TG56 Flashcards
What is TG56?
Brachytherapy code of practise
what are the manual methods of implant design?
Manchester (Peterson-Parker)
Quimby
Memorial
Paris
describe Manchester system
-for interstitial
-peripheral sources define target region
-want dose uniformity within 10%
-source distribution rules tell you how much to put in periphery vs core: 75 % perpheral and 25 % in core is typical
Planar, volume, cylinder, sphere and cube source distributions are used.
Line source spacing ~1 cm.
The stated or prescribed dose is usually 10% higher than the minimum dose within the implanted volume/plane
descibe Quimby system
uses equally spaced, uniform-strength sources distributed over the plane or volume
-results in nonuniform dose distribution, which is higher in the central region of treatment
-line source spacing ~ 1 cm
-volume implants: prescribed dose is min dose within volume
-planar implants: stated dose is max dose within plane of treatment
Typically, for equal dose delivery to similar size planar or volume implants, the total source strength will be higher with the Quimby system than would be required by the Patterson-Parker system
describe Memorial system
- extension of Quimby system
- nomographs to determine total seed strength required to deliver a given dose to a tumour of given dimensions, while also providing guidance on seed spacing (e.g., total seed strength to deliver a peripheral dose of 160 Gy for average dimensions of 3 cm or greater is a power function of target average dimension)
- A nomogrpah is a 2D diagram that allows for approximate math calculation of a graphical function
describe Paris system
- primarily intended for removable implants of line sources with uniform and identical linear source strength
- used for single and double plane implants
- line sources are in parrallel planes with centers of all sources in same plane
- dose specification is based on reference isodose, which is 85 % of basal (average) dose rate
- min dose rates should be +/-10% of basal
where are crossed needles allowed?
Quimby and Manchester, not in Paris
what are crossed needles used for?
enhance dose at implant ends
In cases where crossed needles are not allowed, active length is chosen to be 30-40% longer than target length
describe the “computer system” that evolved through use of computers
sources of uniform strength are implanted, spaced uniformly (1.0-1.5 cm, with larger spacing for larger implants), and cover the entire target volume.
The dose inhomogeneity (hotter in the middle of the implant) arising from the use of uniform (activity and arrangement) sources is accepted with the belief that the central part of the target requires higher sterilization doses than the periphery.
The dose is specified by the isodose surface that just surrounds the target or implant.
It is better to implant a larger volume than to prescribe to a lower value isodose curve to increase coverage (this will result in hotter hot spots). In other words, ideally prescribe to as high an isodose as possible to avoid hotspots.
As in the Paris system, crossed needles are not allowed.
dosimetric pro and con of brachy
more localized
more heterogeneity
compared to EBRT
role of MP in brachy
- design and implement brachy facility
- develop and implement treatment delivery procedures that are safe, effective, meet regulatory requirements
- ensure accuracy and safety of each individual brachy treatment
who governs brachy licenses in US?
Nuclear Regulatory Commission (NRC)
what guidelines does blue book give?
minimum staffing requirements for radiation facilities
HDR dose rate
above 0.5 Gy/min
0.5 Gy/min is common for EBRT
goals of brachy QA
maximize likelihood that each treatment is aministered correctly and that it reflects the clinical intent, and is safe for patients and others
QA program endpoints
safety of the patient, public, and institution positional accuracy (+/- 2 mm relativ eto applicator system) temporal accuracy (2%) dose delivery accuracy (source calibration within 3 %, dose delivery accuracy 5-10%, dose calculation numerical accuracy 2 %)
QA program development focuses
- correct function and physical characteristics of treatment planning and delivery devices
- correct execution of each brachy procedure
what is mgRaEq
- source produces an exposure rate in free space at a large distance on its transverse axis, equal to that for the same mass of radium encased in a capsule of 0.5 mm Pt wall thickness
- large distance: IS obeyed
- only menagiful for hifh energy photons as equivelent strength sources of these radionuclides will yield nearly equal dose rates in tissue along their transverse axis. For low-energy photons, attenuating effect of the tissue is much greater such that an in air mgRa equivalency does not translate to equivalency of dose in tissue
apparent activity vs encapsulated activity
apparent < encapsulated
- because encapsulation reduces dose rate and air kerma compared to if source were free
- apparent actvity is activity of hypohetical point source of the same radionuclide which would produce the same air kerma rate, at the same large distance, as that measured on the transverse axis of a sealed source
unit of air kerma strength
Sk
U
uGym^2/h or cGycm^2/h
Sk= Kl^2, where l s refernce distance at which air kerma rate in free space K is defined
what is air kerma rate constant
dummy parameter
A = kr^2/tau
-many vendors convert air kerma rate to activity using tau, hence just have to make sure we used the same tau when converting back
what is the parameter we should use to characterize the source?
air kerma strength
what parameters should dose calculation be based on?
product of dose rate constant and source strength
-has to be from in phantom measurements or calculations for each source design
For example,with125I sources, the dose rate at 1 cm along the transverseaxis of model 6702 is approximately 8.5% greater than formodel 6711, even though both are identically encased. Thedifference is attributed to fluorescent x-rays from the silverwire in the model 6711
Issue with NIST calibration of I-125
-used a free air ionn chamber. However, was later realized that fluorescent x-rays from the titanium capsule were contributiong. These x-rays don’t contribute to dose in tissue as they don’t penetrate so they should be excluded from air kerma measurement.
geometry of the well chamber
4 pi
source spec is for a point on the transverse axis
chamber is thus sensitive to anisotropy of the air kerma distribution around the source
Thus calibration can be trans-ferred only to sources having essentially the same design asthose used in the initial free-air measurements
how are sources and chambers calibrated?
A customer’s source is calibrated at an ADCL or NIST byplacing it in the well chamber. Alternately, a customer’s wellchamber is calibrated at an ADCL or NIST against a nationalstandard at an ADCL or NIST
direct traceability
source or instrument is calibrated at NIST or NRC
secondary traceability
source is calibrated by comparison with same nuclide or design that has a direct treaceable calibration
secondary traceability by statistical inference
source is one of a group of sources of which a suitable random sample has direct or secondary traceability
how many of a group of seeds do you calibrate?
10%
when to call manufacturer about discrepant source measurements?
- shoud agree within 3% with manufacturere.
- if more than 5 % (individual sources), call manufacturer
interpolative secondary free-air standard for high strength sources (HDR)
measure air kerma rate on transverse axis of HDR source at distance of 10-100 cm in free air geometry
Use ion chamber with build-up cap thick enough to yield seoncdary electron equlibrium for highest photon energies
The192Ir air kerma calibration factor isderived by interpolating between137Cs and hard orthovoltageair kerma calibration factors obtained from the NIST or anADCL calibration service
how to calibrate new source
-use chamber wherew the calibration factor was calculated from NIST or NRC (interpolative secondary free air standard) OR
do the secondary free standard within the institution using an appropriate external beam ion chamber with directly traceable 137Cs and ortho air kerma calibration
-do 2nd check with a tertiary standard (we use matrixx)- should use different electrometer and detector
sievert integral model
most widely usedmethod for modeling single source dose distributions around137Cs tubes and needles. This model consists of integratingthe point source dose distribution over the active length ofthe source, including corrections for photon absorption andscattering in the surrounding medium and oblique filtrationof primary photons through the source capsule.
-user specifies the physical and active source lengths, radial capsule thickness, and effective attenuation coefficient
pros/cons of CT-based vs US planning for prostates
CT: localization of pubic bone and needle angulation for better anterior coverage of large prostates
US: better definition of prostate and keeping seeds within it
matched peripheral dose
- for permanent volume implants
- dose for which contour volume equals volume of the target
- used to be done in absence of custom planning
Lutz box
device used to take radiographs of a brain brachy implant
-affixed to stereotactic frame
max exposure to the public
2 mR in one hour
100 mR anually
why are lead aprons or gloves not used to reduce exposure?
-ineffective for iridium and cesium, and of marginal utility for I-125 and palladium
when can a permanent implant patient be released from the hospital?
when exposure to any other individual from the released patient is unlikely to exceed 500 mR over the lifetime of the implant
4 categories of tests for manually afterloading sealed brachy sources and applicators
1) leak tests, inventories, and surveys
2) verification of source and applicator geometry/construction
3) coincidence of simulation markers and radioactive sources
4) accuracy of source calibration
3 end points of QA for remote afterloaders
accuracy of source selection
spatial positioning
control of treatment duration
