A. Isodose Distributions
1. Measuring Isodose Curves
- Ionisation chambers are the most reliable due to their high accuracy due to well known correction and conversion factors, reproducibility, flat energy response, sensitivity
- Thimble chambers are best for areas of low gradient in photon beams.
- Parallel plate chambers are best in areas of steep dose gradient in photon beams, and should be used for all electron beam relative measurements.
- Other dosimeters are solid state detectors or radiographic films, however these are not as accurate or reliable and they show energy dependence.
- A water phantom is an ideal choice of phantom as it is cheap, attainable and reproducible.
- Water is closely soft tissue equivalent in attenuation and scattering properties as it has similar mass density, electron density and average atomic number as soft tissue.
- One disadvantage of water is that it requires waterproofing of the ionisation chamber. This can be done by a thin well fitting plastic tube or cover.
- Solid or plastic water equivalent phantoms solve this problem and the ACPSEM adaptation of the IAEA protocol provides correction factors for these.
iii) Automatic devices
- Comprise two ionisation chambers, a water phantom connected to a computer that moves one of the ion chambers in measured increments through the water phantom.
- The second ion chamber is to provide a reference dose for each measurement.
2. Displaying Dose Distribution
- There are a number of ways:
i) Central axis PDD
ii) Isodose curve
iii) Isodose chart – family of isodose curves
iv) Beam profile – dose variation across field at a depth, dose normalised to the central axis
v) Cross-sectional isodose distribution – plane perpendicular to the beam central axis
vi) Isodose surface in 3D
3. Features of Isodose Chart
- Dose at depth greatest on the central axis, except for some linac beams which exhibit “horns” near the surface in the periphery due to the flattening filter.
- Spacing between isodose curves of equal increments progressive gets larger due to inverse square law.
- Penumbra at the field edge – area where dose rate changes rapidly as a function of the distance from the central axis of the beam
4. Penumbra and Dose Outside Field Edge
- The physical penumbra is the lateral distance between two specified isodose curves at a specified depth (eg 90 – 20% at Dmax)
- The physical penumbra is comprised of:
i) Geometric penumbra – due to finite size of source. Increases with increasing source size, and decreasing SDD and SSD
ii) Scatter penumbra – this is from scatter from the collimator and air column, as well as internal patient scatter. This is a function of beam energy.
iii) Transmission penumbra – is due to transmission through the edges of the collimating system (jaws, MLCs or secondary blocks)
- Dose outside the beam edge is due to:
i) Within 10 cm – collimator and internal patient scatter
ii) 10 – 20 cm – internal patient scatter
iii) 20 – 30 cm – patient scatter and head leakage equally
iv) > 30 cm – head leakage (collimator transmission)
5. Factors Affecting Isodose Curve
a) Beam quality
- Depth dose greater as quality increases
- Penumbra more sharp with higher beam energy as scatter is more in the forward direction. Alternatively, penumbra is wider in low energy beams due to greater lateral scatter in the medium. This is one disadvantage of orthovoltage beams.
- Affected by source size, SDD, SSD and beam energy
- Methods may be used to minimise the penumbra such as increasing beam energy, or using penumbra trimmers or secondary blocks.
c) Collimation and Flattening Filter
- Horns due to beam hardening in the centre of the field by the flattening filter. 107% of Dmax acceptable to achieve +/- 3% at 10 cm for area bounded by central 80% of geometric field size.
d) Field Size
- Caution should be exercised for small field sizes as a large proportion of the field will be in the penumbra region. Treatment planning with isodose curves is required to ensure adequate coverage.
B. Wedge Filters
- Used to modify intensity across the beam.
- Causes a progressive decrease in the intensity across the beam, resulting in tilt of isodose curves towards thick end of the wedge.
- Degree of tilt depends on slope of wedge. Sigmoid slope produces straighter isodose curves.
- Can use any dense material – lead/steel.
- Dynamic wedges are created electronically by moving an independent jaw within the treatment beam.
- Wedge angle is the angle through which an isodose curve is tilted to the normal of the central axis at a specified depth.
- Due scattered radiation and hardening, the wedge angle decreases with increasing depth.
- The wedge transmission factor is the dose with and without the wedge at a point on the central axis.
C. Combining Radiation Fields
1. Criteria of Acceptability
i) Homogeneous dose distribution to the target volume (95% to 107% of the ICRU reference dose)
ii) Minimise dose to normal tissues
iii) Spare critical organs
i) Number of fields
ii) Beam directions
iii) Beam energy
iv) Beam weights
v) Field size and shape
vi) Beam intensity modifiers
vii) Modalities – photons, electrons, heavy particles, brachytherapy
viii) Total dose, fractions, dose-rate, overall treatment time
3. Single Fields
- Only useful if combination of beams technically difficult or unnecessary irradiation of normal tissues.
- Useful in skin, supraclavicular region, spinal cord, internal mammary nodes.
4. Parallel Opposed Fields
- Pair if fields directed along the same axis from opposite sides of PTV.
- Uniform dose distribution with megaV beams.
- Less chance of geographic miss
- Excessive dose to normal tissues and critical organs on either side of the PTV.
c) Patient Thickness – Tissue Lateral Effect
- As patient thickness increases or beam energy decreases, the maximum dose near the surface will increase relative to the midpoint dose.
- Acceptable uniformity of dose is +/- 5%.
- 6 MV acceptable for 15 cm
- 10 MV to 20 cm
d) Integral Dose
- This is the total energy absorbed in the treated volume (unit joule)
- The higher the photon energy, the lower the integral dose.
- Integral dose should be minimised without compromising the other criteria of acceptability.
5. Multiple Fields
- Reduction of dose to normal tissue and critical organs can be achieved by using a combination of three or more fields.
- By using multiple fields, the ratio of tumour dose to normal tissue dose is increased.
- However, not every beam angle is possible due to critical organs.
- Set up accuracy is also more difficult than with a simple parallel opposed set up.
6. Isocentric Techniques
a) Isocentric Mounting
- Isocentric mounting of a linear accelerator refers to when the source of radiation (the treatment head) is mounted on a rotating gantry that rotates about a horizontal axis. The gantry is capable of rotating through an axis of 360 degrees. The isocentre then is the point of intersection of the gantry axis and the collimator axis. The couch is also made such that it rotates about a vertical axis that passes through the isocentre.
b) Isocentric Technique
- The isocentric technique consists of placing the isocentre of the machine at a depth within the patient, preferably within the target volume. Beams may then be directed from any desired direction and they will be directed at the target volume.
- The SAD remains constant, but the SSD may change depending on the depth of the isocentre within the patient from any given direction.
- Treatment calculations use TAR, TPR or TMR methods which depend on field size and depth, but not on SSD.
- Ease of set up for multiple beams
- Accuracy relies primarily on one set-up and then the accuracy of the machine isocentricity, rather than multiple set up fields, measurements and fiducial marks.
- Permits rotation arcs, 3D-CRT and IMRT.
- Geometric penumbra less as pt closer
- SSD has been used for a long time and is intuitively simple, especially dose calculations using PDD.
- Pt closer to source and hence less dose at depth.
- Higher skin dose due to closer to shadow trays
- SSD required for TBI
- SSD simpler for e- beams
c) Rotation Therapy
- Special isocentric technique in which the beam moves continuously about the patient.
- Suited to small, rounded, deep seated tumours where the tumour is reasonably close to the centre of the patient contour, and the depths from the surface to the tumour do not differ too much from different angles.
- Not indicated if large volume, external surface differs markedly from a cyclinder, tumour is too far off centre.
- For partial arcs the dose maximum is not at the isocentre, but is shallower. Hence, past pointing, whereby the isocentre is placed deeper than the tumour, is employed.
7. Wedge Field Techniques
- Two wedged beams directed from same side of patient, with thick ends of wedges adjacent to each other.
- Results in fairly uniform dose to plateau region of intersection of the two beams, with a rapid dose drop off beyond the region of overlap, which is desirable.
- Hot spots can occur beneath the thin ends of the wedge, and their magnitude increases with field size and wedge angle.
- The hinge angle is the angle between the central axis of the two beams.
- For a given hinge angle, the wedge angle should be adjusted so that the resulting isodose curves for each field are parallel to the bisector of the hinge angle.
- This occurs when:
Wedge angle = 90 – Hinge angle/2
- When this is done, the combined curves will result in a uniform dose distribution in the region of overlap.
i) Superficial tumours extending from the surface to a depth of several cm (0 – 7 cm)
ii) When it is necessary to irradiate from one side of the skin surface
iii) When rapid dose fall off is required
iv) Open and wedged fields can be combined – eg open ant and two lateral wedged beams
v) Penumbra spreaders to imrpove dose uniformity at junctions of adjacent beams
- Measure dose distribution without the wedge.
- Calculate attenuation required to produce the desired distribution
- Use the effective attenuation co-eff of the wedge material to calculate the shape
d) Dynamic Wedging
- Achieved by moving independent jaws during beam-on, progressively decreasing the dose from one side to the other.
- The desired wedge angle can be created by varying the jaw speed
- ADVS of ease of use, less scatter and no beam hardening.
D. ICRU 50 + 62 – Prescribing, Recording and Reporting Photon Beam Theray
- Specification of VOLUME and DOSE is required for prescribing (to be clear direction from RO to physicist and RT), recording (to enable exact dose to be determined if required in the future, to allow RO to check results of their Rx, to allow RO to compare with other ROs and departments) and reporting (to allow the results of one department to be compared with those of others).
- The outcome of treatment can only be interpreted meaningfully if the parameters of the irradiation (esp volume and dose) can be accurately correlated with the clinical and pathological extent of disease.
- The aim is to facilitate communication through standardised prescribing, recording and reporting, and to enable a treatment to be repeated elsewhere without having recourse to the original institution for further information.
- Hence, ICRU requires reporting of:
i) Patient data, disease, staging, aim of treatment (radical, paliative, non-malignant)
i) Gross Tumour Volume
- Gross demonstrable extent and location of tumour (visible, palpable, imaging).
- Cannot be defined if post-op
ii) Clinical Target Volume
- GTV plus any other tissue with presumed microscopic spread, based on type of disease, and ROs experience and judgement.
- CTV must receive adequate dose for cure.
- Two types of CTV – surrounding the GTV (CTV I) and separate, such as lymph nodes (CTV II, III etc).
iii) Internal Target Volume – CTV + Internal Margin
- IM added to CTV to account for physiological movements of internal organs and variation in size, shape and position of the CTV.
iv) Planning Target Volume – ITV + Set-up Margin
- SM accounts for patient movement during treatment, and set-up uncertainties (variation in position, mechanical uncertainties, transfer set-up errors from CT/Sim to treatment).
- IM and SM are not added linearly, but are combined subjectively.
v) Planning Organ at Risk Volume (PRV)
- ORV identified.
- Internal and set-up margin added to compensate for internal movements, patient movements and set-up uncertainty.
NB – The penumbra is not considered when delineating the PTV. However, when selecting the beam sizes, the width of the penumbra has to be taken into account and the beam size adjusted accordingly.
vi) Treated Volume
- The tissue volume that is planned to receive at least the dose specified as being appropriate to achive the aim of treatment (i.e. tends to be 95% of ICRU ref dose).
- Hence, the treated volume is the volume enclosed by the isodose surface corresponding to the dose specified as being adequate to achieve the aim of treatment.
- Dose variation of 95% to 107% is accepted within the PTV, and hence the treated volume is that volume which receives 95% of the ref dose.
- This is generally larger than the PTV and depends on treatment technique.
- The conformity index is the quotient of the treated volume and the PTV, and can be used as part of the optimisation procedure.
vii) Irradiated Volume
- Volume receiving significant dose (eg > 50%).
- This volume will tend to increase relative to the PTV as the number of beam directions increases.
- Treated and irradiated volume can be decreased by conformal shielding.
SUMMARY – ICRU 62 recommends reporting GTV, CTV, PTV, TV, IV, PRV (not internal target volume)
- The volume around the GTV and ORV are safety margins, and it is a judgement call by the RO what compromise should be made if these safety margins overlap such that one must be reduced.
c) Absorbed Dose
i) ICRU Reference Point
- The target dose should be specified and recorded at the ICRU reference point which should satisfy the following criteria:
1. Clinically relevant and representative of dose in PTV
2. Easy to define and unambiguous
3. Dose can be calculated accurately
4. Not in region of steep dose gradient
ii) Other Factors
- Generally should lie within PTV
- For single beam - should be on central axis within PTV.
- For parallel opposed equally weighted – on central axis, midway
- If unequal, then central axis in PTV
- Multiple beams – should be at point of intersection of beams within the PTV
iii) Other Doses
1. Maximum dose – highest dose in the PTV over 2 cm2
2. Minimum dose – lowest dose in the PTV
3. Mean dose, median dose (b/w max and min), modal dose
4. Hot spot – is an area greater than 2 cm2 outside the PTV that receives a dose higher than the ref dose
d) Additional Information
- Dose total, fractionation, overall time
- Technique – beam energy, directions, isocentric/SSD, blocks, beam modification, patient positioning.
E. Introduction to Simulation and Planning
- Once a decision has been made to treat a patient with radiation then the objective is treat the target volume to a tumoricidal dose, while minimising dose to normal tissues and sparing critical organs.
- For radiobiological reasons, a prescribed radiation dose is delivered in multiple fractions over many days.
- Hence, during radiation treatment it is important to ensure that same volume is irradiated each day, and that this is the intended volume.
- This is particularly important with the new highly conformal techniques in which the planned isodose surface tightly surrounds the target volume. If there is a mismatch between the intended and the actual treated volume then the tumour may receive inadequate dose, and/or normal and critical tissues receive excessive dose.
- Once a patient has been accurately diagnosed and staged, and a treatment decision is made to involve radiation therapy (either for curative or palliative intent), then there are a number of steps involved:
iii) Data acquisition including:
1. Target localisation
2. Normal tissue localisation of density
3. Body contours (for dosimetry)
v) Planning (beam arrangements), dose calculation and optimisation
vi) DRRs and documentation
vii) Marking of patient and check films (optional)
viii) Monitor unit calculations
x) Record and verify, EPIDs
- Steps i) to vi) may be considered the “planning” process.
F. Positioning and Immobilisation
- One of the major uncertainties in dose delivery to the patient is the ability to set up the patient accurately and reproducibly each day.
- This requires good positioning, and if necessary immobilisation.
1. Positioning Requirements
iii) Unnecessary structures moved out of the way (eg arms)
iv) If possible, one position only
v) Patient should be explained the importance of positioning
2. Positioning Aids
i) Laser beam lights to aid in alignment of the patient
- Wall mounted laser lights pass through the treatment machine isocentre in the transverse, sagittal and coronal planes.
- These not only allow initial patient alignment, but their intersection points on the patient’s surface (triangulation points) may be marked with tattoos to aid in daily set-up.
ii) Films may be taken also to ensure that the patient’s longitudinal axis is correctly aligned and not rotated
iii) Positioning Devices:
- head rests, breast boards (upper body elevators), pillows, arm/hand poles
- It may be necessary to use an immobilisation device to ensure accurate day to day set-up, and also to ensure that the patient stays in the correct position during the beam on time.
- Precise immobilisation will help to reduce the set-up margin around the CTV and thereby allow the PTV to be as small as possible.
- If an immobilisation device is used, then CT imaging should be obtained with it, and the device should be made of a material that will not cause CT artefacts.
- Devices may be thermoplastic casts, plaster shells, vacuum moulds, and stereotactic frames.
4. Advanced Techniques
- Techniques to account for movements during breathing are being developed and include active breathing control (ABC), deep inspiration breath hold technique (DIBH) and respiratory gating.
- BAT ultrasound for prostate localisation
- Opaque markers inserted into the tumour
5. Treatment Conditions
- The patient should be planned and treated under identical conditions, such as having an empty bladder prior to treatment for prostate irradiation.
G. Disease Localisation
- The patient is placed in the treatment position with all positioning and immobilisation aids.
- Any anatomical or disease landmarks should be marked with radiopaque wire or catheters, such as palpable disease, scars, anatomical landmarks, tentative isocentre for CT.
- If required, imaging may then be obtained, either by the simulator, by CT, or other imaging modalities.
- The radiation therapy simulator is a diagnostic x-ray machine mounted on a rotating gantry with identical geometric, mechanical and optical properties to the treatment machines.
a) Roles of Simulation
i) Tumour and normal tissue localisation
ii) Treatment simulation – that is, after the treatment field directions and shaped have been determined, to take films to ensure that they adequately cover the PTV and avoid critical structures
iii) Treatment plan verification – if CT plans were dose without physical simulation
iv) Treatment monitoring – the set-up can be reviewed during treatment to check reproducibility, if there is a concern regarding a change in anatomy from tumour shrinkage or weight change, and to confirm boost fields.
b) Simulator Necessity
- The need for the simulator arises from:
i) Geometric relationship cannot be duplicated on an ordinary x-ray machine
ii) Image quality is too poor on a treatment machine for adequate target localisation
iii) Saves valuable treatment machine time
c) Simulator Requirements
i) Geometric, optical mechanical properties must closely match those of the treatment machines
ii) Laser localisation
iii) Adequate imaging capabilities
2. CT Simulation
a) Computed Tomography (CT)
- A CT image is reconstructed from a matrix of relative linear attenuation co-efficients measured by a CT scanner.
- Ultimately a CT image detects differences in electron volume density and atomic number (that is, x-ray linear attenuation co-efficients in the photoelectric and compton ranges).
- A relationship between CT numbers and tissue density can be established to allow correction for tissue inhomogeneities in computing dose distributions.
- CTs can generate DRRs (digitally reconstructed radiographs) which are simulated x-rays in any plane.
- For treatment planning purposes:
i) CT requires a flat top couch
ii) the CTs will have to be generated in the treatment position
iii) immobilisation devices on
iv) external markings that are visible in the CT images by using radiopaque markers such as plastic catheters
v) laser localisers to mark a prospective treatment isocentre from which adjustments in position may be made from, depending on the final isocentre position as determined in the planning process.
b) Role of CT
ii) Tumour and normal tissue localisation (incl body contours)
iii) Density data for dose calculations
iv) Treatment monitoring
c) Process of CT Simulation
- During CT simulation, a CT data set is obtained in the treatment position and virtual simulation is carried out.
- The steps are
i) Positioning – Laser lights can assist in alignment, and a tentative isocentre is marked on the patient with radiopaque catheters.
iii) Surface markers – to highlight areas of special interest such as scars, skin nodules
iv) CT Scanning – smaller slices produce better DRRs
v) Image transfer to planning computer
vi) Image segmentation
vii) Treatment planning and optimisation
viii) Isocentre marking – once the final isocentre is determined it should be marked on the patient and simulator films may or may not be taken.
d) Virtual Simulation
- Using computer software, virtual simulation is able to simulate any radiation therapy beam geometry directly onto the CT data, eliminating the need for conventional simulation.
- The ability to view all beams simultaneously in any direction with accurate anatomic data of target volume and critical structures overlaid can increase field placement and shielding accuracy.
- One of the greatest assets of CT sim is that it allows BEV on a DRR which not only allows 3D-CRT, but even for conventional techniques it reduces the inaccuracies associated with a clinician’s ability to mentally translate geometric information from diagnostic scans to treatment portals.
e) Advantages of CT
i) Spatial anatomic accuracy
ii) Good contrast resolution
iii) Data can be reconstructed in any plane allowing 3D images and dose distributions
iv) CT numbers represent linear attenuation co-efficients and can be converted to electron density for the purposes of tissue inhomogeneity calculations
v) Obtains body contour
vi) Faster, cheaper and aperture larger than MRI
vii) Data can be transferred digitally to the TPS to:
- save time
- minimise errors
- maximise digital enhancement
viii) 3D imaging necessary for virtual sim, and use of 3D dose calculation algorithms
f) Disadvantages of CT
i) Aperture size may not allow patient in treatment position
ii) Artefacts from internal prostheses, or immoblisation devices
iii) Time taken to scan – not instantaneous as with sim
iv) Potential errors:
- geographical innaccuracy
- electron density
v) Uses ionising radiation
vi) Soft-tissue contrast is not as good as MRI
i) Better soft tissue contrast
ii) Image can be generated directly in any plane
iii) Obtains body contour
iv) Data can be imported directly to TPS
v) Does not use ionising radiation
i) Aperture size smaller than CT
ii) Poorer spatial accuracy
iii) Does not detect bone
iv) More expensive
v) Scan acquisition time longer
vi) No information regarding electron density
vii) Magnetic interference
4. Errors with CT and MRI
a) Potential Errors
i) Patient not positioned in exact same treatment position:
- Aperture size
- Curved table top
- Lack of positioning aids (lasers etc)
ii) Geometrical inaccuracy of scan data
iii) Innaccuracy of CT number correlation with e- density
iv) Patient movement during scan
v) Breathing artefact
vi) Artefacts from prostheses and immobilisation devices
vii) Too few slices can lead to innaccurate reconstructions
viii) Incorrect transfer of data from CT/MRI to TPS
ix) Spatial innaccuracy of MRI
b) Avoiding/Minimising Errors
i) Patient set-up in exact treatment position:
- Flat table top
- Largest possible aperture
- Laser lights
- Immobilisation and positioning devices
- RTs present if done out of department
- Surface marks highlighted by opaque catheters
ii) QA of spatial accuracy
iii) QA of CT number correlation with e- density
iv) Fastest scan times
v) Direct data transfer to TPS and QA to ensure accurate
vi) CT and MRI fusion
vii) Thin slices, at least through target volume
viii) Staff training
ix) Explain importance of remaining still during scan to patient
5. Other Imaging
- If other imaging modalities are used then an image registration process is required which correlates corresponding points in the two data sets so that they can be merged.
- Planning will require patient surface contours which may be obtained from CTs if they are done, or with solder wires, thermoplastic strips, pen plotters, callipers.
- Decisions regarding compensators, wedges or bolus.
- The following should be done to improve the geometric accuracy of patient set-up during simulation and treatment delivery:
- Comfortable, reproducible, one position, explained to patient, same condition (eg empty bladder)
- Positioning aids
- Optical Light Aids: - field light and cross-hairs, ODI, laser beams
- Fiducial marks
- Same staff
iii) Isocentric technique
iv) Machine parameters – a record and verify system
vi) TV monitoring and breathing control techniques
vii) In vivo dosimetry
viii) Rigorous QA program
vii) Staff training
J. Contour Irregularities
- Dose distribution tables are based on standard conditions of:
i) Perpendicular incidence
ii) Flat surface
iii) Homogeneous density
- If the surface contour is not flat then this may lead to a non-unform dose distribution in the target volume, due to less attenuation in the area of the tissue deficit. This may also result in excessive dose to critical structures.
- This problem may be overcome with:
– is a tissue equivalent material placed directly on the skin surface to even out irregular surface contours (as opposed to buil-up bolus).
- Placing bolus directly on the skin results in the loss of skin sparing.
ii) Compensating filter
– approximates bolus but preserves skin-sparing by placing greater than 20 cm from patient surface.
iii) Compensating wedges
- For oblique beam incidence, or when the curved surface can be approximated by a straight line, standard compensating wedges are convenient.
- Thet are designed to compensate for a missing wedge of tissue.
2. Design of Compensators
a) Tissue Deficit Parameters
- Can be determined by mechanical means or from CT scans
b) Shape and Dimensions
- The shape should be the same as the tissue deficit
- The dimensions and thickness should be adjusted to account for:
i) Beam divergence
ii) Relative tissue density of compensator
iii) Reduction in scatter at depth due to the compensator being at a distance from the patient, and hence the thickness of the compensator must be reduced to increase primary beam transmission
- Thin sheets of lead, block of lucite
- Dynamic compensation may be achieved for wedge compensators with asymmetric jaws moving during beam on time, or for more complex compensation, dynamic MLC movements to create a dose distribution profile which is unifrom in the target volume.
d) Dynamic Wedging
- Achieved by moving independent jaws during beam-on, progressively decreasing the dose from one side to the other.
- The desired wedge angle can be created by varying the jaw speed
- ADVS of ease of use, less scatter and no beam hardening.
- Leaves cover various parts of fields for various periods of time, and dose distributions similar to a compensator can be achieved.
- ADVS – eliminates heavy blocks which can be dangerous to staff and pt, saves cost and time to make individual compensators, faster as automated, less electron contamination and scatter
- DISADVS – Course compensation due to 1 cm leaf width (jaggedness), QA more difficult, MLC breakdown prone and expensive to maintain.
3. Other Applications
- Compensating filters can also be made to compensate for tissue heterogeneity.
K. Tissue Inhomogeneity
1. Effects of Inhomogeneity
- Standard isodose charts assume tissue homogeneity.
- The presence of inhomogeneities (lung and bone) will change the dose distribution depending on the amount of the inhmg and the quality of the beam.
- The Main effects are:
a) Changes beyond inhomogeneity - Attenuation
- In low E beams, photoelectric effect – Z3
- In MV, Compton – e-/cm3
- In high MV, pair production
b) Changes within inhomogeneity
- Within inhmg, dose determined by:
i) Attenuation – as above, which determines electron fluence
ii) Mass collision stopping power – for a given electron fluence dose absorbed determined by the mass collision stopping power.
c) Dose at Interface
i) Source side – backscatter changes – higher with bone, less with lung
ii) Transmission side – bone depends on beam E, lung is initially lower due to loss of secondary electron fluence
iii) Lateral – loss of lateral electronic equilibrium for high E photons in lung due to long e- path length, and hence wider penumbra. For small field sizes this may result in loss of dose on the central axis.
2. Effect of Beam Quality
- For megaV beams, Compton effect is predominant mode of interaction, whereby linear beam attenuation is dependent on e- density (e-/cm3). [NB – Mass atten co-eff dependent on e-/g]
- Hence, for change in beam attenuation, effective depth calculations can be used.
- However, it is more complex close to the boundaries of low density tissue due to loss of electronic equilibrium.
- Photoelectric effect important which is dependent on Z3.
- Hence the major problem is bone. Absorbed dose within or immediately adjacent to bone may be several times higher than in its absence, and increased attenuation results in lower dose beyond bone.
3. Dose Perturbations Due to Lung
a) Beyond Lung
- The density of lung is 25% of the density of soft tissue.
- Hence, beyond lung there is expected to be an increase in dose than without lung due to lower attenuation of the beam.
- This effect is more pronounced for lower beam quality.
- Hence, the increase in dose beyond each cm of lung is 10%/cm for othoV, 3%/cm for 6 MV, and 2%/cm for 10 MV.
b) Dose Within Lung
- Lower lung density gives rise to higher dose within and beyond lung
c) Dose at Interface
- First layers beyond a large thickness of lung will have a decreased dose due to loss of secondary electrons.
d) Dose at Lateral Edge
- Loss of lateral electronic equilibrium with high E photons, due to large path length of e- in lung (low density), more electrons travel outside the geometric edge of the field, causing a wider penumbra. Hence, for high E beams, and small field sizes there will be a dose reduction in the lateral part of the field, and perhaps even on the central axis.
4. Dose Perturbations Due to Bone
- Bone has an average atomic number of Z = 13.8, while soft tissue 7.4
- Bone has a higher density relative to soft tissue, 1.85 for compact and 1.2 for cancellous
- The collision mass stopping power of bone is less than soft tissue
a) Dose Beyond Bone
- In photoelectric range, attenuation proportional to Z3
- However, linear attenuation in the Compton range is related to e-/cm3
- Shielding effect of bone diminishes rapidly as the beam energy increases.
- As bone is more dense than soft tissue, e-/cm3 is higher, and bone continues to attenuate more of the beam than soft tissue, but not to the extent as in the orthoV range.
- As E increases beyond 10 MV, the shielding effect again increases due to the increasing importance of pair production.
- Correction beyond 1 cm of hard bone is: for 1mm Cu -15%, 3mm Cu -7%, 4 MV -3%, 10 MV -2%.
b) Dose to Bone Mineral
- Absorbed dose to bone compared to soft tissue for a given beam energy is determined by the relative energy absorption co-efficients.
- For low E photons, dose 4 times higher (due to photoelectric)
- For 4 MV, dose is 0.98 (as absorbed dose related to e-/g, which is slightly less in higher atomic number materials. As Z increases, ratio of neutrons:protons increases, and e-/g decreases)
- At high MV, ratio greater than 1 due to pair production.
c) Soft-tissue inclusions in bone
- Dose is greater than in soft tissue without bone irrespective of the beam energy, as at all energies bone has higher linear atteniation (higher Z and higher e-/cm3) and therefore STI are exposed to higher electron fluence.
- Dose is higher in STI than in the bone itself at all E, as STI have a higher mass collision stopping power, and hence more energy is deposited in ST for a given level of electron fluence.
- At low E, dose to STI 5 times higher than ST.
- At low MV, dose only 3% higher.
- A very high MV there is an increase (9%) due to pair production in bone.
d) Interface with bone:
- Dose enhancement.
- In MV due to e- backscattering (8%) limited to 2 mm
- Dose perturbation energy dependent
- For < 10 MV, slight initial decrease in dose which then builds up to slight increase
- For > 10 MV, increase at interface of about 5%, which then decreases.
L. Field Blocks
- Depends on beam quality, and allowed transmission, usually 5%.
- Requires 1/2n = 0.05, therefore n = 4.5 to 5.
- That is, require 4.5 to 5 HVLs of the shielding chosen.
- In the orthovoltage range, shielding can belaced directly on the skin.
- In the megaV, the lead is too thick and requires a shadow tray.
2. Block Divergence
- Ideally, blocks should be shaped or tapered so that their sides follow the geometric divergence of the beam so as to minimise transmission penumbra.
- In some situations this is not indicated.
- Is suited for small fields.
3. Field Shaping
a) Custom Blocking
- Uses LMPA such as Cerrobend which is easy to cast into any shape.
- Take sim x-ray, draw outline of treatment field, cut styrofoam with divergent edges, use as cast.
- Sharp penumbra
- Time consuming
- Requires heavy lifting
- Possible errors in incorrect block or placement
- Highest transmission (5%)
- Increase surface dose due to secondary electrons produced by shadow tray
b) Independent jaws
- Rectangular blocking can be achieved with independent jaws. Very convenient when beam splitting.
- Dose distribution slightly different with isodose curves tilting to the blocked edge due to the horns and the loss of scatter from the blocked field.
- Fast, simple
- Sharp penumbra
- Very low transmission (1%)
- Only suitable for rectangular fields
- Consists of large number of collimating leaves that can driven independently to generate a field of any shape.
- Made of tungsten
- 6-7.5 cm thick
- Transmission of about 2%, but can be minimised by blocking with jaws.
- Fast treating multiple fields
- Do not need to lift heavy shadow trays
- Less chance of error using wrong block, or incorrectly placed
- Required for 3D-CRT and IMRT
- Allows dynamic wedging
- Allows dynamic compensation
- Edge not divergent and hence larger penumbra which may be a limitation for small fields
- Jagged field edge which is a problem for field matching
- Unable to achieve island blocks
M. Skin Dose
- MegaV allows skin sparing, however this effect may be lost from electron contamination, obliquity, field size.
a) Electron Contamination
- Surface dose due to e- contam and backscattered radiation.
- e- contam from collimator, air column, and anything else in the path of the beam such as a shadow tray.
- Also as field size increases Dmax becomes more shallow, likely due to e- contamination.
- Extrapolation chambers, PPC, TLD
c) Absorbers (Electron Filters)
- May be placed in the beam to absorb the e- contam from the collimator.
- However, the absorber then becomes a source.
- This can be reduced by moving the absorbed as far from the patient as possible, which helps due to divergence.
- The absorber should be of medium Z material (30-80), such as tin, as these absorb electrons but give less e- scatter in the forward direction than high or low Z material.
- Skin dose increases as a beam become more oblique.
N. Field Separation
- Adjacent fields required for orthogonal fields, and for large fields that cannot be treated with one beam.
- There is the possibility of large dosage errors at the junction (hot or cold).
- Generally try not to have a junction through known tumour
2. Junction Techniques
i) Angling away – along divergence lines
ii) Field separation – gap is calculated by geometric divergence or isodose curve matching so that they match at depth (may require match on surface)
iii) Beam splitting
iv) Penumbra generators
i) Do not place junction in area that has tumour or critical organ
ii) If tumour superficial, then do not separate fields
iii) Fields may be separated for deep seated tumours
iv) Line of field matching should be drawn at each treatment. It does not matter if this moves between treatments, and in fact this is beneficial.
v) Should be verified by isodose distributions, as well as beam alignment with light field.
4. Orthogonal Field Junctions
- For craniospinal irradiation require a lateral brain treatment and a posterior spine. Hence, this requires an orthogonal field junction.
- Three main techniques:
a) Diverging cranial field
- Mark point of inf field on posterior neck
- Sup border of spinal field displaced inf of skin mark t account for divergence
b) Patient prone and immobilised
- Diverging sinal field marks sup border on lateral neck
- Cranial field collimator rotated so that diverging inf border is parallel to sup spinal border
- Couch rotated to match diverging fields
c) Beam splitting
- Treat prone immobilised
- Spinal field diverging and marked on neck
- Cranial field collimator rotated to match diverging spinal line, but beam split so that not diverging, and hence couch does not need rotation.