IMRT/VMAT plan Checking & QA

Question 1

Plan Checking

Answer 1

1. 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
2. Literature/Evidence • ICRU83 • RTOG • QUANTEC Protocols

Question 2

Why is Plan Checking Necessary?

Answer 2

Quality Related

Safety Related

Question 3

Qualitative Eval

Answer 3

1. 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

Question 4

CB-CHOP

Answer 4

C - Contours
B - Beams
C - Coverage
H - Heterogeneity/Hot Spots
O - OARs
P - Prescription

Question 5

IGRT Considerations

Answer 5

Tolerances and Priorities for CBCT matching

Stop and Start angles for CBCT for efficiency

Question 6

Quantitative Plan Evaluation

Answer 6

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.

Question 7

Dose Homogeneity and Conformity Index

Answer 7

CI = 1 ideal 
HI = 0 Ideal

Question 8

IMRT DVH Metrics

Answer 8

• Near Minimum: D98%
  ▪ Near Maximum: D2%
  ▪ Median: D50%

Question 9

ICRU 83 - Remaining Volume at Risk (RVR)

Answer 9

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

1. There could be unsuspected regions of high absorbed dose within the
   patient that would otherwise go undetected.
2. RVR might be useful in estimating the risk of late effects, such as
   carcinogenesis.
3. Especially important for younger patients who can expect a long life span.

Question 10

MU Efficiency

Answer 10

Used in relation with interleaf leakage

Over-modulation = smaller segments = more interleaf leakage = less mu efficiency

Question 11

Modulation

Answer 11

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.

Question 12

Modulation factor

Answer 12

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

Question 13

Modulation Index

Answer 13

▪ 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).

Question 14

Modulation factor

Answer 14

MF is an index that expresses the complexity of the MLC motion.

Question 15

Pre-Planning Checks

Answer 15

Pt is simulated

Primary and secondary datasets are imported into TPS

Question 16

Post-Planning Checks

Answer 16

XRT plan done
XRT plan checked by 2nd RT
XRT plan checked by physicist

Question 17

Evaluation Checklists

Answer 17

Treatment patient, 
◼ Correct site 
◼ Plan matches prescription 
◼ Total dose, fractionation 
◼ Daily dose 
◼ Treatment machine correct
◼ Beam type and energy

Question 18

Plan Checking & QA

Answer 18

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

Question 19

Physics checks (IMRT/VMAT)

Answer 19

1. 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)
2. 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

Question 20

Phantoms

Answer 20

2D Arrays
4D QA
1. Point Dose measurements i.e. normalisation point

Check MUs
Check Fluence
Check Segments
Check Modulation

Question 21

Six –step methodology for PSQA (Patient Specific QA)

Answer 21

1. Verification that the intensity field boundary matches the planning boundary
2. An independent calculation, verification that the machine instructions driving
   the leaves produce the planned absorbed-dose distribution
3. Comparison of the absorbed-dose distribution in a phantom with that
   calculated by the treatment planning computer for the same irradiation
   condition.
4. Comparison of the planned leaf motions with that recorded on the MLC log
   files.
5. Confirmation of the initial and final positions of the MLC for each field by a
   record-and-verify system
6. In vivo dosimetry.

Question 22

In vivo dosimetry.

Answer 22

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

Question 23

In-Vivo Dosimetry

Answer 23

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