
increase or decrease due to these effects based on the polarizing voltage of the
chamber
when do you calibrate the ion chamber?
it is necessary
to have the chamber calibrated when first purchased, when repaired, when the
redundant check suggests a need, or once every 2 years.
When an ionization chamber or dosimeter is sent to a standards laboratory for calibration,
stability check measurements (using a suitable check device) must be done
by the user before and after the calibration. At least two independent checks should
be performed prior to sending the chamber for calibration and the same checks
repeated when the chamber is returned.
