Dosimetry Flashcards
What is in-vivo dosimetry?
Taking measurements of the dos actually delivered to the patient at treatment
Why do in-vivo dosimetry?
Safeguard against significant errors
Function as check of entire treatment
Point dose measurement
Measure dose at a point, using TLDs or diodes (traditionally)
Detectors placed on patient skin, entry/exit dose measured
Must not perturb beam
Only meaningful in uniform dose region: best for conformal treatments
Limitations of point dose measurement
Devices require calibration and maintenance
Require placement at each fraction
How do you verify it was in the correct place if it’s out of tolerance?
Not suitable for VMAT
Advantages of TLD for point dose measurement
No temperature dependence
No dose rate dependence
No directional dependence
No wires
Disadvantages of TLD for point dose measurement
Specialised reader and annealing device necessary
Fiddly to handle
No real time read out
Advantages of diodes for point dose measurement
High sensitivity allows high spatial resolution
Easier to handle than TLDs
Immediately available for re-use
Real time read out
Disadvantages of diodes for point dose measurement
Connection with wire
Temperature, dose rate, directionally, field size dependent
Correction factors needed as a result
EPID for IVD
Full 2D map of dose can be obtained, more suitable for IMRT and VMAT
Response is energy dependent and corrections are therefore required
Size and position of EPID can limit use, need couch clearance, might sag which needs correction, might not get data needed from linac
Is exit dose, so patient set up problems can be identified
Forward projection EPID
Grayscale distribution measured with EID compared with predicted value
Back projection EPID
Measured fluence at PID back projected through CT data to calculated 3D dose in patient
Log files
Look at trajectory log files produced by linac, tells you gantry position etc
Can confirm if they are as expected
Relies on them being correct
No information regarding patient - is this in-vivo?
Transmission detector
Detector mounted on gantry head which measures delivery
Perturbs treatment field - does it need modelling?
No information regarding patient
What errors can be captured with in vivo?
What can be detected depends on type of in-vivo used
Machine related errors (MLC position, collimator angle, beam flatness..)
Plan related errors (errors in dose calculation, delivery of incorrect plan..)
Patient related errors (anatomical changes, positioning errors, movement..)
What can’t be detected with in-vivo?
Problems common to plan and in-vivo system (data transfer, wrong patient dose, wrong plan)
Problems inside tolerance values or difficult to understand
