Characteristics of clinical beams Flashcards
What does the treatment planning system contain and what information does it require?
- The treatment planning system (TPS) contains a description of the physical and dosimetric characteristics of each linac.
- Measurements are required for the dosimetric model but need to be made correctly. Is TPS model right?
- We need to understand the dosimetric characteristics of clinical beams.
- TPS needs the right information about our treatment machines, the beams they produce and how they interact with matter.
- Need to measure lots of aspects of our clinical beams in order to create an accurate model in the software.
What is the dominant interaction with matter at the energies typically used in radiotherapy and how does the interaction vary with e- density and E?
- Compton scatter.
- Attenuation α electron density.
- Attenuation α 1/E.
Draw typical isodose plots for 200KVp, 60Co, 4MV and 10MV beams.
- Isodose lines describe boundaries of different dose levels.
- Lower energy - less forward scattered radiation and greater penumbra.
Draw a diagram of a single beam profile and label it.
- Patient surface.
- Build-up region.
- Point of max dose.
- Penumbra.
- Profile.
- Normalisation point 10cm depth.
- Scatter and leakage from linac and scatter from patient.
- Dose change with depth - PDD.
Describe the central axis depth dose as a beam travels through a patient?
- When photon beam is incident on a patient:
- Dose builds up from surface dose Ds to a maximum Dmax at depth zmax.
- It then falls almost exponentially until dose Dex at exit of patient at depth zex.
Describe and explain the relationship between beam energy and Dmax.
- Higher photon energies produce lower surface dose and deeper Dmax.
- This is because Compton scatter is inversely proportional to photon energy therefore the photons have less chance of interacting with the matter i.e. are more penetrating.
- Results in low skin dose - good for deeper targets but a problem for superficial targets.
Describe how depth dose is measured and draw a diagram.
- Detector Q, shown at depth z, moves up the beam axis.
- Point P is at the depth of maximum dose Zmax.
- Field size A is usually defined at surface f = 100cm SSD.
What is percentage depth dose (PDD)?
-Dose normalised to 100% at Zmax or Xcm.
What is relative depth dose (RDD)?
-Dose normalised to unity at Zmax or Xcm.
What are the components of a depth dose curve?
- Primary beam.
- Head scatter.
- Phantom scatter.
- Electron contamination.
Explain the build-up effect and draw a diagram showing this.
- Photons interact at different depths in the tissue and generate secondary electrons.
- At each interaction, the recoil electrons travel, mostly forward, and deposit dose.
- As more tracks overlap, the dose is built up until charged particle equilibrium (CPE) is reached.
- A steady state would be reached if there were no photon attenuation/scattering.
- Dose > 0 at surface due to some backscattered electrons from patient & contamination from linac.
What is the relationship between beam energy and surface dose, depth of dose max and dose at depth? Sketch graphs showing this.
- As photon beam energy increases:
- surface dose generally decreases.
- depth of dose maximum increases.
- Dose at depth increases.
How dose the PDD curve for a given beam energy vary with beam size and why?
- As the treatment beam gets bigger, the dose at a given depth generally increases due to:
- More photons reaching the patient from the source (extended source of flattening filter).
- More scattered electrons to measurement point from the irradiated volume.
How dose the PDD curve for a given beam energy vary with SSD and why? Sketch a graph showing this.
- Actual dose decreases with distance from source (inverse square law).
- Relative dose, with respect to reference point, increases with SSD.
- Compare two pairs of fixed-separation points (10cm apart) at different places on same inverse square law graph.
- EXAMPLE: D(b)/D(a) = 0.83, D(d)/D(c) = 0.86.
What is the tissue phantom ratio (TPR) and how is it determined? Draw diagrams to help explain.
- TPR(Z,C) = D(Z,C)/D(Zref,C)
- Detector at fixed distance = SAD and overlying material thickness varied.
- Field size AQ is defined at SAD.
- Detector Q remains at SAD.
- Measurement (a) at depth z is normalised to the measurement (b) at the reference depth zref.
- Tissue maximum ration (TMR) is the special name when Zref=Zmax.
Draw the relative depth dose (RDD) and tissue phantom ratio (TPR) curves on the same graph.
- Different shape.
- Not comparable.
- Inverse square law effect.
- Different scatter conditions.
How dose the TPR vs depth curve vary with beam size? Draw a graph showing this.
- Varies similarly to PDD but usually normalised at greater depth.
- Normally separate field size factor for flexibility of data use.
List ways in which an electron beam interacts with tissue.
- Excitation.
- Ionisation.
- Bremsstrahlung.
- Characteristic radiation.
Describe (and sketch) the energy spectrum of an electron beam at the accelerator window, the tissue surface and at depth d in tissue.
- At accelerator window, the e- beam is almost monoenergetic exiting the accelerating waveguide, but the waveguide window, scattering foils, ionisation chamber, air, photon collimators, electron applicators, etc. generate interactions.
- At tissue surface, energy spectra broadens and mean energy decreases.
- Broader spectrum due to collisional & radiation energy loss.
- At depth d in tissue, broader spectrum with more lower energy e-s.
Describe build-up and skin sparing when using an electron beam.
- Electrons deposit energy immediately, hence give a larger surface dose than with megavoltage photon beams.
- Electron path becomes more oblique due to scattering as the beam passes through tissue - dose build-up to depth of max dose.
- Beyond this point, numbers of electrons decline.
- Steep dose fall-off beyond dmax as electrons are not energetic enough to penetrate.
- Range straggling occurs, which increases at higher energies.
Draw and label a graph of absorbed dose vs depth for an electron beam.
- Peak/max dose.
- Surface dose.
- Therapeutic interval (distance between near and far useful dose level).
- Depth of peak dose.
- Depth of 50% absorbed dose.
- Depth of practical range.
- bremsstrahlung tail Dx.
How do PDD curves vary with increased energy for electron beams. Sketch graphs.
- With increased energy:
- Surface dose increases.
- Depth of dose max increases.
- d50, d80 and Rp increase in depth.
- Gradient of fall-off decreases.
- Bremsstrahlung x-ray contamination level increases.
What dose a beam profile describe?
-a beam profile describes the variation across the beam in tissue/water.
How is the beam size defined?
-Beam size is defined as FWHM of beam at 10cm deep.
How is the penumbra defined and why is it useful to know?
- Penumbra usually defined as distance between 80% and 20% dose levels.
- Sets limit on how much of a beam is useful and how much exposes normal tissue.
How does the beam profile change with depth?
- Beam widens with depth.
- Penumbra widens with depth.
- Dose changes with beam size at a given depth (think PDD).
- Flattening filter gives flattest beam at 10cm deep- rounded beyond and ‘horny’ at shallow depths.
How is the wedge angle defined?
-The wedge angle is defined as the angle between isodose line and the normal to central axis at 10cm deep (IEC 1217).
What are the three causes of a penumbra? Draw sketches to explain these causes.
- Geometric (extended radiation source).
- Transmission through collimators.
- Scatter in patient.
What is the equation for the penumbra width, P, at depth d?
-Pd = s(SSD+d-SDD)/SDD
What does the width of the transmission penumbra depend on?
-Depends on energy of beam - higher energy more penetrating.
What does the width of the dosimetric/scatter penumbra depend on?
- Scatter angle is energy dependent.
- Lower energies have more lateral scatter.
- Lower energies have wider dosimetric penumbrae.
- Some contribution from in-air and collimator scatter.
- At higher energies geometric and transmission penumbrae effects dominate.
What are field size factors? Sketch graph of relative dose vs square field.
- As beam size increases, dose to patient increases.
- Compare dose at 10cm deep with reference beam for various beam sizes.
- Can apply field size factors to reference condition.
- ST = D(C)/D(Cref).
What are the two components of dose to the patient in RT?
- Incident beam.
- Scatter within patient.
Explain what head scatter is and state how much it contributes to patient dose.
- Head scatter’ mostly from flattening filter but some contribution from collimators.
- Constitutes about 3-4% of dose to patient.
- Variation from 10x10cm reference field is ~±5%.
What is the head scatter factor and what does it depend on? Draw a diagram to help.
-Sc=Dmp(C)/Dmp(Cref) [Dmp is dose measured in mini-phantom].
Linac-specific, depends on head design.
-Quantifying absolute head scatter value is difficult.
-For dose calculations, need change relative to reference beam size.
-Exclude variation in patient-scatter by constant irradiated volume.
What is the patient (phantom) scatter factor and what does it depend on?
- Sp = ST/Sc
- Not linac specific.
- Beam quality dependent.
- Difficult to measure:
- Infer from ST and Sc.
- Can use published tables (NCS12).
- Variation from 10x10cm reference field is ~±10%.
What are monitor units?
- Ionisation chamber ‘monitors’ beam constantly.
- Amplifier adjusted until 1 monitor chamber unit = 1 TPS dose unit (usually 1cGy) under ‘calibration conditions’.
- Monitor chamber linked to beam control - when requested MUs are reached the beam is switched off.
- TPS calculates MUs required to deliver prescribed dose.
Why do calibration conditions need to be established on linacs and what does calibration define?
- Need a relationship between Dose at Px and MUs required to deliver that dose.
- Linac calibration defines the reference dose/MU for dosimetry system.
- The calibration conditions and the MU calculation (dosimetry system) reference conditions need not be the same but there needs to be a known relationship between them.
what factors result in deviations from the reference condition?
- Depth.
- Treatment distance.
- Collimator setting.
- Shape/size of irradiated area.
- Attenuators etc.
- Patient heterogeneities.
- etc.
Suggest a typical linac calibration condition.
-e.g. 1cGy/MU at dmax depth for 10*10cm field.
Give an example of a relationship between linac calibration conditions and the dosimetry system reference conditions.
-e.g. 1cGy/MU at dmax = 0.0078Gy/MU at 10cm depth for isocentric 6MV 10*10cm field.
What in an equivalent square?
- A square field which has the same central axis depth dose characteristics as a given non-standard field.
- Se = 2xy/(x+y)
- This relationship only applies to photons.
