Treatment Planning and Treatment Delivery (case studies) Flashcards
what are features that are important in a TPS
- Dose accuracy
- Geometric accuracy
- Dose to medium vs dose to water (e.g low density lung or high density bone?)
- CT orientation cube
- Moves from CTRP
- Connectivity to CT
- Connectivity to R&V system
- Integrity of Plan transfer- MU, field size, MLC position, Control point, dose rate
- Integrity of reference data (DRR or reference CT)
- Plan Evaluation tools (DVH, Score cards)
- Limitations in plan (e.g. spinal fields)
MU calculation for 3DCRT benefits and limitations
Benefit
* No measurement required
* Can predict dose to a point at various SSD, depth, field size, wedge, tray,
compensator, energy using reference data
* Independent MU/dose calculation
Limitation
* Flat geometry
* “radiological depth” does not accurately account for heterogeneity
* Heavily shielded fields
* Highly elongated field
* IMRT, VMAT, DCAT
factors to consider when choosing a device
- Absolute vs Relative Dose
- Treatment technique (IMRT, VMAT, FFF) – dose rate?
- Resolution
- Gamma Pass Criteria
- Cost – Upfront and Ongoing
- Compatibility with existing equipment (TPS, Linacs etc)
Common Causes for QA Failure
Plan-Evaluate your plan for potential for plan failure when you see:
* Lower MU Efficiency
* Modulation Factor (MU/cGy)
* Plan Complexity
Detector Limitation
* Elongated fields
* Detector non-uniformity
* Field size limitations
Treatment Equipment failure
* MLC failure
* Beam Symmetry
* Alignment of Optical system to radiation isocentre
High Complexity Fields
- High MU per Gy ~typically a factor of 3 is produces a good plan
- Narrow MLC aperture
- Higher uncertainty during delivery
- Apertures smaller than minimum calibrated field for EPIQA
- Plan Complexity Metrics
- Small aperture score
- Modulation Factor
- Modulation Complexity Score
- Area/Perimeter
daily QA: Planar kV/MV/CBCT imaging
what it checks and the tolerances
collision interlocks –> functional
positioning –> non SBRT <2mm, SBRT <1mm
treatment and imaging isocentre coincidence –> non SBRT <2mm, SBRT <1mm
daily QA: dosimetry
what it checks and the tolerances
x-ray and electron output constancy –> non SBRT 3%, SBRT 3%
daily QA: mechanical
what it checks and the tolerances
laser localisation –> non SBRT 2mm, SBRT 1mm
optical distance indicator accuracy at isocentre –> non SBRT 2mm, SBRT 2mm
light field size –> non SBRT 2mm, SBRT 1mm
daily QA: safety
what it checks and the tolerances
door interlock, audio visual, beam on indicator
function or not functional
collision interlocks
reasons for failure and what happens if it doesn’t get fixed
- Sensors failure
- Electronics board failure
- Gantry/Panel collision into patient causing injury
imaging Positioning and isocentre coincidence (kV, MV, CBCT)
reasons for failure and what happens if it doesn’t get fixed
- Calibration file may have been
corrupted/deleted/changed/recalibrated - Electronics may be incorrect/damaged
- Incorrect source/detector positioning
- Geometric misalignment for treatment
Wedge Morning Check out run one angle
reasons for failure and what happens if it doesn’t get fixed
- EDW STT table may have been corrupted
- All wedge fields would potentially be incorrectly treated
X-ray & Electron output constancy
reasons for failure and what happens if it doesn’t get fixed
- Electronic drift of detector
- Damage/Changes to the MU chamber require replacement or recalibration
- incorrect dose delivered to all patients
Laser Localisation
reasons for failure and what happens if it doesn’t get fixed
- Laser drift, wall vibrations from surroundings
- Someone bumped it
- Patient levelling and positioning off isocentre, large shifts from imaging
Optical Distance Indicator accuracy at isocentre
reasons for failure and what happens if it doesn’t get fixed
-ODI indicator may have
drifted/damaged/bumped
- SSD checks would be incorrect
- SSD setups would be systematically out
Light Field size
reasons for failure and what happens if it doesn’t get fixed
- Light source not on collimator rotation axis
- Light source may not be the correct SSD
- Jaw calibration changed
- Field size could be inconsistent depending on the SSD, border checks shows incorrect information
- Jaw incorrect calibration can lead to all patient field size treated incorrectly
Door Interlock/Beam on Indicator/lights and
audible indicator
reasons for failure and what happens if it doesn’t get fixed
- Electronic component failure
- Circuit board failure
- Safety issue- someone can enter the room during treatment without the knowledge of operator
Audio Visual
reasons for failure and what happens if it doesn’t get fixed
- AV system failure
- Power supply of AV system failure
- Patient safety
- Inability to coach RPM
Australian Clinical Dosimetry Service (ACDS)
- National dosimetry audit program
- Commonwealth Government’s Australian Radiation Protection and Nuclear Safety Agency ARPANSA
- primary standards dosimetry laboratory that provide chamber calibration
- Institutions calibrate the linear accelerators using calibrated ionisation chamber under reference conditions
- All relative dosimetry all relate back to reference condition
Evaluate dosimetry under a range of conditions
Level 1- basic reference condition
Level 2- wedge factors, depth dose, field size, off axis,
Level 3- end to end test
(CT phantom -> planning -> R&V -> treatment delivery)
include 3DCRT, IMRT, VMAT, FFF
Treatment Planning Quality Assurance + tests include
✓Routinely performed (e.g. monthly QA using standardised plans)
✓Post Upgrades
List of tests include:
* Back up and Recovery
* CT data transfer- Demographics, Patient Orientation * CT density and geometry
* Patient Anatomy
* External beam revalidation
* Monitor Units Check
* Plan Transfer
* Patient Specific Quality Assurance (PSQA)
Patient Specific Quality Assurance
- Monitor Unit Calculation for 3DCRT
- Excel spreadsheet manual dose calculation * AAPM TG 114
- IMRT QA
- Dose point measurement
- 2D relative dose fluence maps
- DQA for Tomotherapy
- Cheese phantom for dose point
- Film dosimetry for 2D fluence maps
Patient Specific QA Equipment
- MatrixX
- Delta4
- ArcCheck
- Octavius
- MapCheck
- EPID
Analysis of PSQA
- Measured vs Calculated dose comparison
- Validation of both mechanical and dosimetric uncertainties
- Distance to agreement is a good parameter in the high dose gradient region but not so good in low dose gradient areas
- 3%/3mm Gamma Pass Index Criteria are based on the combined mechanical and dosimetric uncertainty contribution to the measure dose
- Dose to panel vs Dose to Patient
- Higher probability of dose disagreement observed when: * Lower MU Efficiency
- Plan Complexity
- Evaluate your plan for potential for plan failure
EPIQA
- EPIQA is a commercial software that converts a dosimetric image acquired by an EPID into a dose map, and compare with a reference dose distribution from the TPS
- EPIQA was specifically developed for Varian amorphous silicone a-Si500 EPID
- The software can be utilised for verification of static and dynamically modulated fields
- Dosimetry image conversion to dose is based on the GLAaS algorithm
- Pre-Treatment verification tool- incapable of in vivo dose verification
- Identify potential errors in the calculation of the in the dose delivery process
