Radiation Therapy

Percent depth dose

is the ratio of the absorbed dose at a depth d to the absorbed dose at a fixed reference depth d₀ along the central axis of the beam for a fixed SSD

What is the Bremsstrahlung tail

Comes from the bremsstrahlung interaction with the treatment head/patient/phantom

Electrons lose ______ MeV per cm of water

2

For electron beams, as electron energy increases then what else changes (5 things)

Increases: Skin dose, depth of max dose, range straggling, bremsstrahlung tail
Decreases: Sharpness of dose fall off

Electron beam therapeutic range and why

R90 or R80, dose decreases abruptly after R90 due to most therapeutic electrons reaching the end of their range and are no longer available to deposit dose

Dose dose sparing effect exist for electron beams?

Yes but minimal (Electrons undergo frequent Coulomb interactions with atomic electrons and nuclei, causing them to scatter widely and deliver most of the dose to the surface)

Practical range

the maximum depth of penetration

Higher energy electrons are scattered in a ____________________

more forward direction, the end result is less build-up for higher energy electron beams

For electron beams, decreasing the field size (4 things)

Reduces: Depth of max dose, relative skin dose, sharpness of dose fall off
Increases: range straggling

Photon beam skin sparing effect

MV photons do not reach max dose at the surface/skin but build up to a max dose due to compton scattering (As photons penetrate, they undergo Compton interactions that eject fast secondary electrons. These electrons: Are the actual dose‑depositing particles)

Photon beams are ________

polyenergetic

Half Value Layer (used for kV beams)

describes beam quality by the thickness of a given material required to reduce the incident beam's fluence by 50%

Megavoltage X-ray beam quality is usually measured by

PDD(10x)

Depth of max dose is connected to (4 things)

Photon energy, SID, field size and beam profile

Dose fall off is ______ for photon beams

gradual, the slope of the fall off decreases with increasing photon energy

For photon beams, PDD increases as ________ increases

SDD (due to inverse square law), field size

Mass Attenuation Coefficient

The fraction of primary photons removed from the beam per unit distance over medium density

Mass Energy Transfer Coefficient

The fraction of primary beam energy transferred to charged particles in the medium per unit distance over medium density

Mass Energy Absorption Coefficient

The fraction of primary beam energy by the medium per unit distance over medium density

Beam hardening

the average energy of the photon beam becomes higher at deeper depths because lower energy photons are attenuated near the surface

Isodose Charts

provide a 2D map of a dose distribution

Flattening filter

Used to provide flatter dose distribution at treatment depth, this creates horns at the shallow depths

Converting PDD curves measured at one SSD to another SSD can be accomplished using the

Mayneord F-factor - converts exposure in air to absorbed dose in tissue

Tissue maximum ratio (TMR)

Ratio of the dose at a depth in comparison to a reference dose that is measured at a fixed SAD (independent of SSD and ISL)

Tissue air ratio (TAR)

Ratio of dose at a depth in tissue to the dose in air at the same point with same field size and geometry

Wedges

Used to create a dose gradient across a radiation field

Physical wedge

Solid piece of metal with sloped profile

Dynamic wedge

Created by moving the collimator jaw or MLC leaves during beam delivery

Gross tumor volume

gross demonstrable extent of tumor

Clinical target volume

gross demonstrable extent of tumor + true biological extend of the disease

Treated volume

Volume enclosed by the isodose surface that delivers the minimum prescribed dose

Irradiated volume

Volume that receives significant dose (Volume receiving ≥50% of the prescription dose)

Isodose chart

A family of isodose curves drawn at equal increments of PDD

Stopping power

average energy lost per unit path length

Restricted mass stopping power

Average energy loss per unit mass thickness by a CP excluding delta rays above a specific cutoff energy (to account for local deposition)

Restricted mass collisional stopping power

average energy lost per unit path length by a charged particle to collisional interactions (ionization and excitation), excluding energy transfers above a chosen cutoff energy Δ

KERMA

kinetic energy transferred from uncharged particles (like photons) to charged particles (like electrons)

Collisional KERMA

The portion of kerma that goes into ionization and excitation of atoms by charged particles

Radiative KERMA

The portion of kerma lost by charged particles through bremsstrahlung radiation