Ortho Flashcards
what does ortho calibration factor yield?
dose to small mass of tissue in air
why can we use uen/p and not stopping power ratios for dosimetry in ortho?
small range of electrons (order of microns, < 0.5 mm)
-not the case in MV beams
what is uen/p a function of?
- uen/p in air is a function of HVL only, HVL defined for narrow beam conditions. Exception is for bone, where it also depends on depth, field size, and SSD
- in water (for in phantom calibrations), it depends on both depth and field size also
For simplicity, only HVL is used to specify the
beam quality. One should therefore keep in mind that the uncertainty on this quantity is no better than %1.5
where is the effect of photon scattering from the stem icluded?
in the calibration factor NK for the beam quality
and photon field used in the calibration
patient has a lead cutout-what do you need to account for?
backscatter will change
Is Krad significant in ortho?
No, it is neglificle so can say dose = K
What is used to define beam quality?
kV and HVL
does BSF depend on energy?
Yes
daily QA checkmate- what happens if you don;t put space or solid water between the source and checkmate?
dose saturates the detectors
use of short cones vs long cones
- small cones= higher dose rate; large cones= easier to reach hard-to-reach places, but slower dose rate and heavier
- long cones are used for larger FS- easier to get a more uniform field because the IS effect is less significant than it is at 30 cm cone (dose at edge of field vs dose at center). Also, for irregular IS on a patient (say on the face), IS effect is again less significant with long cone vs short cone.
Dose fall off for brachy (400 kV) is faster than ortho (300 kV)
In brachy we are on the steep decline of the IS curve- thus fall off is fast. In ortho we are on the flatter part- thus fall off Is slow (ortho PDD starts on the flatter part of the curve). IS is very important for these small distances and heavily impacts the PDD (PDD depends on attenuation, IS, and scatter).
why do long cones have closed ends?
electron contamination becomes problematic throughout the cone so the bottom is closed
explain virtual source
use virtual source (or isocenter, same with electrons) for IS, as due to forward scatter of electerons (more electrons scatter in than out), the actual source is closer than the physical one. To find the virtual source, have to fit the data to an equation to find the virtual sourced- different for each cone or electron applicator
What does Pq,chamb correct for in-water method?
PQ,cham accounts for the effect of the change in chamber
response due to photon energy and angular variation between chamber calibration
in air and measurement in water, the effect of chamber stem between calibration in
air and measurement in water, and the effect of displacement of water by the chamber
in the measurement in water. In water, dose is measured at 2 cm depth
at what depth is measurement made for in-water method?
2 cm
what detector is preferred for x-ray energies below 70 kV?
calibrated
parallel-plate chambers with a thin entrance window are preferred to minimize
the effect of the chamber wall attenuation. Thin plastic build-up foils should
be added to the entrance window, if necessary, to provide full electron buildup and
to eliminate electron contamination (thicker foil for higher energies, but below 100 kV (i.e. in PP dosimeter range)
-need sufficient build-up to make sure electrons created in wall of chamber are absorbed within that same wall.
can we estimate output factors for different applicators using the ratio of
the backscatter factors corresponding to the respective field sizes.
No
-Since the scatter
contribution from the inside of a cone applicator may be significant, it is not sufficiently
accurate to estimate output factors for different applicators using the ratio of
the backscatter factors corresponding to the respective field sizes. The output factor
for each individual applicator must be measured at each beam quality.
Ho to measure outpt factor for each applicator?
Can do this using ion chamber and BW scatter factors from TG-61 for each sized cone (preferred, BW is needed since we look at dose to surface) or by using solid water with PP chamber- this is essentially measuring your own BW factor. Remember when measuring output factors for long cones that they have virtual sources and thus different IS! Have to correct for IS.
why is there a transparency on PP plate for higher energy ortho beams?
remove contaminant electrons
Why do we commission the # of counts for 100 MU for each cone at commissioning for ortho?
- for MV linacs, the ion chamber in the linac will see different backscatter depending on the FS. It does not correct for this (assumes 100 MU is 100 cGy at 10x10); however, the impact is very small.
- backscatter signal can vary from 1% to 3% for the largest to the smallest fields. The effect of the monitor backscatter is to increase the output per monitor unit with increasing field size.
For ortho, the impact of using different cones is large (backscatter is very dependent, ion chamber sees very different amount of backscatter).
-machine saves this factor as relative to cone C or G- so if we change the output per calibration of cone C, the relative factor will take care of it for all the cones
uncertainties for kV dosimetry
~ 5 %
why does uen/p in air only depend on HVL?
-because it is measured using narrow beam geometry (achieved with distance)- doesn;t depend on scatter factors
Since the long cones have a different filter, could have slightly different HVL.
how to find effective energy of a poly-energetic beam?
find mono-energetic beam with the same HVL as the poly beam
convert from mm Al HVL to mm Cu HVL
-1 mm Al = 0.0278 mm Cu
stand-in vs stand-off factor
stand in- lesion comes out of surface and into cone
stand off- lesion causes surface to dip down under cone
flatness and symmetruy for ortho
at HFX, check ortho symmetry and flatness with profiler- can just compare to previous baseline. Need film to truly assess profile because of IS effect at ends of the cone, and where these fall on the profiler detectors. Beam is continuous not pulsed- profiler doesn’t like this.
why is Y-axis less symmetric than x-axis?
Y-axis is always less symmetric because of the anode heel effect. Aim for about 5 % flatness due to heel effect and IS (much more significant at 30 cm vs 100 cm).
Pol wrt depth
verify Ppol doesn’t change with depth- more likely change with depth in a phantom versus just increasing air gap. PP plates are more sensitive to electron contaminations compared to cylindrical chambers
do you typically need the saran wrap to absorb contaminant electrons for cylindrical chambers at energies > 180 kV?
typically no
- more of an issue with PP chambers
- cylindrical chambers absorb the contaminant electrons
for what energy is error associated with longer SSD more significant?
lower energy because more scatter
describe in-air method
Dw,z=0 = M N BW Pstem,air (uen/p) water to air
The backscatter factor Bw must include the effect of end
plates in close ended cones, if used, on the determination of
water kerma at the phantom surface.
describe in-water method
-cannot be used for E < 100 kV
-2 cm depth
Dw,z=2 cm = M Nk Pq,chm Psheath (uen/p) water to air
why is reference depth 2 cm?
If the
reference depth is too small there may not be enough buildup
material in the upstream direction to cover the whole chamber.
If the reference depth is much larger than 2 cm, the
ionization signal in the chamber may be too small. Therefore,
this protocol has adopted a reference depth of 2 cm.
what is Psheath?
correction for photon absorption and scattering
in the waterproofing sleeve if present
-depends on HVL and the material
What Pion equation do we use?
continuous beams
describe end effect
amount of time that is not
accounted for by the machine timer mechanism during the
x-ray beam delivery
This amount of time usually describes
the time difference between when the timer mechanism starts
and when the desired mA and kVp is achieved, or the finite
time required for the shutter to move from the fully closed to
the fully open position.
- 0.5 to 3 s - can be siginificant for short treatments < 3 min
- measure with graphical extrapolation method
what is Pstem,air
accounts for the effect of the change in photon
scatter from the chamber stem between the calibration in a
standards laboratory and the in-air measurement in a user’s beam
What does BW depend on?
field diameter, SSD, and HVL. The multiplicative factors for the close ended cones depend on field diameter and HVL.
what does Pq,chm depend on?
chamber type and HVL
How to get Nk from Nx
Nk = Nx (W/e)air * (1-g)
how to calculate virtual SSD
plot square root of (reading with no gap/reading with gap x) vs air gap on x-axis; 1/slope – dmax (0 for ortho) is the virtual SSD
