Brachytherapy Quality Assurance
Brachytherapy requires a high degree of manual labor which, owing to steep dose gradients and variable source strength, must be performed to high standards of precision. The majority of brachytherapy medical errors are preventable with an appropriate quality assurance program.
AAPM TG-40 provides general guidance on brachytherapy quality control. This report states that a precision of +/-15% is realistic for brachytherapy dosimetry and may be lower for complex multiplane interstitial implants. Regulatory requirement for radiation safety are found in NRC 10 CFR.
Data from IAEA Report No 17
| Cause of Accident | Incidence Rate |
|---|---|
| Incorrect Source Strength | 22% |
| Dose Calculation Error | 19% |
| Equipment Failure | 13% |
| Error in Quantities or Units | 6% |
| Other | 41% |
| Total Accidents | 32 |
Important QA Points
- A vendor supplied source calibration may be accepted but must be verified to +/- 3% (batch) and +/-5% (individual deviation from mean)
- At least 10% of a batch of sources should be assayed.
- Source inventories must be maintained
- List source type and radionuclide
- Total number of sources, their strength, and location.
- Log use of sources and track their movement into and out of safe storage.
- Patient name
- Attending physician
- Source disposal
- Treatment Planning Software
- Source models and dosimetry as per TG-43
- Correctly assigns source location and dwell time, especially for HDR afterloader based planning.
- Film verification or imaging (CT) may be helpful here.
- Radiation surveys should be performed before, during, and after procedures.
Required QA Equimpent
QA Equipment
- Well-type ionization chamber
- Electrometer
- Wipe test equipment
- Thermometer
- Barometer
- Records management system
- Length test ruler
- Film
Tools and Safety Equipment
- Area radiation monitor
- Hand detector (Geiger Counter, etc)
- Source retrieval equipment
- Pliers
- Wire cutter
- Shielded “pig”
Key Point: Well chambers typically exhibit a strong energy dependence in the brachytherapy energy range. Therefore, it is important to calibrate the well chamber using the correct source nuclide and source model as source construction impacts attenuation and energy spectrum.
Quality Assurance for High Dose Rate Afterloaders
10 CFR Part 35 - Afterloader QA
Key Point: While the AAPM TG-40 provides guidelines for quality assurance of remote afterloaders, most states require the quality assurance mandated in 10 CFR Part 35.
Full Calibration (10 CFR 35.633)
Full calibration is required prior to first afterloader use, after source replacement, after significant repair, and at least quarterly.
Required QA
- Output +/-5%
- Position accuracy +/-1mm
- Emergency source retraction under power failure
- Verify source transfer tube lengths
- Timer accuracy and linearity
- Verify applicator length
- Verify function of transfer tubes, applicators, and all interfaces.
Daily/Periodic Checks (10 CFR 35.643)
Spot checks are required prior to first treatment in a given day.
Required QA
- Verify vault entrance interlock
- Verify source exposure indicator light at control console, on the afterloader, and at the vault door
- Verify audio visual system function
- Verify presence of emergency response equipment
- Verify function of radiation monitor equipment
- Verify timer accuracy
- Verify computer clock, date and time
- Verify source strength is accurately presented in the computer
AAPM TG-40 - Afterloader QA Recommendations
Download: AAPM TG-40: Comprehensive QA for Radiation Oncology (External link)
Daily Checks
- Interlock function
- Audiovisual system
- Area radiation monitors
- Catheter function and locking
- Presence of emergency tools
Weekly Checks
- Source and dummy positioning: +/-1mm
Annual Checks
- Dose calculation algorithm: 3% and 1mm
- Simulate emergency conditions
- Source Inventory Audit
Source Change Checks
- Calibration of source: +/- 3%
- Verify timing accuracy: +/- 1%
- Verify source positioning: +/- 1%
Source Change Quality Assurance
Source change is performed by a qualified field service engineer. Appropriate afterloader preventative maintenance will also be performed during a source change. Quality assurance focuses on assuring source strength and positional accuracy of the afterloader.
- A source chance is typically performed every 3-4 months corresponding to a drop of activity by 60-70% (3-4Ci assuming an initial activity of 10Ci).
- Source will arrive in shielded shipping container and prior to source change and must be stored in secure room until.
- Source packaging must be inspected for damage and surveyed via wipe test upon receipt.
- On the day of source exchange, the service engineer will safely remove the old source and insert the new source.
- Physicist performs quality assurance after source change.
- Verify appropriate source received compared with documentation.
- Verify source activity using calibrated well-type ionization chamber.
- Verify source length and positioning.
- Update treatment planning system with source activity (around 10Ci).
AAPM TG-41 - Afterloader and Applicator Acceptance Testing
Download: AAPM TG-41: Remote Afterloading Technology(External link)
Remote Afterloaders
The following tests are recommended by AAPM TG-41 for the acceptance testing of a remote afterloader unit.
- All console functions and indicators are operational
- Source retracts appropriately when:
- End of preset time
- An interrupt is engaged
- Loss of power
- A blockage is encountered
- The battery is adequate to power source retraction
- Timer accuracy
- Accuracy of source decay calculation
- Indexer (which selects which channel to send the source) functions properly
- Backup systems function during power/pneumatic failure
- Mechanical (hand) source retraction system is functional
- Radiation detectors function
- System memory correctly stores information
- Device shielding adequately limits leakage
Applicators
The following tests are recommended by AAPM TG-41 for the acceptance testing of an HDR applicator
Key point: The primary source of potential error in a transfer tube/applicator system would be a systematic positioning error.
- Guide tube integrity
- Applicator integrity
- Source and dummy positioning
- This can be measured using gafchromic film
- Accurate movement of source through the applicator producing expected dose distributions
- This can be measured using gafchromic film
- Applicator attenuation (if dose calculation will be attenuation corrected)
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