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What is Positron emission tomography (PET)?
- PET creates functional images of blood flow or metabolic processes
- Tumours have higher than normal metabolic rates, therefore increased uptake of FDG
- FDG 2-fluoro-2-deoxy-D-glucose usually tagged with 18F
- Measures important body functions such as blood flow, oxygren use and sugar (glucose) metabolism to help evaluate how well organs and tissues are functioning
- Helps to accurately diagnose the extent of the disease
How is PET used in Nasopharynx Ca patients?
- Very useful in NPC for detecting local and regional disease as well as distant mets, can be considered for initial evaluation and staging especially in N3 patients
- Must be fused with a CT study set if used for planning purposes
- Use of PET combined with CT & MRI information in the planning process can lead to a change in disease staging in NPC patients and often this may change the treatment volume
- Evaluation of response early during treatment using PET will give the possibility to adapt radiotherapy to the PET findings.
- Note FDG-PET/CT is not absolutely specific to tumours and false positive findings can occur in inflammatory states.
- Pet may have a role in the treatment planning of:
- - Brain - H&N - Lung -Oesophagus -Pelvis
Advantages of PET-CT in 3D tmt planning
- 1) Anatomical and functional information
- Improved visualisation of tumour volumes
- Improved tumour volume delineation
- Conformal radiation therapy & intensity modulated radiation therapy require precise target volumes.
- PET-CT inter-observer variability is minimised
- Provides standardised tmt
- Significant alteration in target volume has been found by incorporating PET information into treatment planning:
- Volume found to decrease:
- > Benign tissue exclusion
- > Reduces dose to dose limiting structures
- > Opportunity for dose escalation
- Volume found to increase:
- > Improved tumour volume definition
- > Improved tumour coverage
- > Improved tumour control
- 2) Treatment Intent:
- PET can identify:
- Metastatic sites in lymph nodes
- Distant metastases
- Sites of occult tumour
- Therefore, potentially change treatment intent from radical? palliative
- Biological aspect of the disease, which is not detected by CT, plays an important role in 3D treatment planning.
- 30% unexpected distant metastasis detected by FDG-PET
- 5% unknown 2nd malignancies diagnosed
- 3) Evaluation of treatment outcome
- PET-CT may also play a role in the role in the response of the tumour to different treatment modalities, both during and after treatment.
- Especially evaluation of response early during treatment will give the possibility to adapt radiotherapy to the PET findings.
Limitations of PET in 3D tmt planning
- > FDG-PET/CT is not absolutely specific for tumour, and false positive findings can occur in inflammatory states.
- > False negative findings
- > Cost
- > Exact tumour borders are not well defined
- > Scanning protocols and SUV levels are not standardised
- > SUV influenced by may factors; reconstruction algorithm, kind of PET scanner used, resolution, noise and biological factors.
- > Subclinical disease not adequately detected
- > Relative low spatial resolution of 6-7mm
- > Organ & tumour movements influence PET findings
- > In the thoracic region respiratory motion causes image artefacts.
- > Artefacts also seen in bowel & bladder
- > CT data acquired more rapidly, therefore, may lead to fusion problems
- ? CT represents snapshot breathing position
- ? PET scan data acquired in a mean respiratory position
Does PET improve the outcome for RT patients? Potentially by:
- - Dose escalation
- - Improved tumour volume delineation
- - Improved tumour coverage
- - The use of IMRT
- - Knowledge on biological characteristics of the tumour