PET Flashcards
What is the role of PET in RT?
- image fusion to identify extensive disease
- SPECT-CT images used in RT planning to precisly delineate the tumour volume
What are the advantages of PET in RT?
- high accurate image fusion
- good indication of metastasis
- indication of LNs involvement
What are the disadvantages of PET in RT?
- long waiting time
- not widely available in RT department
- expensive
- patient required to fast
How does a PET scan work
Uses radionuclides which are bonded to carrier molecules such as FDG (modified glucose).
Cancerous cells absorb more glucose.
Tracer injected into patient - travels through the body
The radioactive element releases positrons which annihilates with electrons. Causing gamma rays to be released and go in the opposite directions.
Gamma rays picked up by gamma ray cameras
What is imaged
Gamma ray from single photon emitters (from the radionuclide) - measured by spect
Positron Decay - yields 2 x 511 kEV photons - by pet
Able to penetrate greater than 10cm of tissue hence imaged with either a gamma camera or PET scanner
Whole body sweep - like scout scan
Typical Radionuclides for imaging
Technetium (Tc99m) - 140 kEV gamma rays - 6 Hr half life
I131 - 364 kEV - 8 days half life - single photon + therapeutic: Beta minus decay. Used in RT
F18 = 511 kEV = 110 minutes = positron decay PET tracer
Problems
When administring something with long half life, i.e. more than 2 hours. If 511kEv, mindful of the activity to administer. Limit activity to reduce radiation burden to pt. Takes longer time to image. Patient compliance is reduced, oar motion, patient position motion. Reduced Resolution
SPECT
Radionuclide injected beforehand. 3D camera rotates around pt. Useful for reducing artefacts. Cross-sectional image.
Combines low resolution of spect with high resolution of ct. Ranges froim 6 to 64 slices - purpse: image attentuation correction, localisation and registration.
Important for RT = tumour and anatomical delineation. Planning
SPECT Alone Pitfalls
Low resolution leads to decreased reporting confidence
Acquisition times long, motion a problem
Lack of anatomical landmarks
Developement of PET/CT
- Identify areas of increased (metabolic information) with patient-specific anatomy under identical “conditions”.
PET - radioactive sources for attenuation correction was problematic - expensive and slow
The CT measured attenuation correction suffered from far less noise and variations and methods were developed to correct the Hounsfield units into an attenuation correction “map”
PET/CT - uses CT based attenuation correction
Attenuation Correction
When photons from in the body are detected outside the body. They are absorbed. Need to correct for depth = otherwise image will have cold centres.
CT-based Attenuation Correction
To correct for depth, need to scale images for 511 kEV.
Originally PET used rotating rod source (radioactive)
Standard Biodistribution of 18F-FDG
The Brain High Uptake - Cortical and cerebellar grey matter, nuclei, and thalamus Low/No Uptake - White matter and ventricular system Body All Patients Liver > Spleen Mediastinum (blood vessels) Kidneys, Ureters and Bladder Muscles and Bone Marrow Variable in Patients Heart Stomach and Intestines Uterus and Ovaries Palatal Tonsils, Adenoids and Vocal Cords Thymus Secretory Mammary Gland Normal Esophagas Extraocular Muscles
Pitfalls of PET/CT
- Differences in breathing patterns
- CT – breath hold
- PET – tidal breathing
- Mis-registration
- High density contrast agents (eg barium) and metallic objects (eg pacemaker, hip replacement) - pet algorithm overcorrecting for the artefacts
-overestimation of FDG activity if CT data is used for attenuation correction
511 keV vs CT energies.
The Value of PET/CT in Staging and Re-Staging
Standardized uptake value (SUV) is the most commonly used quantitative analytical parameter in clinical practice.
SUV allows comparisons of 18F-FDG uptake to be made between the target tissues and normal tissues.
Region of Interest ROI compared to Liver
Liver ~ 1.5-2.5 SUV
SUV used to check image quality issues
Pt internal quality control
ROI compared to liver = 1.5 - 2.5 SUV
Basic Patient Prep for 18F-FDG Studies
Oncology
6 Hour Fast
Nothing to eat or drink other than plain water with no additives - if pt eats or drinks coke etc, increases insulin and glycogen storage. Voiding the pet scan.
Refrain from strenuous exercise 24-48 hours prior to scan - muscle repair = increased glycolysis
Diabetics must discus current therapy and special requirements with the department to insure insulin levels are correct
Lorazepam
- Reduces muscle activation within the neck and relieves tension. Also used for patients who are claustrophobic
Head Rests
Can use similar head rest + equipment as RT sim for PET/CT
What do we need to consider for PET for radiotherapy planning
Accurate spatial localisation
Diagnostic imaging position and RTP position
Correctly registered the image data sets to the RT position
Registration
Rigid vs Deformable
Rigid Registration
LDCT from PET session is fused to the simulation/planning CT
Spatial transformation is then applied to the respective PET
Translation and rotation the PET scan to match the CT scan
Deformable Registration
Allows for registration beyond translation and rotation
Ulitmately, the deformable algorithms “warp” the PET data set to match the reference image (small vectors used maximise the similarities between data sets)
Rigourous QA is needed when applying deformable solutions
Check PET and LDCT perfectly aligned
Warps the PET scan to match similar vectors to reference image (CT)
Important for PTs who arent in the correct position
Motion
Blurring in PET images can be used to RT advantage to define the GTV
Breathing => 0-3cm movement depending on location or greater
Apex vs base lungs
Is not just translational
Lots of blurring due to motion, due to respiration. Movement includes rocking, sup-inf
Useful for defining GTV margins to account for motion due to respiration.
PITFALLS FDG IMAGING
Treatment Adaptation – modification to improve clinical outcome
FDG – poor at early response to assessment
Inflammation - used 12 weeks after last fraction of RT for accurate results
Morphologic Imaging also slow to demonstrate response
Other tracers FLT FDOPA FMISO Ga68 DOTATATE
Radiopharmaceutical and Application
18F-FDOPA - large neutral amino acid analogue Glioma, Congenital Hyperinsulinism (CHI), Parkinson’s Disease, neuroendocrine tumours
Ga68 DOTATATE
used for diagnosis and staging of well differentiated neuroendocrine tumour
FMISO
Radiotherapy stereotactic boost to hypoxia regions - hypoxic areas can be demonstrated by FMISOUse of radiosensitisers
ga68 PSMA
Used for prostate cancer, image tumours with increased prostate specific membrane antigen (PSMA).
Increased PSMA expression with prostate malignancy such as adenocarcinoma
Renal CC
Colorectal
Not all tumours have PSMA expression
PSMA selection criteria
Alot of prostate cancer, scarce resources.
High gleasons and PSA scores - useful for pts with LN or metastes
Biochemical recurrence - high psa lvls post prostatectomy or RT
PET for prostate
We need a diagnostic tool which is capable of detecting disease recurrence when PSA levels start to rise in order to guide early salvage treatment
Bone scans, CT scans and MRI scans alone commonly underestimate disease burden.
Nuclear Medicine
Can be used to demonstrate physiology, i.e. for lung tissue = healthy or not. Can affect RT if pt has poor lungs = affects quality of life.
