Brachy Flashcards
Advantages of brachy
High dose to target with rapid ISL fall off - low integral dose and can deliver radical dose in less fractions
Higher dose to CTV for same OAR dose compared to IMRT etc.
What is margin around CTV?
None - PTV = CTV + nothing
When is brachy particularly good?
For aggressive cancers with rapid proliferation and tumours where alpha/beta might be similar to normal tissue, high conformality.
Clinical indications
Post surgery to remove remaining clonogenic cells
As boost alongside EBRT
As monotherapy if surgery is not on option, EBRT not suitable, brachy sufficient alone and lower side effects
Site must be accessible
Disadvantages
Implants/insertions require a theatre session, time consuming and lots of resources, GA
Strong ISL fall off, misplacements affect treatment a lot
Large volumes (>5cm) lose ISL advantage
Need specific radiation protection and security requirements
What legal aspects need considering
Management, security, transport, disposal
Sealed source environmental permit, what is it, who regulates
Permit to hold
To be allowed to hold sealed sources, required under RSR and regulated by EA under EPR.
What want to hold, how long, use
Send off form to EA, reapply if adding source or changing premises
ARSAC
Have employer licenses (what procedures and radionuclides are allowed on site) and practitioner licenses (what can CCOs prescribe, procedures and radionuclides) under IRMER
License to administer
Renew every 5 years
Administration of radioactive substances advisary committee
SRSR
Written declaration for import of sealed sources
Permit to receive
Consignee declares they have complied with UK requirements. Applies to anything EU –> UK
Shipments of Radioactive Substances (EU exit) Regulations 2019
HASS
High activity sealed source
Security requirements
Sealed source classed as HASS if exceeds a an energy threshold. Differs for different isotopes
Need control measures, site security plan, level depends on isotope and activity
What do you need to consider for sources (8)
Photon energy spectrum
Charged particle emissions
Source size
Specific activity
Half life
Source strength
Decay scheme
Physical form
Where is interstitial implanted?
Implanted directly into tissue
Where is intracavity implanted?
Inserted into applicator in natural cavity
Where is intraluminal implanted?
Bronchal, oesophagus
Where is intravascular implanted?
Arteries to prevent restonosis
When is surface mould used?
Adjacent to superficial lesions
Different durations
Temporary - HDR or PDR, uses for short length of time
Permanent - left in site, dose delivered exponentially to complete decay
Source loading techniques
Pre-loading - sources inserted in theatre (prostate seeds) or applicators contain sources during insertion (eye)
Afterloading - applicator applied first and sources loaded later
Dose rate classifications in brachy
LDR 0.4-2Gy/hr 10 Gy/day
MDR 2 - 12Gy/hr 10 Gy/hour
HDR >12Gy/hr 10 Gy/minute
Afterloading benefits
Improves radiation protection of staff, more time spent positioning needles, applicators etc, imaging possible with dummy sources
How is dosimetry optimised
Varying source position and dwell time
Why is Ir-192 used?
High specific activity
Ease of manufacture
Photon energy spectrum
Easy shielding of charged particle emissions
Reasonable half life
No gaseous emissions
Features of Ir-192
~370GBq when installed
Dose rate ~ 8Gy/min
Decays mostly via beta (95%) and EC (5%)
Average energy 370keV
Ir vs Co
Co costs less over time - more expensive but half life means less replacement.
Less time out of service and less QA required
Ir has higher maximum dose rate x2, quicker treatments
Co requires thicker shielding
Safety features of an afterloader (6)
Shielded source safe
Dummy source on separate cable (ensures unimpeded progress prior to treatment)
Back up battery for source retraction with no power
Optical detection for source leaving and entering
Manual drive cables for retraction
Locking mechanism for transfer tubes
Safety features of room (5)
Multiple GM detectors in treatment unit, room, wall mounted probe, hand held for emergences
Gamma ray illuminated sign
Shielded door with interlock and IR curtain
Emergency stops
Audio/visual systems: CCTV, intercom, visual and audio signals that source is out of treatment unit
Measuring RAKR
Based on NPL primary standard
Secondary standard calibrated with primary by irradiation with same source used to determine primary
NPL give hospital calibration coeff Nkr for their secondary standard well chamber
Eq. used by NPL for air kerma
Ka = Q/m . W/e . F/(1-g)
Routine afterloader QC (7)
Source strength
Source positioning
Dwell time accuracy
All safety systems
Faculty testing
Wipe tests
Dosimetry
What formulism is used for dose calculation
TG43 - used to calculate dose rate in water from brachy source, updated in 2004
Terms in dose rate eq
Air kerma strength
Dose rate constant
Geometry function
Radial dose function
Anisotropy function
Air kerma strength
Sk = .K(r) r^2 (r^2 removes distance dependence, Sk becomes function of source not location)
.K(r) is air kerma rate in free space at r
Why use KERMA in calculation & what is difference between KERMA and dose in brachy?
Can easily be measured
Difference between KERMA and dose is energy lost at point of interaction, only 0.3% as brem in brachy
What is used for sealed source strength measurement
Secondary standard well chamber. Chamber should be traceable to NPL
Dose rate constant
Dose rate constant = Dose to water at 1cm / unit air kerma strength
TG43 formalism normalised to this reference point for all data
Geometry function
Accounts for variation of dose rate around the source due to spatial distribution of activity within source (ignores effects of photon absorption/scatter within surrounding water or self filtration)
Geometry function values
ISL far from source. Point source
beta/L r sin(theta) if theta not zero
(r^2 - L^2/4) if theta = 0. Line source
Radial dose function
Curve fit of measured dose fall off along a line perpendicular to the centre of the long source axis due to attenuation and scatter. Relative to dose at 1cm.
Given in TG43 papers
Anisotropy function
Does everything else - it is change in dose with distance and angle not accounted for by other terms. Includes self filtration and scatter around source. Given as lookup table
Anisotropy factor, constant
Factor is 1D approximation averaged over all angles for a given distance, can be used for checking calculation but not for planning.
Constant is function averaged over all angles and distances of clinical interest.
Finding source data for calculations
As TG43 is function of exact source geometry it varies between sources need to find data for type of source used
NIST 99 changes
Introduced new primary standard for measurement of lower energy sources in 1999. Wide angle free air chamber (replaced Ritz low energy fre-air chamber, which was significantly affected by low E photons), has thin Al filter between source and aperture to remove low energy photons, reduces by 10%, removes photons which don’t contribute to dose in water past 1mm.
Paris Dosimetry System for Interstitial Implants
Set of rules for good geometric coverage and homogeneity without large hot volumes or cold spots.
Designate dose calculation points, prescription isodose that gives desired clinical result
What is basal dose and reference dose?
Mean of doses in local dose minima within implant. Reference dose to prescribe to is 85% of mean basal dose.
Basic principles of paris system (10)
Parallel
Straight
Equidistant
Perpendicular to plane through which mid-points lie
Uniform activity within and between wires
5-20mm separation, 1.5-16cm length
Volumes should be equilateral triangles or squares
No obtuse angles
No rectangles
Variation of BDR about mean <10%
Units of dose rate constant
cGy hr-1 U-1
1U of air KERMA strength
1U = 1 microGy hr-1 m^2
= 1cGy hr^-1 cm^2
What are descriptors of brachy?
Radionuclide positioning
Duration of radiation
Source loading technique
Dose rate
What were changes in TG43 update?
Revised definition of AKS - has low energy cut off
Elimination of apparent activity for specification of source strength as it is a derived quantity
Anisotropy constant dropped for factor (but function preferred)
Why consider photon energy spectrum of source?
Want local dose coverage with minimal whole body dose and shielding
0.2 - 0.4 MeV ideal
Why consider charged particle emissions of source?
Want to avoid them usually as they give very high local dose in comparison with distant photon dose (there are exceptions, such as purely beta emitters if maximum range is advantageous, such as occular)
Why consider source size?
Less trauma to tissue if source is smaller
Why consider specific activity of source?
Higher specific activity suggests smaller sources
Why consider half-life of source?
Long half life can be used many times but also means low specific activity, shorter half life preferable if permanent implant
Why consider source strength?
Affects treatment time and radiobiology
Why consider decay scheme of source?
The daughters need to be considered for how appropriate they are, we don’t want gaseous emissions for instance
Why consider physical form of source?
Can they be used as we want to use them, are they seeds, wires etc? Encapsulation shields against charged particles, prevents leakage and provides strength and rigidity
What is needed to calculate dose at point from permanently implanted seed?
Air kerma strength
Dose rate constant for exact source type (isotope, manufacturers type, from peer reviewed publication)
Geometry function (line source fn), need active length
Radial dose function (from peer reviewed publication)
Anisotropy function (note not needed if along line perpendicular, will be 1)
Half life
What would preclude implant from using Paris system?
Triangular implant with angle between sources greater than 90 degrees
One or more BD point greater than 10% different from mean basal dose
What is the Manchester system?
For gynae treatments
Gives loading ratio rules for uterine and vaginal sources to achieve same dose rate to point A regardless of size/shape of uterus and vagina
Where are points A and B in Manchester system?
A is 2cm up from base of uterine tube and 2cm lateral to uterine canal
B is 2cm up from base of uterine tube and 5cm laterally
Which ICRU report is used for cervical brachy?
ICRU89
How is cervix brachy CTV prescribing done?
CCOs draw high and intermediate risk CTVs based on ICRU89, GEC-ESTRO guidance
Goals for CTVs and constraints for OARs considered during adaptive planning
What is PDR?
Replaces LDR brachy with series of pulses o HDR, treatment delivered for a fraction each hour and then source returned to shielded safe
Advantages of Radium
Long half life, used repeatedly
A lot of experience with it over time
Disadvantages of Radium
Gaseous decay product which is radioactive Radon 222
Low specific activity - bulky sources
High energy beta particles, a lot of screening needed
High energy photons mean thick shielding for staff
Advantages of Iridium
0.37MeV, radiation protection easier
Beta energies low, so 0.1mm Pt filtration adequate
Decays to stable isotope with no gaseous products
High specific activity, small sources
Disadvantages of Iridium
Half life of 73.83 days, source needs to be replaced every 3 months
Advantages of I125
Half life suitable for radiobiological outcome and seed density
Low energy means surface dose rates low enough to not restrict patient movement after insertion
Disadvantages of I125
High anisotropic dose distribution around individual seeds makes non-trivial dosimetry
What equation is used for RAKR at hospital?
.Kr = M.kion.ksg.Nkr
where M is measurement
ksg is source geometry factor relative to that used at NPL
Nkr is NPL calibration constant
How is RAKR measurement taken at hospital?
Secondary standard in middle or room on low scatter surface
Initial measurement compares RAKR to manufacturers certificate, should be less than 3% but must be less than 5 for use
Independent check must be within 1% of initial assessment
Can manufactureres RAKR be used?
If their determination is tracable to a primary standard, if not then own measurement must be used
Measurement of source strength of 125I seeds
Currnt only in order of pA, low leakage essential
To improve SNR and maintain integrity of stranded seeds, often measured as strand of multiple seeds to give air kerma strength as average per seed
