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.
