IMRT/VMAT plan Checking & QA Flashcards
Plan Checking
- Departmental protocol • Plan must be generated on an
appropriate CT data(quality,
slice thickness)
• Plan generated as per protocol • Contouring, Beams, Dose
prescribed…
• Scorecards/Goal sheets - Literature/Evidence • ICRU83 • RTOG • QUANTEC Protocols
Why is Plan Checking Necessary?
Quality Related
Safety Related
Qualitative Eval
- Key isodose lines
* Coverage
✓ ICRU/dept protocol
✓ High dose and low doses
✓ Location of max dose
✓ Volume of max dose
- Homogeneity and Conformity
- Transverse/Sagittal/Coronal views
CB-CHOP
C - Contours B - Beams C - Coverage H - Heterogeneity/Hot Spots O - OARs P - Prescription
IGRT Considerations
Tolerances and Priorities for CBCT matching
Stop and Start angles for CBCT for efficiency
Quantitative Plan Evaluation
DVH Analysis
• Reminder-No. of Bins, accurate contours …
• Scorecard tool-Pinnacle3 TPS
• Dosimetric Criteria- Monaco TPS
• ICRU 83- Use of Metrics
• Dose homogeneity (HI) and dose conformity(HI) are independent
specifications of the quality of the absorbed dose distribution(ICRU 83,p34).
• Formulas and definitions of these indices.
• Limitations.
Dose Homogeneity and Conformity Index
CI = 1 ideal HI = 0 Ideal
IMRT DVH Metrics
- Near Minimum: D98%
▪ Near Maximum: D2%
▪ Median: D50%
ICRU 83 - Remaining Volume at Risk (RVR)
RVR - The difference between the volume enclosed by the
external contour of the patient and that of the CTVs and OARs on
the slices that have been imaged.
Essentially the NTT
- There could be unsuspected regions of high absorbed dose within the
patient that would otherwise go undetected. - RVR might be useful in estimating the risk of late effects, such as
carcinogenesis. - Especially important for younger patients who can expect a long life span.
MU Efficiency
Used in relation with interleaf leakage
Over-modulation = smaller segments = more interleaf leakage = less mu efficiency
Modulation
Modulation- The process of varying one or more properties of
a beam.
▪ In VMAT delivery – there is modulation of dose intensity using
multileaf collimators (MLCs) while synchronizing with the gantry
rotation
▪ Term also used in Linac Based IMRT/ Helical Tomotherapy
planning.
Modulation factor
When a small value is set as the modulation factor (MF), that is one
of the parameters, delivery time shortens; however, a small MF
value results in poorer dose distribution.
▪ Therefore, it is necessary to set MF with a good balance of the
delivery time and dose distribution
Modulation Index
▪ In Helical Tomo Planning, the user sets a value (1.0–5.0) as MF in
the design of a treatment plan
▪ MI (Modulation Index): The modulation of the beam fluence, a low MI
value is associated with a beam with low complexity (good thing = higher chance of passing QA).
Modulation factor
MF is an index that expresses the complexity of the MLC motion.
Pre-Planning Checks
Pt is simulated
Primary and secondary datasets are imported into TPS
Post-Planning Checks
XRT plan done
XRT plan checked by 2nd RT
XRT plan checked by physicist
Evaluation Checklists
Treatment patient, ◼ Correct site ◼ Plan matches prescription ◼ Total dose, fractionation ◼ Daily dose ◼ Treatment machine correct ◼ Beam type and energy
Plan Checking & QA
Critical organ dose not exceeded
◼ Isocentre moves in relationship to landmarks
◼ Individual shielding (MLCs)
◼ Appropriate inhomogeneity correction applied
◼ Correct bolus used where prescribed
◼ Target volume and field size correlate
◼ DRR generated to the correct Isocentre
Physics checks (IMRT/VMAT)
- 3DCRT Plans
▪ Independent method of calculation and MU check including investigation
where independent and planned MU differ from the calculated by x %. (Independent check of the MU) - IMRT/VMAT plans
◼ The need to verify a number of parameters both in the planning and delivery phase (patient-specific QA). Check what is planned vs to what is measured on phantom
Phantoms
2D Arrays
4D QA
1. Point Dose measurements i.e. normalisation point
Check MUs
Check Fluence
Check Segments
Check Modulation
Six –step methodology for PSQA (Patient Specific QA)
- Verification that the intensity field boundary matches the planning boundary
- An independent calculation, verification that the machine instructions driving
the leaves produce the planned absorbed-dose distribution - Comparison of the absorbed-dose distribution in a phantom with that
calculated by the treatment planning computer for the same irradiation
condition. - Comparison of the planned leaf motions with that recorded on the MLC log
files. - Confirmation of the initial and final positions of the MLC for each field by a
record-and-verify system - In vivo dosimetry.
In vivo dosimetry.
In vivo dosimetry directly monitors the radiation dose delivered to a
patient during radiation therapy.
It allows comparison of prescribed and delivered doses and thus
provides a level of radiotherapy quality assurance that supplement
port films and computational double checks
Diodes placed at various points of interests i.e near the lenses to measure the dose - real time. TLD, silicon diodes, MOSFETs
Used to estimate dose to lenses, pacemakers, foetal dose and testicular dose
In-Vivo Dosimetry
RTs could be involved in
◼ Placing diodes,
◼ Records the results,
◼ Performs simple calculations to compare
measured with expected results, and
◼ Informs the physicist if a result exceeds
