Card Set Information
PET information in detail
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
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
> 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