Week 3 Quality Assurance Flashcards
quality assurance
Program that is designed to control and maintain the standard of quality set for that program
Rad Onc: set of policies and procedures to maintain quality of patient care
set collectively by the profession
Dosimetric Accurary
+/- 5%: equipment calibration, treatment planning, patient setup
Errors: random and systematic,
Ion chamber uncertainty 1.5%
Estimated overall machine calibration uncertainty 2.5%
Acceptance testing required
New piece of equipment- linac, sim, brachy sources
Linac Acceptance testing
Radiation survey-does not exceed permissable doses, formal: head leakage, area survey, interloks, warning lights, emergency switches
jaw symmetry (error less than 1mm)
coincidence: collimator axis, light beam axis, cross hairs
light field with xray beam–+/-2mm, 1% on the side
Mechanical iso: collimater, rotate with rod attached, noting distance of rod to iso= tol 2mm diameter circle
Gantry rotation: same rod, with ring, rotate 360, not displacement, tol +/- 1mm
Acceptance: radiation isocenter collimator
collimator: place film on couch, open 1 set of coll wide, other small slit, rotate coll, do 6-7 exposures, repeat for opposite jaws with new film. intersection tolerance 2mm diameter circle
Acceptance: radiation iso treatment table
film on table, open small split in lower jaws, rotate couch, multiple exposures. tol 2mm diameter circle at intersection
Acceptance: radiation iso Gantry
film on table perpendicular, slit opening, 12 exposures at 30 degree intervals, 2mm circle
Acceptance: multiple beam alignment check
more than 1 beam
causes: focal spot displacement, assymmetry of col jaws, displacement in coll or gantry rotation
Split field test
Acceptance: MLCs
TG40
Projected leaf width at iso: film
Cal of leaf positions: film at iso with buildup, expose up to 8 fields, matchlines every 5cm
Leaf travel: max range of leaf travel in both directions
transmission: intra and interleaf, and beneath jaws/leaves
leakage: leakage when mlc leaves closed, check w ion chamber or film
Field shaping: series of irregularly shaped fields
IMRT additional tests
Acceptence Electrons: Energy
Depth dose distribution: practical method of specifying clinical beam energy
CAX curve is measured with ion chamber in a water phantom and compared to published data to specify the energy
10 x 10 cm field size, 100cm SSD, and 10cm depth
Acceptence Electrons:Field Flatness
Traditional definition: the transverse variation of dose relative to the CAX over the central 80% of the field size at 10cm depth plane perpendicular to the CAX
Tolerance: ± 3%
Flattening filter & penumbra
Acceptence Electrons: Field symmetry
Beam profiles for flatness can be used here
Tolerance: 2% at any pair of points situated symmetrically with respect to the CAX
Acceptance: wedge
10 x 10 field
Tolerance: ± 2 degrees of values specified
The angle between the isodose curve and the line perpendicular to the central axis at a specified depth (often 10cm).
Side Note: Wedge Angle = (180o – hinge angle)/2
Acceptance: Conventional Sim
Geometric and special accuracies
Performance evaluation of the x-ray generator and the associated imaging system
If used for mimicking the LINAC, then geometric accuracy should be comparable
Acceptance: CT sim
Accuracy of image from the CT scanner
Alignment of simulation hardware
Lasers
Couch alignment under typical load conditions
Accuracy of target localization & accuracy of DRRs
+/- 5HUs for daily tolerance
Acceptance: brachy
.Source Identity:Check physical length, diameter, serial number, and color-coding of all sources
.Source Uniformity & Symmetry
Autoradiograph: distribution of activity & active length
.Source Calibration: Individually calibrated to check source strength
Well-type ionization chamber
National Institute of Standards & Technology (NIST)
Tolerance: ± 5%
.Applicator Evaluation
Applicators hold sources in specific geometry and some have shielding
Orthogonal radiographs
Dummy sources
Shields compared to vendor’s drawings
Mechanical function (source loading and removal)
.Interstitial Sources
Needles (Cs-137)
Sources tested the same as intracavitary
Short-lived sources (seeds, wires, or seed loaded ribbons) can be tested by visual check from behind a leaded glass window
Acceptance: Remote Afterloader
\+Source Positioning Radiograph of dummy & autoradiograph of sources Position Tolerance: ± 1mm \+Source Calibration =Well ionization chamber LDR sources HDR sources –must be able to measure large currents =Cylindrical ion chamber LDR & HDR sources Farmer-type, using free-air geometry Must have appropriate build-up cap
Commissioning
Most equipment ready after acceptance testing, but some need additional data before using with patient treatments
LINAC- calibrated, beam data
Treatment Planning Computer- isodose lines
Once all the beam data is acquired and in the treatment planning system, the machine is commissioned for clinical use
Linac Commissioning: Central Axis Depth Dose Tables
PDD & TPR/TMR
Manually interpolation
Tolerance ± 2% (preferably ± 1%) of manually measured & computer generated
Linac Commissioning: Isodose Curves
Central Tolerance: ± 2% (up to about 1cm from field edge)
Penumbra region Tolerance: 2mm (between 90% & 20% ISL)
Linac Commissioning: MU calculations
MU- deliver a certain dose at a point of depth on the CAX
Number of dosimetric quantities have to be measured
Linac Commissioning: MLC
Transmission Average leaf and interleaf < 2% CAX Depth Dose Spot-checked to show agreement Penumbra Dose distribution profiles of MLC-generated fields and conventional collimators should be compared
Treatment planning commisioning: Hardware
Accuracy and linearity of input digitizers, output plotters & printers
Treatment planning commisioning: Software
Accuracy of dose distribution
Treatment planning commisioning: Algorithm verification
Accuracy, precision, limitations & special features
Treatment planning commisioning: brachytherapy software
Testing of linear source and seeds
Dose distributions
Strength, filtration, tissue attenuation, source anisotrophy
Periodic Quality Assurance
Maintain system within it’s acceptable limits
Tests similar to acceptance testing on a regular basis
LINAC
AAPM TG-40 & AAPM TG-142
Must log QA measurements
Procedure for variations
Linac Tolerances:
TG- 142: Tables 17.8 A,B,C (Daily, Monthly, Annual)
AAPM Task Group 50: Table 17.9 (MLC)
Refer to Khan to study these
Treatment planning algorithms
Based on physics of radiation interactions within tissue
Complex- so uses simplifications for speed
Uncertainties
Monte Carlo is the gold standard
Provided by manufacturer
Require basic radiation data
Specific to each energy & each machine in a department
Patient-Specific Information
CT, MRI
Dose calculations are usually performed for each beam independently, then summed
Treatment planning process: Patient Data
CT
Patient positioned to identically to the eventual treatment
Data must be transferred correctly into the TPS
Conversions of CT numbers to electron densities must be correct (phantom with known electron densities)
Treatment planning process: Display of Patient Data
Manipulate display, slices, reconstruction of images
Treatment planning process: Display of Beam-Related Information
Field size, beam direction, collimator rotation
Field shape: jaws, MLCs, auto surround
Treatment planning process: Dose Calculation and Display
Accuracy of the geometric correctness of isodose lines are difficult to assess
Treatment planning process: Output
Printing the plan, distribution, magnification
Measurements: In Vivo
Measurements on or in the patient with the patient in treatment position
TLD, OSL, diode or MOSFET devices
Used when there is concern about critical structures (eyes, gonads, or a fetus)
Or for coverage
Measurements: In Vitro
Measurements on a special-purpose or anthropomorphic phantoms
Helpful in new treatment techniques
Uses TLD’s or OSL in the phantom that has humanlike tissue densities and composition (Rando)
Agreement should be 3-4%, since dose delivery to the patients should be ≤ 5%
