Planning - ICRU margins Flashcards
What are the three ICRU reports considered for planning margins?
ICRU 50 (1993) ICRU 62 (supplement Nov 1999) ICRU 83 (2010, IMRT)
Define the GTV
Gross Tumour Volume
The gross palbable or visible extent and location of malignant growth
Define the CTV
Clinical Target Volume
A volume containing a GTV and/or subclinical microscopic malignant disease to be eliminated.
How is the GTV to CTV expansion defined and created?
Often based on biopsy and post-mortem series
Isotropic margins often applied
May be constrained by anatomical barriers
Define the PTV
Planning Target Volume
A geometrical concept defined to select appropriate beam sizes and beam arrangements, taking into consideration the net effect of all the possible geometrical variations and inaccuracies.
Define the Treated Volume
The volume enclosed by an isodose surface selected and specified by the radiologist as being appropriate to achieve the purpose of tx
Define the OAR
Organ at Risk
Normal tissues whose radiation sensitivity may significantly influence tx planning and/or px dose.
What is the internal margin defined in the ICRU supplement?
Margin added to the CTV to compensate for expected physiologic movements and variations in size, shape and position of the CTV during therapy
What can affect the relative position of the CTV?
If the CTV lies next to or near the digestive system it may move day-to-day
Changes in weight of the patient
What is the setup margin?
A non-anatomical margin to account for other uncertainties such as:
- Variation in pt positioning
- Mechanical uncertainties
- Dosimetric uncertainties
- Transfer/setup errors
- Human-related uncertainties
Define the PRV
Planning Organ at Risk Volume
A margin added to the OAR to allow for movement within the volume
What are the three types of organs as defined in ICRU 62?
Serial
Parallel
Serial-parallel
How is a serial organ defined?
If any volume of a serial organ gets above the tolerance dose, the organ’s function may be totally impaired.
A “serial string” of subunits.
How is a parallel organ defined?
A parallel string of subunits.
For these organs, the main parameter impairing its function is the proportion of the organ that receives a dose above a specified tolerance.
How is a serial-parallel organ defined?
A serial-parallel string of subunits.
These organs are a combination of the two other types.
Give an example of a serial, parallel and serial-parallel organ.
Serial: Spinal cord
Parallel: Lungs
Serial-parallel: Heart
How does ICRU recommend dose volume reporting of PTVs?
ICRU 83 recommends:
Median absorbed dose, D50%
Near-maximum absorbed dose, D2%
Near-minimum absorbed dose, D98%
Absorbed dose to a PTV with uncompromised margins
Define a systematic error in the context of treatment planning and delivery?
Treatment preparation errors which influence all fractions
Define a random error in the context of treatment planning and delivery?
Treatment execution errors which influence only a single fraction
What can be used to reduce margin errors in tx planning and delivery?
IGRT
Adaptive planning
Fiducials
Gating/tracking
Why are the near-maximum and near-minimum dose reporting methods used rather than max and min?
The minimum or maximum doses reported by the TPS are not representative due to their small size and movement, so D98% and D2% are used instead.
Give an example of a systematic error in the context of tx planning and delivery
Laser alignment and precision in CT Dimension/position/alignment of tattoos CT imaging parameters Transfer error from CT to tx planning Transfer error from tx planning to tx unit Laser alignment and precision in tx room RT beams geometric stability Tx unit geometry stability
Pt positioning at time of CT
Tu and OAR position at time of CT
Target volume and OAR delineation
Give an example of random errors in the context of tx planning and delivery
Organ shape changes between #s
Physiological mvts during each #
