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
