Imaging during Radiotherapy and Quality control

IMAGING DURING TREATMENT

CT – linear attenuation coefficient for each pixel is compared to water and gives us Hounsefield number 

CT numbers represent Electron density of material 

Compton attenuation is proportional to Electron density 

MRI better for soft tissue contrast. But no electron density information. Prone to geometric uncertainty.  

PET : F18 is a positron emitter 

F18 FDG cannot be distinguished by the body from glucose 

positron annihilated by an electron and 2 photons in opposite direction produced. Detecting these photons helps understand where the original position, i.e. glucose uptake was 

Patient Set up for External beam RT:

tattoos and fixed room lasers 

Immobilisation devices 

treatment field – corresponds to a light field from treatment head and centre seen with cross wires 

checking SSD (source to surface distance)

imaging protocols or cone beam CT verification 

Immobilisation considerations

Reproducibility 

Comfort 

Limited normal tissues radiated (wing board in lung to protect arms) 

Technique and allowing gantry rotation freely 

Verification imaging

Anatomy comparison between planning and treatment times 

2D MV or kV images can be acquired during each pretreatment episode 

2D verification images for comparison can be a DRR (digitally reconstructed radiograph) from planning CT 

3D CBCT (Cone beam CT) 

Captured with a  single rotation of X ray tube and detector while patient is on the treatment couch.

Detector is a single square panel (as opposed to diagnostic CT which is a thin linear ray of detectors and reads helically through patient slice by slice) 

Increased noise of CBCT due to detector geometry 

CBCT has good bony anatomy and some soft tissue information 

good resolution in all axes 

Patient breathing motion correction

Decrease motion – abdominal compression.

Active breathing coordinator, (block nose, breath into a tube which is attached to a machine through mouth. Has a resistance that stops you from breathing out and allows treatment to be delivered during the time that fixed volume of air in lung – inspiration and breast moved away from lung) or gating (using a marker on the chest and as it moves can only treat when the marker is at a certain range – it will move with chest wall) 

What is a 4D CT

Low pitched CT scan (slow movement through scanner) 

Images each slice of the patient through all phases of respiratory cycle 

Essentially a series of respiratory phase correlated 3DCTs

Respiratory waveforms are identified and split into bins and images grouped accordingly to make a series of 3DCT scans 

CT scanner identifies phase of respiration either by an external marker that moves with chest wall, or implanted marker in tumour 

MIP – max intensity of image at each point of breathing cycle combined 

used for contouring 

AVG is time averaged density of tumour-  used for planning calculation 

Min IP = shows location where the tumour is ALWAYS present. Might be Good for negative contrast contouring (liver tumours). NOT USED currently.  

4DCBCT also rotates only once around patient – but very slowly! to capture all phases of respiratory cycle. Can take 5 min to take one scan. Uses often an internal marker to assess phase of respiration (diaphragm vs lung interface) 

QUALITY ASSURANCE IN RT

Every machine and patient failure significant events go to CQC (Care Quality Commission). Machine errors also goes to HSE (Health and Safety Executive)

Significant Overexposure – 10% overexposure of dose for whole treatment or 20% for one fraction 

Quality assurance vs Quality control

Assurance: Is a system in place or a list of instructions that must be followed to assure maintain quality 

Quality control – is a measurement/means to check quality of a system 

Quality control for LINACS 

  1. Radiation output – checked daily for linac output to ensure the monitor units are constant as locally defined 

2. Beam ENERGY

Determined by electron gun current and power of microwaves in wave guide 

Measure of beam quality and how penetrative the beam is = TPR 20/10 

TPR 20/10 : Tolerance is 1%

3. FLATNESS AND SYMMETRY

Beam profile affected by gun current, wave guide acceleration, beam steering 

Tolerance 2% (feedback system so slightly more lax tolerance)

4. FIELD SIZE

Corresponding Light field from linac head – checked every morning. Easy to measure with graph paper – can just see the field.  

X ray field size measurement – using MV image panel – EPID (Electronic portal imaging device)

5. Mechanical distance checks

Isocentre checked with cross wires

Source to surface distance checked with Optical distance indicator 

6. IMAGING TESTS

Check imaging system for quality of image 

Imaging data influences accuracy of treatments – geometric and dosimetric both 

ELECTRO GUN influences: Beam Energy, symmetry and flatness 

RF wave guide influences: Beam Energy, symmetry and flatness 

STEERING SYSTEM influences: symmetry, flatness, leakage 

Checks prior to treatment plan delivery 

  1. Patient check, prescription check 

2. Isocentre check 

3. Machine parameters – is it physically deliverable 

4. Optimisation check ( DVH) 

5. MONITORING UNIT CHECK (different from output check!)

MU calculated by TPS should always be independently checked  

(tolerance of about 3% as physical checks for simple plans assume all patient is water. National standards are 3% but for iMRT often dosimetric checks or independent computer verification can be used to check MUs and often achieves a much tighter tolerance) 

Pre-treatment plan verification for IMRT

Can’t do is physically by physicists as too complex! 

Physicists re-create a similar mock-plan in a phantom and measure dose with ion chamber or Thermoluminescent diode.

2D Film measurements  

Radiographic or gafcrhomic films and measure dose exposure to film 

Arrays of small diodes or ionisation chambers in an array that can be 2D or 3D 

Initially phantom dose measurements done for every patient plan – but as more confident with Treatment Planning System, can decrease these 

RECORD AND VERIFY SYSTEM

Data can be transferred from Treatment Planning Systems to treatment machines electronically 

Records treatment data delivered automatically 

GEOMETRIC ERROR SOURCES 

1. Patient position : 

Set up machine parameters corresponding to correct patient position

Couch angles, rotation etc can all be recorded and auto set 

Treatment cannot be delivered if machine positions are not within set tolerances of treatment plan 

Patient position verification 

Upto 2mm tolerance

Tighter for SRS or SABR

2. internal motion 

3. imperfect immobilisation 

4. laser/machine instrument error 

5. Human error 

Can be systematic ( measurement device error, wrong immobilisation mask) 

or random (patient movement) 

Pre treatment imaging protocol – ONLINE IMAGING PROTOCOL while patient on bed 

Can correct for shifts with CBCT prior to each fraction 

OFFLINE IMAGING PROTOCOL 

independent check of images taken after treatment completed  

Set up checked frequently (every few fractions) and look for consistent shifts – then can correct for these 

CBCT 

Good contrast, wide field of view, 3D information, low dose  

Poor resolution, takes time 2 min and patient might change position between CBCT and treatment, cannot show you treatment beam (only contours and patient position) 

2D kV imaging  

Same machinery as CBCT but only 2D images at 2 orthogonal angles 

Quick, good bony anatomy, large field of view, good contrast  

Often instead a DRR can be constructed from CBCT 

2D MV imaging 

Treatment head and EPID 

Poor contrast (Compton predominant) 

No extra dose is treatment dose is used to image a patient 

Can see treatment field 

IN VIVO DOSIMETRY 

Diodes or TLD 

Very rough estimate 

Generally performed as check for gross errors at first dose 

Not generally even used now as pretreatment verification is very good. 

Can identify gross MU calculation errors, transfer of information from TPS to machine errors, gross patient set up error 

Diodes positioned on patients skin during treatment  

3-5% accuracy  

Delivered dose behind diode is less  (shadowing) 

TLDs 

rarely used as a lot of time for calibration of processing 

small size – can be used to measure dose to small areas such as lens of eyes (over eyes) 

Transit Dosimetry 

Use exit dose to EPID (Electronic Portal Imaging Device) as a measure of dose delivered to patient. Back project exit dose into what would be expected in the middle of the patient. Very difficult in a practical sense.  

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