online and offline correction strategies Flashcards
Correction strategy aims to
reduce systematic and/or random set-up errors
IGRT strategy may be:
*Online daily imaging
* Offline – No Action Level (NAL) and extended No Action Level (eNAL)
daily online
-Image daily – match, shift and treat
-For all IMRT/VMAT treatments – usually recommended.
-Accounts for both systematic and random errors
Offline Set up Analysis Task (SAT)
*Daily pre-treatment imaging fractions
1–3 with online correction (apply all
shifts and treat)
* In offline setting SSE calculated based
on fractions 1-3
* Fraction 4: calculate SSE from
fractions 1–3 and apply this
correction to isocentre position,
followed by pretreatment
verification.
* If set-up is within tolerance, continue
treatment with weekly imaging.
sources of systematic errors-
-immobilisation devices - incorrect headrest documented or knee fix instead of foot fix used.
-shifts incorrectly written in set up notes
systematic erros for :
long
lat
vert shifts
-patient not well imm on bed ie heels touching ff
-tat not in stable position
-arched or clenched at ct
No Action Level (NAL)
fraction 4 = image taken to verify SSE and SAT
IF IN TOLERANCE= NO MORE FURTHER IMAGING
extended No Action Level (eNAL)
IF IN TOLERANCE = WEEKLY IMAGING
JUST SYSTEMATIC ERRORS CAN BE CORRECTED
tolerance
= allowed variation in a measurement or setup.
If the variation goes beyond this limit, it could affect the radiation dose delivered.
Each treatment site and protocol should have its own specific tolerance values.
what are Action levels
-specific measurements or limits that, when exceeded, require action to be taken.
When online image review is used, anything that goes beyond the action level should be corrected immediately.
In most modern systems, automatic couch corrections are made.
After these corrections, extra verification imaging is usually not needed.
type of action depends on:
How important the measurement is
The risk of not correcting it
The workload involved
examples of action level
- action level for rotations in a CNS patient whereby a patient should be re set up and reimaged before treating.
- For online ART – action levels whereby volumetric changes should be addressed
tolerance level use
if the SSE corrections are done and patient is in tolerance —– weekly imaging
intrafraction errors
changes in the position and internal anatomy during delivery of single fraction
(ie breathing or target motion between localisation and treatment)
intrafraction correction strategies
- Considered in CTV-PTV margins – should be realistic
- Use of real time patient monitoring (+/-fiducials), respiratory motion management strategies
- Use of intrafraction imaging e.g. SABR treatment
*Intrafraction tumour deviation evident after 34 minutes
