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CSH059 > PET > Flashcards

PET Flashcards

1
Q

What is the role of PET in RT?

A
  • image fusion to identify extensive disease

- SPECT-CT images used in RT planning to precisly delineate the tumour volume

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2
Q

What are the advantages of PET in RT?

A
  • high accurate image fusion
  • good indication of metastasis
  • indication of LNs involvement
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3
Q

What are the disadvantages of PET in RT?

A
  • long waiting time
  • not widely available in RT department
  • expensive
  • patient required to fast
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4
Q

How does a PET scan work

A

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

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5
Q

What is imaged

A

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

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6
Q

Typical Radionuclides for imaging

A

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

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7
Q

Problems

A

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

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8
Q

SPECT

A

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

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9
Q

SPECT Alone Pitfalls

A

Low resolution leads to decreased reporting confidence
Acquisition times long, motion a problem
Lack of anatomical landmarks

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10
Q

Developement of PET/CT

A
  • 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

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11
Q

Attenuation Correction

A

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.

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12
Q

CT-based Attenuation Correction

A

To correct for depth, need to scale images for 511 kEV.

Originally PET used rotating rod source (radioactive)

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13
Q

Standard Biodistribution of 18F-FDG

A 
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
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14
Q

Pitfalls of PET/CT

A
  1. Differences in breathing patterns
    • CT – breath hold
    • PET – tidal breathing
    • Mis-registration
  2. 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.
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15
Q

The Value of PET/CT in Staging and Re-Staging

A

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

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16
Q

Basic Patient Prep for 18F-FDG Studies

Oncology

A

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

17
Q

Head Rests

A

Can use similar head rest + equipment as RT sim for PET/CT

18
Q

What do we need to consider for PET for radiotherapy planning

A

Accurate spatial localisation
Diagnostic imaging position and RTP position

Correctly registered the image data sets to the RT position

Registration
Rigid vs Deformable

19
Q

Rigid Registration

A

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

20
Q

Deformable Registration

A

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

21
Q

Motion

A

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.

22
Q

PITFALLS FDG IMAGING

A

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
23
Q

Radiopharmaceutical and Application

A 
18F-FDOPA
- large neutral amino acid analogue
Glioma, 
Congenital Hyperinsulinism (CHI), 
Parkinson’s Disease, 
neuroendocrine tumours
24
Q

Ga68 DOTATATE

A

used for diagnosis and staging of well differentiated neuroendocrine tumour

25
Q

FMISO

A

Radiotherapy stereotactic boost to hypoxia regions - hypoxic areas can be demonstrated by FMISOUse of radiosensitisers

26
Q

ga68 PSMA

A

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

27
Q

PSMA selection criteria

A

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

28
Q

PET for prostate

A

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.

29
Q

Nuclear Medicine

A

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.

CSH059 (6 decks)

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