IMRT/VMAT Plan Checking And QA Review Of ICRU83 Flashcards
CBCHOP
- Contours
B- beam arrangement
C – coverage
H – heterogeneity
O – organs at risk
P – prescription
Qualitative evaluation
Low, intermediate and high dose regions
Use ICRU83 metrics of dose homogeneity and dose conformity
RVR
The difference between the volume encolsed by the external contour of the patient and that of the CTVs and OARs on the slices that have been imaged
Similar concept to NTT
Helps in estimating risk of late effects such as carcinogenesis
MUE
Monitor unit efficiency (MUE) is related to MLC leakage and patient total body dose
Using a relatively large number of small apertures drives MUE
Fluency
sum the contributions from each beamlet modulation factor
Complexity parameters IMRT/VMAT
• Max MUs
• Number of control points
• Minimum segment size
• Maximum fluence: a fluence distribution with many high tops and deep valleys of values is associated with high complexity. Those high amplitude fluctuations might be more likely ti be found in a beam with a high maximum value of the fluence than in a beam with a low maximum value of the fluence. Low maximum fluence is believed to be less complex.
Modulation factor
when a small value is set as the modulation factor, that is one of the parameters which shortens delivery time. However, a small MF value results in poorer dose distributions. Need to choose good balance between delivery time and dose distributions
Expresses the complexity of the MLC motion
Max open time/ average open time
Modulation index
in tomotherapy planning, user sets a value (1-5) as MF in the design of a treatment plan
The modulation of the beam flence, a low MI value is associated with a beam with low complexity
Pre-planning checks
• Patient is simulated
• Primary and secondary datasets are imported into TPS
• RT checks prior to planning
Post planning checks
• Treatment plan done
• Treatment plan checked by second rt
• Treatment plan checked by physicist
3D-Portal Dosimetry solutions- rationale:
Patient specific QA
Evaluate agreement between predicted and measured images
Components of 3D-portal dosimetry solutions
The portal dose image prediction software
The portal imager to measure the image
QA tasks using portal imager:
Evaluate the agreement between predicted and measured images similar to 2D dosimetry software such as the mapcheck
Improves effeciency
In-Vivo dosimetry
Monitors the radiation dose delivered to patient during RT
Allows comparison of prescribed and delivered doses and thus provides a level of radiotherapy quality assurance that supplement port films and computational double checks
Thermoluminescent dosimeters(TLDs), silicon diodes, and new detectors such as metal
oxide silicon field-effect transisters (MOSFETs) are currently
available for in vivo dosimetry
Estimate dose to normal structures outside of the treatment fields such as
eye lens, pacemakers, foetal dose and testicular dose.
◼ Diode in vivo dosimetry for TBI and TSET
What to check in a plan
Critical organ dose does not exceed
Isocentre moves
Individual shielding
Inhomogenity correction
Correct bolus
Target volume and field size correlate
DRR generated to the correct isocentre
Six step methodology for PSQA
- 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.
Quantitative plan evaluation
DVH
Scorecard
Use of ICRU83 metrics- dose homogeneity and dose conformity
Interpolate button
In 4 degree gantry spacing, can click interpolate button to approximate an additional control point between each current control point
Leads to a 2 degree spacing
HI
HI of 0 is the most homogenous
(D2-D98)/d50
CI
VP/PTVp
Modulation complexity score MCS
High value of MCS means low complexity
Take into account the relative variability on leaf positions, the area of the beam opening and the number of MUs
Plan QA checklist includes
Couch removal and laser localisation
Isocentre position and moves
Oar contouring and naming
ROIs with correct densities
Bolus
Energy and modality
Gantry strat and stop
Collimator angles
Jaws set to 1 dp
Arc sequence
Dose gris and resolution
Calc algorithm
Prescription
DVH and scorecard
VMAT smartarc parameters
ROI objectives
Max dose and position
Visual analysis and scorecard
Dose reporting D2 D50 D98
MU and delivery time
Excess trials deleted
What is modulation
Modulation of dose intensity through the use of multi-leaf collimator while synchronising gantry rotation
What physics checks are done in IMRT and VMAT
Leaf position drives dose distribution so this is vital to check
Point dose measurement
Verification of dose delivery for each beam
IMRT plan evaluation
Max dose and position
Scorecard and DVH
Visual dose distribution
Max MUs and number of segments per beam
Evaluate segment shape
