Special Procedures and Equipment Flashcards
define QA
QA refers to administrative policies, quality control (QC) measures, and consideration of quality improvement objectives that ensure a consistent and safe fulfilment of the treatment prescription
brachy QA
- All clinical applicators need to be comprehensively QA prior to clinical use.
- Physicist must determine that the source can travel accurately to intended locations in the applicators and must confirm that the total treatment length (applicator plus transfer tube) coincides with the planned one
- The constancy of the ionization chamber and electrometer used for calibrating the HDR source(s) must be checked upon receipt, after repair, and prior to each use.
- The ionization chamber and electrometer must be calibrated at least every 2 years at an ADCL facility.
- The sensitivity, linearity, and reproducibility of the instrument must be documented at least annually
the goal of QA in SRS/SRT
is to reduce the uncertainty in treatment and to set an error tolerance limit for SRT treatment.
The overall uncertainty of an SRT system is the combination of uncertainties resulting from several
components, which mainly consist of uncertainties in the:
- target location
- dosimetry in the TPS
- radiation treatment delivery system
- image guidance system
what is the radiation isocenter
is the point in space where radiation beams intersect when the Gantry is rotated during beam-on.
QA of Radiation Isocentre - SRT technologies
- The radiation isocentre is typically identified by exposing film dosimeters to multiple exposures with slit-like fields.
- The collimator, gantry or couch is then rotated, and the field is re-exposed.
- Performing this test on multiple angles for each collimator, gantry, or couch allows the determination of the isocentre and the error in its positioning.
- Typically, the radiation isocentre is kept within 1mm movement in all planes.
Isocentre accuracy- SRT technologies
- The high dose delivered during stereotactic radiotherapy
necessitates accurate patient positioning by performing the Winston-Lutz test. - Modified- Winston-Lutz tests (different phantoms)
what is the Winston-Lutz test
- It is a procedure developed for verification of the LINAC isocenter for cranial SRS.
- Also used for non-dedicated systems to improve accuracy.
the process of the Winston-Lutz test
The test involves determining the coincidence of the isocenters of the linear accelerator and associated imaging system.
- A small metallic ball phantom, which is representative of the planned target, is fixed to the Linac couch by a locking mechanism.
- The ball center is positioned at the Linac isocenter defined by the lasers of the treatment room.
- The ball is then irradiated with several beams at different angulations of the gantry and couch, and each shot is imaged using a film placed perpendicular to the beam direction on a stand behind ball or using the electronic portal imaging device (EPID) of the linac.
evaluation of the Winston-Lutz test
the measured distances for all shots between the center of the ball shadow and the radiation field center reveal isocenter movements. Ideally, differences should be less than 1 mm to avoid important dose errors during the treatment delivery
Gamma Knife - Daily checks
- Functionality of the unit on a short test run
- Checking the daily decayed estimate of standard
- Absorbed dose rate computed by the TPS.
- Elekta Gamma Knife Perfexion is supplied with a diode test tool (consisting of four diodes) that
measures output during a 4-min exposure for comparison against pre-computed profiles
Gamma Knife - weekly checks
The emergency alarms and interlocks (a weekly basis)
Gamma Knife - monthly checks
Regulatory agencies require an output measurement be performed by a “spot check.” and should be within 1 -2 % of that predicted by the last annual measurement
Gamma knife - annual check
- The physicist should perform every test required in the site radioactive material license.
- It should include performance of all the manufacturer’s recommended tests, rigorous checking of all interlocks and emergency warning systems and a full calibration of the output of the unit
gamma knife - dosimetric QA
The Dosimetry Phantom is used for accurately measure absorbed dose and dose rate in Leksell Gamma Knife.
- Three adapters for measuring with ionization chamber or other detectors are included.
- The Dosimetry phantom can also be used to verify dose 3D distributions by means of film dosimetry.
- The dose distribution position is defined by accurately positioning the films in the phantom using two rods
tomotherapy - daily QA
- Beam output
- MVCT image quality consistency
1 - DQA
2 - slice width and field width
3 - Output
tomotherapy - Monthly, quarterly, and annual tests includes
- Beam parameter consistency,
- Couch mechanical movement,
- Gantry rotation
cyberknife - daily QA
- Output
- MLC QA
- Flatness
- Symmetry
Performed by RTs, checked by MPs
Varian Halcyon Unit
- The unit has a ring-mounted 6MV linac with a beam-stopper. The head leakage specification is 0.1% and the beam stopper is specified for 0.1% transmission of 6 MV
- The unit has a ring-mounted 6MV linac with a beam-stopper. The head leakage specification is 0.1% and the beam stopper is specified for 0.1% transmission of 6 MV
- The meteorological equipment (thermometer and barometer) should also be of good quality and should have calibrations or inter comparisons of not more than 1 year old.
Why is QA important is SRS treatments?
Higher degree of error because of the higher dose per fraction and small target volumes with these treatments
List some brachytherapy daily QA checks
- Time + date to check source strength
- Timer check to verify dwell time
- Obstruction test to ensure source will retract if obstruction in channel
- Audio visual
- Safety interlocks (beam off, light indicator)
- Emergency procedures are visible
- Survey meter is functional
- Turret lock test
- Visual inspection of the brachytherapy unit
- Source will retract using battery if power outage
causes of MRI distortions
Patient movement
Signal ghosting
Metallic implants
Chemical shift
Magnet susceptibility
tomotherapy QA - DQA
delivery quality assurance
- the procedure is integrated into into the tomotherapy planning system. the patient plan is recalculated in a new CT anatomy. This new CT anatomy is typically a phantom. the DQA plan can then be delivered and the measured dose in the phantom can be compared to the calculated dose for quality assurance
tomotherapy QA - slice width and field width
- the longitudinal extent (ie in y direction) of the fan beam is frequently referred to as field width in the literature
tomotherapy QA - output
- tomotherapy plans are based on time rather than on monitor units. the output of the machine is therefore measured in dose per unit time
list the safety checks done on the Halcyon
- door interlock
- audio
- video minitor
- radiation minitor
- radiation off at console
- stop treatment with timer
- collision interlock - end of couch
- collision interlock - bore
