Superficial RT Flashcards
What are the conventional energy ranges and their depths
10-20kV Grenz rays (seldom used)
40 - 50kV contact therapy (intraoperative) Tx depth - 1-2mm
50-150kV superficial Tx depth = 5 mm
150-500kV orthovoltage therapy, Tx depth = 20 mm
Give details of two modern units
Xstrahl : 50 - 150 kVp, 6 - 18mA
Gulmay: 20 - 220 kVp, 0 - 20 mA
How do orthovoltage units differ from SXT units?
Potential difference is greater, so to overcome insulation challenges, anode is held at high positive potential and is oil cooled
What are the properties of the cathode, how are electrons produced?
Tungsten filament: high melting point and high atomic number means low binding energy of outer shell electrons
Thermionic emission.
Filament electrically heated
What does filament current determine?
Tube current is proportional to number of electrons accelerated per unit time
X-ray intensity (no photons per photon energy) I proportional to mA
What does peak kilovoltage impact?
Max energy attainable by electrons is kVp
Higher kVp produces more x-rays I proportional to kVp ^2
What are the anode’s properties and how is energy lost?
Non rotating reflection target (don’t need small focal spot as in diagnostic systems so non-rotating target feasible)
De accelerates beam of electrons
Energy loss via collision (99%)
Radiation 1% (characteristic radiation ~ z^3, brem ~ Z^2)
What is the heel effect
Feature of reflection type targets which results in inhomogeneous distribution along cathode anode axis
Result of differential self attenuation within anode
What is the extent of the heel effect dependent on?
Angle of anode
Initial x-ray spatial distribution
Why is filtration used and what inherent filtration exists?
Preferentially absorb softer xrays which are not therapeutically used and only contribute unwanted skin dose
Lowest energies are removed within the anode itself, and 2.2mm Be window
What properties of material are considered in filtration?
Mechanical stability
Minimal reduction in overall intensity
Minimal production of characteristic (k edge) energies
What materials are used for filtration?
Typically medium Z filters
Al (z=13) 0.5 - 3 mm thick
Cu (z=29) 1-4 mm thick
Al for superficial energies
Cu or compound (compounds fitted with highest Z near source to remove characteristic x-rays of higher Z filters) for orthovoltage
What is shape of x-ray spectrums and general characteristics?
Brem: continuous spectrum
Characteristic: discrete energies which are a function of target and filter materials
Max energy = kVp
Mean energy ~ kVp/3
Range = kVp - min KeV
What are the typical SSDs for superficial and orthovoltage?
SXT: 15 & 25 cm
Ortho: 30 & 50 cm
What are the safety systems used in the units?
Ortho: unsealed ionisation monitor chamber to determine when dose has been delivered
SXT: primary and backup timers determine when dose delivered
Mechanical interlocks in filters and applicators to ensure they are correct and on doors so they can’t open during treatment
What is contained in BJR supplement 25?
Reference data sets for different beam qualities, PDD, Bw, TARs
Applicable to most kV systems
What are the primary features of the PDD and how can the PDD be measured?
Little to no build up: dmax at or near surface
Steep fall off with depth
Ionisation chambers or GafC (radiographic unsuitable due to high Z components)
How is beam quality defined in kV beams?
HVL
Thickness of material required to reduce intensity to 50%
Homogeneity factor = 1st/2nd HVL
What is beam quality used to determine and what is good geometry for measurements?
Water to air mass energy absorption coefficient ratios
Backscatter factors
Good geometry is narrow beam and scatter free conditions
How is HVL determined?
Limit beam with aperture roughly halfway between source nad detector
Take a reading with no absorber
Add absorbers until reading has halved
What sections are in the kV CoP, and what is discussed?
Very low energies 8-50kVp
Low energies 50-160kVp
Medium energies 160-300kVp
Each section discusses method of cross calibration and determination of absorbed dose
What is k_ch and its values?
Chamber correction factor
Assumed to be 1 in original CoP in very low energies due to lack of data, this has been updated
No required in low energies because measurements are taken in air
Medium: table of values provided
What was added in 2005 addendum to CoP
k ch for very low energies
Updated mass energy attenuation coefficient ratios
Updated backscatter factors
Alternative in air cross calibration method for medium energies
What is the primary standard?
Free air ionisation chamber
Quantity is air kerma measured in air
NPL has two, small which goes up to 50kVp and large goes up to 300kVp
What field instruments are recommended in kV CoP?
Very low kV: any parallel plate ionisation chambers with volume 0.02 to 0.8 cmc
Low kV: ny thimble ionisation chamber with volume < 1cmc
Medium: NE2571
What are the cross calibration methods?
V low kV: at surface of full scatter phantom, surface at standard SSD, applicator size allowing 2cm margin
Low kV: in air, at least 5cm from end of 7cm diameter applicator
Medium: 2cm deep in water or equivalent phantom, surface at standard SSD, 10 x 10 applicator OR in air at least 5cm from end of 7cm diameter
How is cross calibration done?
Chambers held in jig attatched to applicator, side by side, perpendicular to A-C axis
Chamber positions swapped between sets of readings
Average ratio of std to fld calculated
What are terms in conversion to absorbed dose eq for very low and medium energies?
Field instrument reading
Field instrument air kerma calibration coefficient
Field chamber correction factor
Mass energy absorption coefficient ratio
What are terms in equation for determining absorbed dose in low energy?
Field instrument reading
Field instrument air kerma calibration coefficient
Mass energy absorption coefficient ratio
Backscatter factor
ISL
What is checked during QA?
Daily: interlocks and warnings, filter interlocks, output measurement
Monthly: dose control linearity, HVL constancy, end effect consistency
Yearly: focal spot alignment, beam quality, radiation field size
What are the benefits of superficial units over electrons?
Treatment unit is cheaper to buy and maintain
Smaller room footprint
Cheaper room design for radiation protection
How much can shielding reduce dose by?
2 mm of shielding can reduce dose to < 5%
What are the features of the dose distribution?
Little or no build up
Sharp discontinuity at beam edge
Rounded isodose lines due to ISL effect over applicator
Significant low dose penumbra beyond geometric field edge
Heel effect in anode-cathode direction
What are clinical uses of superficial?
Superficial lesions
Tumours where rapid dose fall off is required
Small fields
How can underlying bone impact dose?
Dose to underlying bone increases substantially due to greater PE effect for high Z materials
What are some clinical indications?
BCC
SCC
Keloid scars
Dermatological conditions
Mycosis fungoides
What is BCC
Basal cell carcinoma
Malignant tumour
Most common skin cancer
Grow slowly and metastasies rarely but can be locally destructive if allowed to grow
What is SCC
Squamous cell carcinoma
Malignant tumour
Can metastasise
Surgery often used and RT used when difficult to define margins
Less common than BCC but more aggressive
What are keloids?
Scar which continues to enlarge as a result of excessive formation of collagen during repair
Can cause itching, sensitivity, stiffness
Patient has surgery and then RT
What is mycosis fungoides?
A class of cutaneous T cell lymphoma affecting the skin
Lesions can be treated with low energy x rays to palliate symptoms
What is intra operative radiotherapy?
Fraction of RT delivered during surgery
For selected low risk breast cancer patients, but not recommended by NICE
What clinical beam modifications need to be considered when determining treatment time/MU?
Applicator dimensions
Cut outs
ISL
internal/external shielding
When might ISL correction be necessary?
Inner canthus
Tumour protruding into applicator
How is ISL correction done?
Difference in distance is measured with a dipstick and ISL calculation made to account for difference
SOC = (FSD/(FSD+d))^2
Why are cutouts used and how do they impact the dose rate?
The purpose of the cutout is to conform the beam to the irradiated area and avoid geometric misses with patient motion, because immobilisation tools are not used like on a linac
Cut out will reduce the dose rate, accounted for with reduced backscatter factor
Why is backscatter factor necessary?
Without the patient in place, the dose at the end of the applicator is from primary radiation only, this is how outputs are measured
With patient in place, dose increases due to backscatter, time to deliver dose decreases
What is backscatter equation and swhat is BSF a function of?
BSF = (absorbed dose to surface of water phantom) / (absorbed dose to same point in a water equivalent detector in air)
= (primary dose + scattered dose)/(primary dose)
BSF is a function of the HVL, FSD, field size
How is BSF measured?
Use a LiB TLD, placed flush with surface of water equivalent phantom and obtain TLD reading
Rotate applicator so open end pointing up, place LiB on thin piece of mylar and obtain TLD reading for same time
BSF = measurement 1/measurement 2
How does BSF change with irradiated area at superficial energies?
Initially increases due to larger fraction of photons coming from edge of field and contributing to CAX dose
Plateaus at larger beam diameters when photons beyond a certain distance won’t reach CAX
How does BSF change into orthovoltage energies?
Increases through superficial beam qualities because Compton scatter to PE ratio increases
Decreases through orthovoltage beam qualities because scattered photons are more forward peaked
How does cut out impact BSF?
Presence of cut out reduces dose rate from that of open applicator by BSF ratio (BSFR)
How does internal shielding impact dose?
2mm PB reduces dose beyond shielding to < 5%
Upstream reduced due to lack of backscatter
Tissue equivalent covering is needed to prevent enhanced dose from low energy backscattered photons/photoelectrons
How is internal shielding measured?
Place chamber in WT1 above mylar and then gold, vary the amount of mylar and take measurements
How would you consider different dose rates from shielded/non shielded tissue
Different dose rates to shielded eyelid (eg) and unshielded area
Can only set a single treatment time, so reduced dose above shield is accepted so that areas not shielded are not overdosed
There will be scatter from non shielded tissue to shielded tissue which reduces effectiveness
Why is time end error necessary?
There is a ramp up dose while superficial machines turn on, during which dose rate is not stable, dose delivered is not rate x time.
How is time end error accounted for?
Calculated by taking 4 measurements of t and one measurement of 4t and calculating end time, add this on
What factors are used to adapt treatment time?
BSFR, SOC, timer end error
When is time end error not necessary?
In orthovoltage units: not using time to determine dose, using monitor chamber
How do we calculate BSFR?
BSFR = BSF for cut out / BSF for open field
Time/MU will be increased by same ratio
For irregular field:
BSFR = average BSF of all sectors / BSF for open applicator
Why might applicator have end plate?
To generate scatter and remove small build up at largest kV in orthovoltage systems
Why can we assume water kerma in water is absorbed dose in water?
Fraction of energy of secondary charged particles lost to brem is negligable.
All energy loss is by secondary charged particle collisions at point of CP production: if energy released per unit mass absorbed in same unit mass, KERMA = dose
What is clinically attractive about kV distributions?
Sharp discontinuity at beam edge (discounting low dose penumbra) allows for small margins and small / tightly conforming treatment areas
Easily shielded
Why is LiB detector used in BSF calculations?
Lesser energy dependence
What is the quantity used at NPL for the free air ionisation chamber?
Air kerma measured in air
What do we need to consider when treating two fields close to each other?
The depth at which the two fields coincide and the dose at this depth. Consider the sum of the doses and whether this is unreasonably high, speak to a clinician.
Consider the separation between the beams, the SSD, and the diameter of the beam.
