Use of verification data Flashcards
What are the four verification techniques used?
Pre-treatment
Off-line
Online
Real-time
When is pre-treatment verification performed and how?
Before the treatment course is started the reference images are compared to the treatment plan
When is off-line verification performed and how?
Compare reference images with images taken in the room at some point after the treatment
When is on-line verification performed and how?
Compare reference images with images taken in the room immediately before the treatment
When is real-time verification performed and how?
Compare reference images with images taken in the room as the treatment is being delivered
What are gross errors in the patient set-up?
An unacceptably large error which could underdose part of the CTV or overdose part of an OAR
This is not accounted for in the CTV to PTV margin
What are the possible causes of gross errors?
Incorrect patient, anatomical site, or patient orientation
Incorrect field size, shape, or orientation
Incorrect isocentre position of unacceptable magnitude
What are systematic set-up errors?
A deviation that occurs in the same direction of the same magnitude throughout treatment
What are random set up errors?
A deviation that can vary in magnitude and direction throughout treatmeant
What are the two types of systematic error?
Individual: mean error over the course of a treatment
Population: weighted standard deviation of the distribution of the mean errors for each individuals treatment
What is the target delineation error?
The difference between the true and delineated CTV. This is the largest error in RT
What is the target position and shape error?
Changes in the target position and shape between delineation and treatment. Can be due to regression or growth, bladder filling, or rectal distension
What is the phantom transfer error?
The error which accumulates when transferring image data from initial localisation to the TPS to the linac
How is the phantom transfer error tested and what are the possible causes?
Measured using a test phantom Causes: difference in laser alignment between CT and linac CT couch longitudinal position indication Image resolution Margin growing algorithm Field edge and MLC leaf position Isocentre location SSD indication Gantry/collimator angle accuracy
What are the possible causes of the patient set-up error?
Anything under gross error
Changes in patient position, shape or size
Displacement of target relative to skin due to the use of different couches
