2nd Year: Brachytherapy Flashcards
How does the AAPM recommend specifying source strength?
Air kerma strength
Why is Air Kerma Strength recommended over mg Ra Eq, activity, or exposure rate constant?
Air Kerma Strength can be esrtablished directly from primary standard measurements
The other terms cannot be
Why do we use Air Kerma Strength instead of dose rate in tissue for source strength specification?
Because air kerma strength is directly and absolutely measurable in a wide-angle free-air chamber
and
Because air kerma strength is already proportional to dose rate in tissue. So the conversion is easy enough
What kind of detector does NIST use to calibrate LDR lower energy sources (such as I-125 and Pd-103)?
Wide-angle free-air ionization chamber with 2pi area geometry
This chamber is large enough volume so electronic equilibrium can occur
What is the difference between a 2pi geometry and 4pi geometry chamber?
2pi geometry chambers are able to measure in a complete radian, 2D circle
4pi geometry detectors can measure in complete 3D sphere
2pi geometry detectors are inherently limited to a maximum possible efficiency of 50% (since they see only one side of a source)
What setup does NSIT use to calibrate LDR low energy soruces (I-125 and Pd-103)?
WAFAC to detect exposure rate
Filter wheel with various aluminum filters to absorb contaminant characteristic photons from titanium encapsulation of seeds
Seed rotated along longitudinal axis to average out axial non-uniformity
Source placed on nylon base to limit photon scatter by base

What correction factors does NIST include in their calibration of LDR low energy sources?
Temperature and pressure
Decay correction
Ion recombination
Photon attenuation in air
Filter and front electrode attenuations
Inverse-square
What is the traceability workflow for calibrating your well chamber for low-energy LDR sources?
NIST WAFAC –> ADCL transfer well chamber –> your well chamber
At each step, a calibration coefficient is created
As an alternative to calibrating well chamber for LDR low energy sources using traceability, what else can the ADCL do for your chamber?
Providing you with a standard, NIST traceable calibration source, so you can calibrate your well chamber manually
Which source is used for LDR low energy source calibrations at NIST?
Trick question. It is not one singular source. ADCL’s perform well chamber calibrations for specified seed types, not any one general isotope
In the past, it used to be only Cs-137. But the manufacturer builds of seeds have evolved throughout the years and they directly impact measurements, so each calibration is dependent upon the seed itself
True or False
As of yet, NIST has no standard for calibrating HDR sources, so instead ADCL’s perform a extrapolation based method published by Goetsch et al
False
It’s 90% correct. It should just read this instead…
As of yet, NIST has no standard for calibrating HDR sources, so instead ADCL’s perform a interpolation based method published by Goetsch et al
What transfer chambers does ADCL use for calibration of HDR sources?
Either a exradin A3 spherical ion chamber or a thimble chamber
Describe the general workflow that ADCL use to calibrate your well chamber for HDR sources
- Retrieve calibration factor for a transfer chamber (either spherical ion chamber or thimble chamber)
- The calibration factor is valid for Cs-137 (662 keV) and a medium-filtration x-ray beam (250 kVp)
- Interpolate to get a calibration factor for Ir-192 (energy between Cs-137 and medium-filtration beam)
- Calibrate activity using the “seven distance measurement technique”
- Once activity of the Ir-192 source is confirmed, use the source to derive a calibration factor for the well chamber at the sweet spot of the chamber
What is the purpose of the “seven distance measurement technique” used by ADCL to calibrate HDR source strength? How does it work?
Measure source from seven distances in air using the chamber and an equilvane tplastic buildup cap applying a distance correction at each position
Technique removes effects of room scatter from measurement
What are the units of air kerma strength?
uGy m2 / hr
What is the relationship between air kerma strength and air kerma rate?
Sk = air kerma rate * d2
What are we trying to find during a source calibration measurement?
Activity of the source
When performing a source calibration, where should you place your well chamber and why?
Center of room (or atleast 1 meter away from any wall or scattering material), on a plastic cart
To limit backscattering (affect can be as large as 1.1%)
Per TG 56, what two chambers are allowed to be used for seed assays?
Well-type chamber or farmer chamber in a jig 10 cm away from source
At what positions in the well chamber is the ADCL supplied calibration factor valid?
Sweet spot for HDR sources
or
Source holder holding position for LDR sources
True or False
For low energy LDR source calibration, the calibration factor must be obtained for specific models of the seed used
True
True or False
For high energy sources, such as Ir-192, calibration factor must be obtained for specific model of the seed used
False
You can use the calibration for whatever seed model, as long as it is the same isotope
True or False
For LDR well-type chamber calibrations, the source holder must be sent with the chamber?
True
What is the main difference between a pressurized and non-pressurized well-type chamber?
Pressurized chambers have greater signal, but must be checked for constancy to ensure that there has not been a leak
Non-pressurized chambers are open to air, so they must have a pressure correction component








