Beam Energy Flashcards
T/F: you CAN find beam energy on the treatment prescription/plan
TRUE
beam energy choice depends on what factors? (3)
- thickness of patient
- depth of tumor
- if skin sparing is desired
generally for patient separation/thickness LESS than 22 cm, what beam energy is used?
4-6 MV
generally for patient separation/thickness MORE than 22 cm, what beam energy is used?
10-18 MV
as beam energy increases, what happens to DMAX?
DMAX also increases
what is dmax for a 4MV beam?
1 cm
what is dmax for a 6MV beam?
1.5 cm
what is dmax for a 10MV beam?
2.5 cm
what is dmax for a 15MV beam?
3.0 cm
what is dmax for a 20MV beam?
3.5 cm
what is dmax for a 25MV beam?
5 cm
what is dmax for a 6MeV beam?
1.5 cm
what is dmax for a 9MeV beam?
2.2 cm
what is dmax for a 12MeV beam?
2.8 cm
______ = point at which 100% of dose is distributed within patient
dmax
as beam energy increases, so does dmax - what happens to the skin sparing?
increases as well
–dmax increases, so does beam energy, and skin sparing –> cause penetrates further into body and not as close to skin surface
what is the green region on this photon PDD and depth graph?
build up region
what is NOT spared when using electron?
the skin is NOT spared
what is the green region on this drawn diagram of a patient getting a photon trt?
build up region
which curve is representing electrons and which curve is representing photons?
PINK VS BLUE
pink = photons MV
blue = electrons MeV
once an ___MeV/MV___ beam hits skin surface, it will begin to build up until it hits dmax releasing 100% dose there
MV (photon)
what type of beams are very superficial?
electrons or photons
electrons
nearly ___% of electron beam is delivered at skin surface
100%
for photon beams, ___-___% of dose is delivered to skin surface and then 100% at dmax
**this can change/depends on beam energy selected
25-35%
which beam energy participates in skin sparing effect?
PHOTON (MV)
once an electron hits dmax, what happens?
they abruptly fall to zero; zero %
–this is how other tissues are spared during e- treatments, unlike photons
unlike photons, electrons start off high meaning they do not need a _______ region
build up region
–there is no build up region for electrons
which curve correlates to electrons, and which for photons?
PINK = photons (MV)
BLUE = electrons (MeV)
which isodose curve belongs to electrons… and which for photons?
A. deeper curves- PHOTON
B. shallow curves - ELECTRONS
higher beam energies yield ___greater/lesser___ PDDs
GREATER
higher energies have ___smaller/larger___ gaps between skin surface and dmax
larger
which isodose curve is a higher energy photon beam, and which belongs to a lower energy photon beam?
A. lower energy isodose curve
B. higher energy isodose curve
**we know by looking at dmax and gaps between skin surface and dmax
concept check:
For whole brain treatments we use lower energy beams, what would happen is we used higher energies?
we’d miss tissue right next to skull, as beam would penetrate straight on through
T/F: there IS a shape difference between electron and photon isodose curves
true
which isodose curve is a rough sketch of a photon curve and which is an electron?
A. electrons
B. photons
on electron isodose curves … higher energy lines ______ laterally
constrict laterally
low energy electron isodose curve lines are ___closer/farther___ - whereas high energy e- isodose curve lines are __closer/farther__
low energy lines —> closer
high energy lines –> farther
**safe to assume same for high/low energy photons beams
what is Bowing Effect on electron isodose curves?
when low energy isodose lines expand laterally
–Bowing Effect is due to beam scatter! –> e- scatter easy
what causes Bowing Effect for electron isodose curves?
beam scatter
Photon or Electron isodose curves –
penetrate in a more forward direction
PHOTONS
Photon or Electron isodose curves –
are very superficial
ELECTRONS
Photon or Electron isodose curves –
have greater energies available at greater depths
PHOTONS
CA = _____ ______
Central Axis
what is equation to find 80% central axis depth dose of electrons?
E/3
what is equation to find 90% central axis depth dose of electrons?
E/4
what/where is Therapeutic Range of electrons?
90%
___-___% isodose lines of electrons are most useful because after them, the dose falls off/electrons lose energy at a high rate
80-90%
electrons lose ___ MeV per cm in water or soft tissue
2 MeV/cm
once dmax is hit for electrons where does any dose after that come from?
mainly from contamination, not the primary beam
what does “dose fall off” mean?
losing energy at a high rate
________ are heavier, charged particles that scatter at smaller angles
protons
protons compared to electrons, which have a larger scatter angle?
electrons
protons, compared to electrons and photons, are slowed by __lower/higher___ Z materials
lower
what is another name for proton dmax?
bragg peak
a proton bragg peak is typically too narrow to cover a whole tumor/ enough tumor volume - SO what happens
SOBP ; several beams of different energies are used to spread out the Bragg Peak
what does SOBP stand for?
Spread Out Bragg Peak
protons:
a _______ ensures that the most superficial/proximal, to the most distal/deepest part of tumor is covered
SOBP - Spread out bragg peak
an SOBP creates what
a flatter, longer line we can use to treat larger areas w/ protons
the following is an example of what?
A =
B =
A –> Bragg Peak
B –> SOBP - spread out bragg peak
