Inverse square law
Intensity of radiation from a point source is inversely proportional to the distance from the source.
I = 1/D^2
Percentage Depth Dose
Percentage depth dose is a way of expressing the dose at a particular depth. It is defined as the ratio of absorbed dose at a depth (d) to the absorbed dose at a reference depth (dr ) along the beam central axis.
Why does beam intensity reduce as it pass through the body?
- Attenuation processes
- Beam quality or energy, depth, field size and shape, SSD
The inverse square law
- Beam quality or energy, depth, field size and shape, SSD
Skin Sparing effect
The region between the patients surface and the point of maximum dose is called the dose buildup region. Gives rise to skin sparing effect
Isodose Curve
Lines passing through points of equal dose
Drawn at regular intervals and expressed as a % of the dose at the reference point
Dmax
Depth of maximum equilibrium for single field photon beams
Define Field Size and maximum, minimum F.S.
- Defined as the lateral distance between the 50% isodose lines at a reference depth, this coincides with the light field (symmetrical field only)
Maximum = 40 x 40 cm field Minimum = 4 x 4 cm field
Beam effects
- Greater lateral scatter with low-energy beams causing the isocurves to bulge out
- Orthovoltage beams have increased scattered dose to tissue outside the treatment region
- Megavoltage beams, the scatter is mainly forward
What is penumbra
- Penumbra is the area at the edge of the radiation beam where the doserate changes rapidly.
- A radioactive source size can have a greater effect on the degree of penumbra, larger source size=larger penumbra
- In an image penumbra is seen as the area of unsharpness at the edge of the beam
Penumbra is defined as the area between the 20% and 90% isodose line at the depth of D max
Factors impacting penumbra
§ Source size (e.g. 2cm Cs source)
§ SSD (Source to Skin Distance)
§ Position of the collimators, the closer the collimator is to the patient
the smaller the penumbra, composition material
§ Geometric and transmission penumbra
Effects of scatter in the patient
Transmission penumbra
occurs as the radiation passes through the edge of the primary collimators, therefore secondary collimators are added to reduce this effect
Geometric penumbra
this is where the lack of sharpness or “fuzziness” occurs at the edge of the field
Exit Dose
the amount of radiation at the surface of the body opposite that to which the radiation is directed
Treatment Planning Process
- Patient positioning & immobilisation
- Image Acquisition and input-CT/MRI/PET
- Anatomy Definition-target & OAR
- Beam Technique
- Dose Calculations
- Plan Evaluation
- Plan Implementation
- Plan Review & QA
- Treat the patient
Decisions to be made for planning
-Using fields of appropriate size
- Increasing the number of fields
- Selecting appropriate beam angles
- Adjusting beam weights (contribution from each field)
- Selecting appropriate beam energy
Utilising beam modifiers
Uniformity depends on
Patient size
Tumour position
Beam energy
Different tissue densities
Why should multifield plans contribute different dose
To improve dose uniformity
The PTV may not be centrally located
There are different tissue densities in the volume
Reduce dose to organs at risk
Normalisation
This is just the process where the dose distribution is normalised to the reference dose or PTV dose or max dose- usually so that the dose can be easily recorded and reported and we can see by looking at it what it represents
What factors affect choice of beam arrangements
PTV location, size and shape
Structures in entrance and exit beam
OAR location and dose limits, beam energy
External contour shape
What factors affect isocentre placement
PTV location
PTV size and shape
Where do we define the field size
at 100cm. 100cm can be placed on skin surface (SSD) or within patient (SAD)
