planning 1 Flashcards
how do we decide what dose to give to patient
do not generally measure dose in patient
accurately predict dose
data measured in water equivalent phantom
2 aspects of dosimetry
absolute absorbed dose - amount of energy imparted by ionising particles to a unit mass of irradiated material at a place of interest (GRAY)
reletive dose - sptial distribution of dose relative to point of determination of absolute dose
what is %DD
- Relative spatial distribution of dose
- Relative to absorbed dose at reference depth
- %DD or PDD
- Distribution of dose related to the point of determination of absolute absorbed dose
What is Central axis PDD?
- Dose distribution on central axis
- 3 regions:
- Build up region –lack of electronic equilibrium
- Attenuation region –loss of beam intensity
- Depth of maximum dose –electronic equilibrium
What is a beam profile?
- Measurement of dose at different positions in a cross sectional plane
- Measured at a defined depth
- Produces a beam profile
What are the characteristics of isodose curves?
- Lines joining points of equal PDD
- Maps variation in dose as a function of:
- Depth
- Transverse distance from central axis
- Curves related to the reference point on the central axis
What are the characteristics of an isodose chart?
- Set of isodose curves
- Show incremental PDD
- Typically maximum to 10%
- Plotted at 10% intervals
- Normalised to maximum dose
- Represents distribution of dose in one plane –map of dose distribution
What is build-up region?
- Also known as skin sparing
- At MV energies
- Rapid increase in dose, maximum at specific depth
What is penumbra?
- Rapid fall off of dose
- Lateral distance from the central axis
- Width dependent on:
- Distance from source
- Scatter
What are the differences between percentage depth dose and tissue maximum ratio?
- %DD used for fixed FSD treatments
- Measured in a water tank
- Chamber moves, hence measuring point moves
- TMR used for isocentric treatments
- Measuring point constant
- No ISL correction needed, independent of distance
- Measures absorption
- Calculated from %DD values
What are the factors affecting %DD and hence dose distribution?
- Beam type
- Beam energy/quality
- Field size
- Field shape
- Shielding
- Effective distance
- Filtration
- Collimation
- Wedges
- Compensators
- Build up, bolus
- Attenuating medium
What are the isodose curve characteristics?
- Build up depth
- Shape of curve
- Penumbra
- Penetration
What is beam energy?
Beam quality i.e. penetrating power of beam
What happens when beam energy increases?
- Increases primary photon penetration depth before interaction
- Increases %DD (ratio of ‘dose at depth’ to ‘dose at BU’)
- Scattered radiation increasingly in forward direction
- 100% depth increases –greater skin sparing
- Useful range increases
- Exit dose increases
What is half value layer?
- Thickness of material needed to reduce incident radiation intensity to half it’s original value
- Shows the energy characteristics of beam
- Low HVL →lower energy beam →less penetrating
- HVL characterises the hardness of the x-ray beam
- HVL is measured in mm of Al, Cu or Pb
what is beam hardening
- In a polyenergetic x-ray beam, low energy photons will be removed from the beam when passing through matter
- When the x-ray spectrum shifts to higher effective energies as the beam travels through matter, this is called beam hardening
- Low energy x-rays won’t penetrate most tissues in the body, so their removal reduces the exposure to patients without affecting the diagnostic quality of the exam
What is field size and what effect does changing it have?
- %DD affected by field size due to scattered radiation following primary interaction
- As field size increases scattered radiation at depth on central axis increases
- Not much change at build up depth
- Change more important at small field sizes
- Dependent on how far 2oscatter can travel
- Limitation at larger field sizes
- As energy increases scattered radiation is more in the forward direction
- Hence effect of field size on %DD reduces
What is field shape and what effect does changing it have?
- Shape important but not on a geometrical basis
- Area of beam not important
- Example:
- 10 x 10 cm field = 100cm2area
- 5 x 20 cm field = 100cm2area
- BUT there is greater %DD from 10 x 10 field compared to 5 x 20 field
- This results from the range of scattered radiation
How does equivalent square affect field shape?
- Square field resulting in same value of %DD as rectangular/irregular beam of same radiation quality
- Takes into account the contribution of scatter on the central axis
- Focus is on the range of scattered radiation
- Usually takes the form of a table
- Accounts for distance the scattered photons have to travel to reach the central axis
- Can use formula to calculate (2AB/A+B)
- If field elongated >5:1 eq. sq. formula does not work efficiently, use dosimetry measurement technique
- An elongated field contributes less scatter to the central axis than a square field of the same area
- Rectangular field backscatter will be similar to a square field of a smaller area
What is shielding and what effect does it have?
- Blocks, MLC’s
- Will change the equivalent square of a field
- Hence will alter output and depth dose
- Can be calculated
