Brachytherapy Flashcards
Give some examples of interlocks that need to be checked in an HDR suite
Guide tube length ALMO radiation monitor Interrupt buttons Emergency stop Door interlock Battery backup
What does RAKR stand for
Reference Air Kerma Rate
Define RAKR
Reference Air Kerma Rate
The kerma rate to air, in air, at a reference distance of 1m, corrected for air attenuation and scattering
Where is RAKR defined in the literature?
IPEM COP for HDR BT
Why is using a well chamber better than using a thimble chamber for calibration?
Less sensitive to source positioning errors due to 4pi geometry.
Much smaller uncertainty
Hard to get reproducible positions when using a thimble
Much quicker to take measurements with a well chamber
Transfer uncertainty between well chambers smaller than between Farmer chambers
What is the primary standard for RAKR at the NPL?
Graphite-walled cavity ionisation chamber
What are the two steps for the calibration of secondary standard chambers?
RAKR of Ir-192 source determined using NPL primary standard
Calibrated source used to calibrated SS instrument
What is the source calibration set-up used at NPL?
Source inside a lead collimator with aperture such that the direction from the source centre to the reference point is at right angles to the long axis of the source.
Collimated beam directed towards primary standard
Centre-to-centre source-to-chamber distance 1433mm
Why are the source-to-chamber distance and aperture sizes chosen as they are at the NPL calibration?
To give a uniform field over the whole of the ionisation chamber
What correction factors are applied to determine the RAKR
Temperature and pressure Normalised to 1m Air attenuation Scatter correction factor Primary standard sensitivity
Reference time and date
What is the secondary standard calibration set-up that should be used?
Well chamber 1m from any wall and 1m above floor level on a low scatter surface.
Time for chamber to reach thermal equilibrium.
Bias voltage s.t. central collecting electrode is positive wrt outer electrode.
What is the position of the source that is used in the chamber for calibration?
The “sweet spot”, or position of maximum response .
What is the equation for RAKR in terms of Gy/s at 1m?
Kdot_R = M x k_ion x k_sg x N_Kdot_R
What correction factors are applied to the instrument response?
Electrometer correction factor
Decay correction factor to correct the measured current to a reference time
Temperature and pressure
At what threshold should discrepancies between the manufacturer’s source certificate and the measurement be investigated?
3%
At what threshold discrepancy between calculation and certificate should a source not be used clinically?
5%
How can the ion recombination be measured for a well chamber?
kion = (4/3 - (I300V/3xI150V))^-1
Two polarising voltages used
It is the reciprocal of the ion collection efficiency
How is the HDR source position checked?
Source step viewer - can see using the CCTV camera and where the source wire lines up.
Use Gafchromic film, tape the applicator to it at known positions and then expose each region a known distance apart.
What is transit time?
The time for the source to reach the dwell position; the time it is moving along the guide tube
What is the tolerance on the dwell position time?
0.5s
What is the tolerance on the difference between measured time and actual time - the transit time?
<2s.
How is linearity measured for HDR BT?
Charge vs actual dwell time is plotted and should be linear
What is the dose rate range for HDR?
12Gy/h or more
What is the dose rate range for LDR?
0.4 to 2 Gy/h
What is the definition for PDR?
Dose “pulses” given for about 30mins separated by gaps in the order of hours.
What is the level 1 standard for reporting BT plans?
The minimum of data required to perform BT in an efficient and safe way.
Total Reference Air Kerma
Point A dose
Recto-vaginal reference point dose
D0.1cc, D2cc for bladder, rectum
What is level 2 reporting for BT?
Must contain all the information of L1 plus information to perform a state of the art tx Dose reporting for defined volumes: D98, D90, D50 for CTV_HR D98 for GTV_res D98, D90 for CTV_IR if used for px
Dose reporting OARs
What is the definition for the TRAK?
Total Reference Air Kerma
The sum of the products of the RAKR and the irradiation time for each source.
What are the units for U?
uGym^2/h
or cGycm^2/h
What is the equation for total air kerma strength?
S_k = A x N x exp((-ln(2)/59.4) x t)
What is the equation to convert total air kerma strength to dose?
D = S_k x T(x) x (100/d)^2 x f x h x 1/1000
T(x) = tissue transmission factor d = distance from point of interest to centre of implant f = conversion factor from air kerma to dose in tissue h = time in hours of the surgery
What are the two types of calculations for sources given in TG43?
Line and point sources
What are some limitations of TG43?
Assumes full scatter conditions Assumes homogeneous water Source to source shielding not accounted for Applicator shielding not accounted for Doses close to sources to accurate
What does the dose rate constant, Lambda, define, and what are it effects in the TG43 2D formalism?
The dose rate to water at 1cm per unit air kerma strength.
Includes effects of: source geometry, activity distribution, encapsulation, self-filtration, scattering in water, changes in S_k.
What does the geometry factor (G(r,\theta)) account for in the TG43 2D formalism?
Variation of relative dose due only to the spatial distribution of activity within the source.
Different value for point source or line source approximation.
Purpose for interpolation between tabulated data points so the simplistic forms still yield accurate results.
What does the radial dose function, g(r), account for in the TG43 2D formalism?
The effects of absorption and scatter in the medium along the transverse axis of the source.
What does the anisotropy function, F(r,theta), account for in the TG43 2D formalism?
Anisotropy of the dose distribution around the source, including the effects of absorption and scatter in the medium.
Includes effects of: self-filtration, encapsulation, scattering in medium.
It is unity for a point source.
What are some advantages of the TG43 model?
Specific data for each source model
RAKR/Sk traceable to primary standards
Rigorous process to get the parameters
Fast computation
What regulations regarding security do HDR users need to be aware of?
EPR 2016
HASS regulations
Transport of Dangerous Goods
What brachytherapy TPS QA needs to be done?
Compare TPS calculated dose to manually calculated at a number of points
Check source decay
Recalculation of standard source arrangement to check consistency of calculated source dwell times.
How would you go about verifying dose calculation of a TPS?
Scan phantom of well-defined geometry
Position source at px points inside phantom
Compare TPS calculated dose rates/dwell times with independently calculated values
