Patient Dosimetry Flashcards
How does inhomogeneity affect dose deposition?
High-density (high Z) tissue increases attenuation while low density tissue decreases it. Thus, the dose distal to an inhomogeneity will be different. Thus, there is a loss of electronic equilibrium:
- high density = more attenuation but more secondary electrons
- low density = less attenuation bur fewer electrons
Thus, going from low>high means there will be underdosing of the distal aspect. From high>low means there will be overdosing of the distal aspect
Without homogeneity corrections, what happens to the dose of a lung tumor?
At the anterior side, the tumor will be underdosed whereas the posterior lung tissue will be overdosed
In an AP-PA setup, why is the summed dose higher more superficially vs at midplane?
What increases/decreases this effect?
Dmax is constant no matter the beam approach whereas Dexit (from the opposed beam) decreases at a slower rate. In order to get a dose to midplane, you will always have the contribution of the Dmax and Dexit, so there will always be a superficial hotspot.
Increased beam energy will decrease this effect;
What are two unintended dosimetric impacts of physical wedges?
1) beam hardening – increases penetration
2) generate scatter, increasing dose outside the field
How the the following wedge angles affect hotspots?
1) under-wedged
2) over-wedged
3) optimal
1) causes a large hotspot at the heel
2) causes 2 large hotspots at the toes
3) 3 small hotspots at the toes and heel
When treating a larynx, how would you wedge and why?
Would consider underwedging to cause a hotspot at the heel (anterior) to avoid underdosing anterior commissure
What is the equation for wedge angle?
WA = (180-HA)/2 = 90 - HA/2
How is the dose at field edge defined?
50% of central beam
What is the equation to find skin gap?
g1/d=y1/SSD1
g1 = (d)(y1/SSD1)
g1 + g2 = (d)(y1/SSD1) + (d)(y2/SSD2)
What is the equation for obliquity factor (OF)?
OF = surface dose (oblique)/surface dose (en face)
What are the differences between the “treated volume’ and “irradiated volume”?
The TV should encompass the target and what gets high dose; the IV should encompass everything within a toxicity (low) dose
What is the conformity index?
CI = TV/PTV
Do we use a reference point in modern treatment planning?
For 2D/3D (non-IMRT) fields, a reference point is appropriate as the limited beams means one is able to identify and isodose curve and prescribe relative to a reference point. For IMRT/VMAT, we use dose-volume histograms to evaluate a plan (instead of a reference point)
How do you calculate dose to a reference point?
Depends on the prescribed dose and the prescribed IDL. For example, if someone says “60 Gy to the 90% isodose line”:
Dref = Presc D/IDL
Dref = 60/0.9
Dref = 66.7 Gy
