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.