Photon Beams Flashcards
What is the equation for intensity/attenuation?
I(x) = Io(e^-ux) where ‘u’ is the linear attenuation coefficient and x is depth
How does the attenuation coefficient relate to the HVL?
I(x)/Io = 1/2 ln(.5) = ln(e^-ux) 0.693 = ux x (HVL) = 0.693/u note that this is similar to half life (t1/2 = 0.693/lambda)
What is needed for ‘good geometry’ when measuring a beam?
1) narrow beam (no scatter or field size effects) 2) long source-target distance (scatter, field size) 3) long target-detector distance (no secondary particles)
How many HVLs approximate a tenth value layer?
TVL = 3.3 HVLs
Which is more penetrating: the photon beam that goes through the heel or toe?
Tricky question…while more of the beam makes it through the toe, the *heel* is more penetrating due to beam hardening. Thus, as you get deeper in tissue the heel flattens out.
How is the wedge angle measured?
Angle of the isodose line at 10cm depth in water
Describe a Thoraeus filter and its utility
The filer is constructed with layers of metals from high Z to low Z. This is so the characteristic x-rays from the high Z materials are filtered out by the low Zs. Sn>Cu>Al. The purpose is to remove low energy photons and make a more penetrating beam.
How do we describe beam energies in the orthovoltage range? MV range?
Ortho: HVLs MV: PDD
How is PDD defined?
Measured in water, at 10cm with a 10x10 field at SSD = 100 –> “%dd(10)x
Rule of thumb: what is the effective energy of a beam?
Approx 1/3 peak energy (kVp). Ex: effective energy of a 4MV beam is 1.33 MeV
What is the equation for hand calcs (SSD setup)? How does it change for SAD?
Essentially Dose = MU x K x ISF x PDD x ScSp x WF x TF….you’ll rearrange it so MU = Dose/[[]], but essentially you’re taking the output factor K (cGy/MU) and hitting it with a bunch of different correction factors SAD: just using TPR or TMR in place of PDD
What is the inverse square factor, what is it used for?
As you get further away from a radiation source, the output decreases (output is related to energy deposited by a photon…as you get further away, the photons spread out in space, and less dose is deposited per unit area). So if you wanted to correct for this, the Intensity at point 2 vs point 1 would be inversely related: I2/I1 = (distance 1/distance 2)^2… that is, the intensity at point 2 is “inversely” related to the “square” of the ratio d1/d2
In practical terms, how do you calculate the inverse square factor?
I2/I1 = (d1/d2)2. At calibration, I1 is based on SSDref + dmax. Thus I2/I1 = (SSDref+dmax/SSDexp+dmax)^2; dmax is constant for any given energy
How is the PDD measured and defined?
PDD is simply the [dose at depth x/dose at dmax]. It is experimentally derived in a water phantom (at reference SSD and reference field size) and a table created
How do the following affect PDD: 1) beam energy 2) SSD 3) field size 4) depth
1) higher energy = more penetration = higher PDD 2) longer SSD = decreased dose rate (InvSq) BUT relative impact of fluence decreases (Mayneord) = increased PDD 3) larger field = more lateral scatter = more central dose = increased PDD 4) deeper = beam attenuation = decreased PDD
Describe the effect of increased SSD on dose in tissue
Increased SSD will decrease the dose rate in tissue (Inv Sq Law) –> less photons per cross-sectional area = less dose in a unit of time –> more beam time needed to deliver a prescribed dose. HOWEVER, what is 10cm depth relative to a 20cm SSD, what about 200cm SSD? 0.5 vs 0.05. Thus, a given depth at an extended SSD is relatively smaller. Thus while attenuation is the same (depth is the same), Cross-sectional fluence decreases relatively less. Again, think of the impact of the inverse square law when comparing intensity of 30 cm vs 20 cm (for SSD 20, depth 10) vs. the intensity of 210 cm vs 200 cm (for SSD 200, depth 10). So the *dose falloff* is relatively smaller, more homogenous. In total: increased SSD means lower dose rate in tissue (more beam on time), but also a more homogeneous dose distribution (shallower falloff).
What is the utility of the Mayneord F-factor and how do you calculate it?
The Mayneord F-factor will account for the *relative* difference in fluence as a function of depth with changing SSD. F = [(SSD1+d)/(SSD2+d) x (SSD2+dmax)/(SSD1+dmax)]^2 “old and deep” (SSD1+d) x “new and shallow” (SSD2+dmax) “over the opposite, squared”
What is Sc? What changes it?
Collimator Scatter; for a narrow collimator opening, less scattered beams are allowed through. Thus, when you increase a field size, the Sc contribution to dose increases (output in cGy/MU increases)
What is Sp? What changes it?
Phantom Scatter; for a narrow area of tissue treated, the scatter is minimal. As you open the field size and/or go deeper, the scatter increases
What is the equation for eq sq? eq circle?
Sq = 4A/P Cir = sqrt(pi*r)
How do you determine the dose influence of scatter vs. primary beam?
Can take a dose measurement at central axis for a particular field size, then take a dose measurement with a pencil beam. The difference between the two is due to scatter: SAR(d,r) = TAR(d,r) - TAR(d,0)
What is the leakage of radiation through jaws? through MLCs?
0.1% for jaws 1-3% for MLCs
Where is the dmax for a Co-60 beam?
0.5 cm
Where is the dmax for a 4MV beam?
1.0 cm
Where is the dmax for a 6MV beam?
1.5 cm
Where is the dmax for a 10MV beam?
2-2.5 cm
Where is the dmax for a 18MV beam?
3-3.5 cm
Where is the dmax for a 25MV beam?
4-5 cm
What are 5 ways to increase the surface dose?
1) lower energy beam 2) bolus 3) larger field size (electron contamination from head) 4) introduce a beam spoiler (intentionally generates secondary electrons before patient) 5) oblique/tangential beams
What is geometric penumbra?
Due to source size
What is transmission penumbra?
Due to transmission through block or MLCs (1-3% for MLCs)
What is physical penumbra? What factors contribute to it?
Physical processes leading to deposition of dose outside the field:
1) Lateral electron scatter – more for lower energy beams
2) Photon scatter
3) Photon leakage, electron contamination, neutron contamination
