Radiotherapy Flashcards
Why use CT for treatment planning
Geometrically accurate in 3D
HU map to electron density
Good resolution
Good bony-soft tissue contrast
Low distortion
Limitations of CT and other options to get around this
Poor definition of soft tissue - MR has good soft tissue contrast
No functional information - PET CT or functional MRI would give this
Additional requirements for RT planning CT over normal CT
Needs to be in consistent and reproducible position, the same as for treatment
Flat top couch to mirror that of the LINAC
Scan patient in treatment position with any immobilisation that will be there
Lasers establish reference system, ball bearings define orthogonal points on patient surface
What affects shape of TCP curve and how
Total dose or dose per fraction - increasing moves to left
Initial number of clonogenic cells - moves to right
Parameter alpha - moves to left (and gets steeper)
Alpha/beta ratio - curve moves to right
Spread of alpha - gradient decreases
Properties of an ideal radiation detector in RT
Accurate and precise
High sensitivity
Appropriate spatial resolution
Linear response with dose
Minimal angular dependence
Dose rate independent
Energy independent
Temperature independent
Define primary shielding
The primary collimator absorbs any radiation that may have gone astray, before it scatters around the room and irradiates some part of the patient that should not be treated.
Define secondary shielding
The secondary collimators or moveable jaws/ MLCs define the field size.
How and why is electron energy reduced when using electron beam instead of photon beam
o Electron energy is reduced by reducing the current
o Lots of energy lost as passes through the target as heat and attenuation through flattening filter etc
o (Flattening filter also hardens the beam and this does not occur for electrons-because of this need to alter the energy of the beam)
Uses of CT in planning
4D reviews - lung movement
Contouring
Used during patient set up
Can be fused with other modalities
Can check dose distribution through patient
What is on axis of cell survival curve
x: dose
y: surviving fraction (logorithmic)
Larger alpha/beta ratio and smaller: types of cell
Larger ratio: tumours, early responding tissue
Smaller: late responding tissue
5 R’s of radiotherapy
Repair (of sublethal damage) - hyperfractionation
Repopulation (following irradiation) - accelerated fractionation
[more resistent]
Redistribution (of cells within cell cycle) - chemotherapy
Reoxygenation (of surviving cells) -drugs
[more sensitive]
Radiosensitivity (intrinsic per cell)
6th? Reactivation of immune response
Tumour control probability
TCP = exp(-k) = exp(-k0 SF)
= exp(-k0 exp(-alpha BED))
BED
BED = D[1 + d/(alpha/beta)]
Systematic vs random error in setup
Systematic: an error propagated through the process
Random: error at single event that won’t be reproduced
Treatment prep errors
Systematic:
Doctors delineation
Organ position and shape at localisation
Error in TPS
Image registration
Treatment delivery errors
Systematic:
Linac geometry errors
Systematic set up error
Random:
Daily set up error
Daily position and shape of organ
Calculating random and systematic errors
Systematic: difference to mean positions day to day
Random: standard deviation, spread of points from mean
Reducing systematic and random errors
Systematic: off line verification, acquire images on successive days and calculate and correct for mean
Systematic and random: online verification, acquire image each day, calculate displacement and correct for this
What is 2D verifiation
Checking patient position or check treated and shielded position with beams eye.
Quality of match depends on type of image used
Might image for 3 days and see out of tolerance and shift
Methods of matching during verification
Bony registration
Soft tissue registration
Online verification: register localisation and verification CT data, overlay 95% isodose on verification and ensure CTV still enclosed.
QA vs QC
QA: ensuring standards are established and maintained
QC: undertaking of tasks to ensure defined parameters remain in tolerance
What are two parts to QC
Geometric accuracy
Dosimetric accuracy
ICRU 83 recommends 3.5mm on geometric and 3.5% on dosimetric
Pre treatment imaging QA
Check geometric accuracy (couch movement, laser alignment), CT number, image quality (SR and CR)
Need geometry to be correct for planning - use phantom of known geometry
Need lasers/ couch position to be correct because this is how we set the patient up
Need CT number correct because its how we end up calculating the dose
TPS QA
Need patient model and linac model
Some systems use check sums - database checks, some perform routine dose calculation algorithms.
Daily - recalculate standard plans - ensures consistency of CT –> ED
Routine testing of functionality - ROI contouring, algebra, DVH statistics.
Process/data transfer QA
Test process using phantom - scan on CT, data transferred to TPS, planned here, plan exported and transferred to linac, treatment delivered and check it’s correct.
QC in patient pathway
Photo/DOB/name/address.
customised immobilisation
Comprehensive plan checks
Daily vs monthly checks
Daily: output consistency, interlocks, laser alignment, light field and cross wire set up
Monthly: photons, electrons, mechanicals, on-board imaging
Radical vs palliative radiotherapy
Palliative: symptom relief not cure. Quick, pain relief, simple fields, POP/ single field, critical structures less of a concern
Radical: aiming to treat. Immobilisation, CT, planning system, dose distribution, plan check
What is a monitor unit
Ionisation chamber ‘monitors’ beam
constantly.
* 1 monitor
chamber unit=1 dose unit
(usually 1cGy) under ‘calibration
conditions’.
* Monitor chamber linked to beam control - beam is switched off at correct MUs.
* TPS calculates MUs required
What is PTV
Margin added to account for geometric considerations and patient motion. Treating the PTV ensures coverage of the CTV with setup variability
What happens below saturation voltage
- If voltage not high enough, measured charge proportional to voltage
- At a particular saturation voltage (Vsat) the current is saturated for a given exposure rate. i.e. nearly all ions formed are collected.
- All ionisation chambers must operate under saturation conditions if their response is to be linear with exposure.
