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
