Amanda C notes II Flashcards
TG142 MLC test tolerances
-picket fence (weekly)
-setting vs radiation field- 2 mm (monthly)
-loss of leaf speed < 0.5 cm /s (monthly)
-leaf positioning accuracy 1 mm (monthly, picket fence)
-MLC transmission (0.5 % from baseline, annual)
-leaf position repeatability 1 mm (annual)
-MLC spoe shot radius 1 mm (annual)
-coincidence of light field and xray field 2 mm (annual)
-gamma test, < 0.35 cm max, error RMS 95% of error counts < 0.35 cm
Co-60 dmax, PDD10
0.5 cm, 59%
surface dose for 5x5 cm2 PFF vs 40x40 cm2 PDD
20% vs 40%
SSD effect on PDD
2%/cm
temperature error/degree
0.3%
pressure error/kPa
1%
how often are wedge factors checked?
daily
CPQR gamma tolerance
tolerance is 95% < 3%/3 mm
action is 95%< 5%/3mm
key points about scanning water tank
-centered
-orthogonal
-level
-accurate
tolerance for ion chamber extra cameral signal, signal reproducibility, stem effect
0.5% tolerance, 1 % action
tolerance for ion chamber leakage
0.1%, 0.2%
tolerance for ion chamber reproducibility
1%,2%
Pion should be less than…
1.05
2.-use another chamber if this isn’t the case; don’t increase voltages just to reduce it
internal shielding- how to protect against electron backscatter
coat in Al around Pb shield and dip in wax to form 1-2 mm coating around Pb. Reduces low energy backscattered electrons to patient
In ortho, layers of Pb required to reduce transmission to <5% of max dose
1 layer reduces 100 kV to 1 %, 180 to 3.5% and 300 to 21%
2 layers reduces 100 to 0.1%, 180 to 0.6%, and 300 to 8.6%
3 layers reduces 300 to 4.2%
sources of uncertainty in RT
-patient localization
-imaging
-anatomical definition
-beam geometry establishment
-dose calculation
-dose display and plan evaluation
-plan implementation
things to test in TPS
-Contour dimensions
-CT to RED conversion
-dosimetric cases for:
-standard geometry
-olbique, lack of scattering, tangential fields
-significant blocking
-four field box, three fields, wedges
-customized blocking
-non-coplanar beams and collimator rotation
-standard geomet
how to evaluate 3rd party software?
-cost
-evaluate what you do in clinic (can it do VMAT, IMRT. multiple iso etc)
-QA each modality
-can it produce reports
-dicom compatible
-import/export
main responsibilities of RSO
-education (radiation safety training) and for non NEW staff as well
-monitoring and recording
-licensing
-respond to emergencies and incidents
-develop, maintain, and enforce procedures
-radiation safety program
-must promote safety culture
how to estimate dose to fetus
out-of-field profiles based on distance from field edge
-10% at 2 cm
-1% at 10 cm
-0.1% at 30 cm
5 cGy is lowest dose with little risk of damage
risk of damage to detus from RT
< 5 cGy little risk
5-10 cGy uncertain
10-50 cGy sigificant risk if during first trimester
>50 cGy high risk during all trimester
considerations for RT of preganant patient
contributions for fetal dose:
> > 10 MV- contribution from neutrons coming from head
-photon leakage through treatment head
-scatter radiation from collimators and beam modifiers
radiation scattered within patient from treatment beam
-imaging
-reduse high energy use
-modify technique to reduce dose
-special shield (bridge over patient)
-shielding designed to reduce head scattering
-low dose imaging where [possible
issues with small field size dosimetry
-direct photon beam source is occluded- get overlapping penumbra and lowers output- widens FWHM of profile- FWHM might not be proper description of FS
-lack of lateral electronic equilibrium
-detector is large- can’t resolve the prnumbra and also perturbs fluence at point of measurement
types of sites being treated by brachy
Intracavitary: Uterine cervix, Corpus uteri, Vagina
Interstitial: Head and Neck, Breast, Prostate Gland
Interaluminal: Bronchus and Oesophagus
Superficial brachytherapy: Surface or mould
brachy sources
ideal brachy source characteristics
Energy: should be high enough to be penetrating and not to have too many photoelectric interactions, but should not be too high so that radiation protection requirements are minimized
Half-life: Should be long enough so it is practical to use the source and that the decay is a minor effect on the treatment time calculation, but it should be short enough so not to be a radiation safety hazard
Charged particle emission: Should be minimized so that the dose rate very close to the source is not too high (α and β particles are too short in range and only result in very high doses to small volumes around the source)
Gaseous disintegration products: i.e. should not give off Radon
Should be insoluble and non-toxic
No Powdering or dispersion if incinerated
Should be available in a variety of shapes and sizes: ease of use in clinical situations
Should be resistant to damage during sterilization
shielding of remote afterloader must meet what condition?
air kerma rate 1 µGy h-1 at 1 m
Cs-137 activity for LDR
10 mCi
Acceptance and Commissioning of a HDR unit:Acceptance: Purpose of acceptance is to test that the HDR unit:
Meets safety standards:
Interlocks, signage, emergency functionality, radiation surveys of afterloader
Meets contractual specification of the unit:
Positional accuracy, timing accuracy, source activity, TPS functionality and integration
Scope of acceptance tests generally determined by vendor, unless previously agreed upon
Commissioning:
Acquire and test accuracy of all system-specific parameters in the treatment planning process
Entry of acquired data into to TPS and testing of dosimetric accuracy and end-to-end (E2E) process Development of operational and quality control procedures
Training of all staff involved
Image guidance system, e.g. ultrasound, should ideally be commissioned prior to afterloader for E2E testing
brachy TPS tests
-overall system test
-geometrical accuracy (source input and display)
-optimization performance
-time and dose calculation accuracy
-isodose accuracy
-DVH and figures of merit
-dose and position constancy under plan rotation
-coordinate reconstruction accuracy
-input parameters (Verify parameters for all pre-calculated single-source arrays against publish recommendations and source vendor’s mechanical drawing)
-verify consistency of printed plan documentation
-dicom import from ct
dose drop-off in brachy
IS for high E source
faster than IS for low E source (I125, Pd103)
evolution of TG43
pre TG43- specific source construction not addressed, problems at low energy, calculations based on apparent activity, sievert integral
Many problems with above formalism (specific source construction not addressed, problems at low energy)
TG-43:
Use dose rate constants that are specific to source design & directly measured/calculated (MonteCarlo). Also use consensus data for low E sources
brachy radial dose functions
Behaviour of F(r,theta)
While F(r, θ) on the transverse plane is defined as unity, the value of F off the transverse plane typically decreases as
r decreases
as θ approaches 0° or 180°
as encapsulation thickness increases
as photon energy decreases
brachy program design considerations
A multidisciplinary team is required: nurses, RT, physicists, dosimetrists, radiation oncologists, administrators and engineers. The physicist serves as the leader of the team with respect to planning and treatment delivery, determining which tasks and quality assurance checks can be delegated to team members.
Need to establish the expected patient population and the types of procedures that will be performed
Facility design begins with specification of sources, applicators, delivery systems, imaging, treatment planning software etc. Allocated space must meet the shielding requirements
Physicist are also responsible for acquiring the licensing for the proposed facility with the appropriate regulatory agency
Acceptance & commissioning
Develop treatment planning and QA protocols
Training
Shielding
physics role in brachy program design
Design and implement a brachytherapy facility that meets the clinical needs of the institution
Develop and implement treatment delivery procedures (for each clinical site and type of brachytherapy procedure) that accurately realize the clinical intent of the radiation oncologist, protect the patient from treatment delivery errors, maximize safety of the patient and staff, and, finally, minimize the legal and regulatory liability of the institution
Ensure the accuracy and safety of each individual brachytherapy treatment through review of calculations, monitoring treatment team compliance to established procedures, and adapting procedures to meet the needs of unusual patients
LDR prostate constraints to urethra and rectum interface
urethra < 360 Gy
rectum interface < 90 Gy
heterogeneities in prostate brachy
calcified deposits in prostate gland
rectum and bladder dose in manchester gyne brachy
keep <80% of pt A dose
high risk CTV
GTV + whole cervix
intermediate risk CTV
high risk CTV +0.5-1.5 cm margin
low risk CTV
intermdiate risk CTV + uterus + parametria + vagina
radiation safety committee
advise RSO and manaement on quality and effectiveness of radiation safety program
Design of SBRT program
-assemble inter. team
-determine clinical goals (site, fractionation)
-assemble documents (ASTRO, TG etc) and read liteature
-delivery technique, energy, motion management immobilization, IGRT
-proper dosimetry equipment
-design QA program, protocols and procedures
-training
-external audit
-regular review for improvement
-also maybe visit a site doing the technique
ALARA in IGRT
-protocols specific to pediatric
-local imaging protocols
-kV lower than MV
-use collimation to reduce tissue exposed
ICRP risk of inducing a fatal cancer from a single radiograph
5*10^-5/mSv
effective dose from CT head and CT body
2-4 mSv head
5-15 mSv body
versus 50 mGy CTDI head and 15-25 mGy CTDI body
effective dose xray
fluoro 0.01- 0.05 mSv
chest xray 0.01-0.05 mSv
skull xray 0.1- 0.2 mSv
mammo 0.05 mSv
abdo xray 1 mSv
effective dose kVCBCT
1.1-24 mSv for trunk
0.04-9.4 mSv for head and neck
