Exam Flashcards
Beam Measurements
Taken in a water tank, with a waterproof ion chamber attached to a robotic arm. Which moves in all three directions. The chamber can follow an isodose curve around. The path follows traces an isodose curve.
Beam Profile
Scans that are taken in one direction:
- cross-plane scans (right/left)
- In-plane scan (sup/inf)
- Depth-dose scan
Types of field sizes:
- Geometric field size
- Light field size
- Dosimetric field size
Geometric field size
Cone defined by the target at the apex and it widens the further one moves from the target and field size is the base of the cone a certain distance (s) from the source. (*Reality is not so simple as the target is not a point source)
Light field size
The light field is defined as the width of the light field as defined by the shadow of the collimator at a distance (s) from the source.
Similar to geometric field size. The target is replaced by the filament of a lamp. The filament is not physically where the target is, but through mirrors and optics, it is optically in the same location as the target.
Dosimetric field size
A water tank or patient is in the beam. Interested in the horizontal dose profile at the depth (d) distance (s) from the target.
Defined as 50% of the central axis dose.
Collimator setting
Field size at isocenter:
100cm for linacs, commissioned as 10x10 at 100cm
Field size at skin: Fixed SSD vs SAD (isocentric)
Fixed SSD: 100cm set to skin, therefore field size is also collimator setting.
SAD (isocentric): field size on the skin is smaller than the collimator setting.
Beam flatness
The horizontal profile is flatter at 10cm (or where it is calibrated) due to the flattening filter. Photons on the sides of the field are attenuated more readily than photons on the central axis due to the slightly lower energy spectrum to the sides. As you go deeper the sides are attenuated at a faster rate than in the center and the horns gradually flatten out.
Looks at the max/min ratio in the middle 80% of the field
Change is seperation
Increasing separation decreases coverage of the 100% isodose
% dose at Dmax increases with increasing separation.
The dose gradient along the central axis increases with increasing separation
For larger separations advantageous to use higher energy. Assuming the tumour is mid-plane.
For smaller separations advantageous to use lower energy as coverage of the 100% is adequate and they are usually more superficial and so increased skin-sparing is a disadvantage.
Dose variations
Depth of Dmax increases with increasing energy
Depth of maximum buildup is greater for higher energies due to higher energy electrons having a longer range
For fixed energy, the % dose at Dmax will increase as the depth of the isocenter increases (because 100% is still defined at the isocenter) and more dose has to be pushed to get 100% of the dose at the isocenter therefore more dose is also at Dmax.
For the isocenter at a fixed depth greater then Dmax we find the dose % at Dmax increases with decreasing energy (skin-sparing, less dose has to get pushed if the beam penetrates more)
Penumbra
Refers to the transition area from where there is a full radiation field to where there is almost no radiation.
Sources of Penumbra
- Geometric: size of source (small on linac, larger on cobalt)
- Jaw Transmission: partial transmission of radiation through the jaw widens the penumbra
- Photon side scatter: dominates at lower energy
- Electron side scatter: dominates at higher energy (higher the energy the more energetic the electrons are, therefore the penumbra is larger for 20mV then for 6mV)
Penumbra Width
Typically defined between the 10% and 90% dose points.
Influenced:
- Energy
- Source geometry
- Depth (the deeper you get the larger penumbra)
- Jaw location (half beam block is sharper)
Types of algorithms for TPS
- Pencil beam
- Triple A
- Monte Carlo
Monte Carlo
Gold standard. Calculates individual trajectories of particles. Determines where the energy is deposited for each electron.
Based on probability distributions.
Takes too long, not used in TPS. Mostly used to QA IMRT and VMAT plans.
Limitations of pencil beam
Doesn’t model scattered electrons well such as:
- lateral scatter and backscatter
- scatter from LINAC components (the jaws, etc)
Regions of uncertainty
- skin
- build-up regions
- penumbra
- close to shielding
- tissue/lung interface
- tissue/bone interface
Inhomogeneity
Refers to any part of the internal irradiated volume having different densities, atomic numbers or both.
The presence of inhomogeneities not only affects the dose to the inhomogeneity but also the dose to the other soft tissue adjacent to the inhomogeneity.
Homogeneity corrections
Modified Batho is common. Only one dimension, the effect of laterally adjacent homogeneities not accounted for. Can be turned on or off.
Off: assumes the patient to have the electron density of water.
On: takes into account different densities
Situations when to turn it off:
- artifacts due to metal (can cause HU calculations to be inaccurate)
- Presence of contrast
- simplify calculations for palliative patients
Anisotropic Analytical Algorithm (AAA)
AAA calculates dose as a sum of contributions from primary photons, scatter photons, and electrons scattered off LINAC components.
More accurate the pencil beam but still has limitations.
Limitations in same areas as pencil beam but more accurate:
- skin
- build-up regions
- penumbra
- close to shielding
- tissue/lung interface
- tissue/bone interface
Homogeneity correction can be turned on or off.
Beam weights
The proportion of dose delivered by each beam.
Isocentric techniques
For pencil beam:
- isocentric weighting - weight applied to isocenter
- a beam weight of 1.00 places 100% at the isocenter
For AAA:
- not normalization for AAA
- weighting applied to a specific reference condition (depth & FS)
Isocentric dose does NOT depend on:
The relationship between beam weight and relative isocenter does not depend on: - depth - prescription - energy - field size - heterogeneities - addition of a wedge A beam weight of 1 will always give 100% to the isocenter
Isocentric dose does depend on:
The relationship between beam weight and dose does not depend on:
- field symmetry
- blocking
- off-axis point of interest
- missing tissue
Under these conditions, a beam weight of 1 no longer delivers 100% to the isocenter
