PET scanning is utilized in skin cancer staging by detecting regional lymph node metastases as well as distant organ metastases. Its specificity is superior to CT and MR for N-staging, particularly with melanoma. It can be the superior modality for primary tumor staging as well.
PET scan imaging is based on the increased metabolic activity of cancer cells that more rapidly take up 18F-FDG, a glucose analog with a positron-emitting radioisotope fluorine-18. The sensitivity is limited by the minimum size of the tumor – 4mm or greater. The specificity of PET scan is limited by false-negative readings with well-differentiated tumors or by proximity to the primary tumor where resolution of imaging may not be able to visualize the lymph node as separate from the tumor. Additionally, inflammatory and infectious lesions can demonstrate increased 18F-FDG uptake, although usually at an SUV (standard uptake value) below that of cancer.
Most PET scans today are performed as fusion images with a concurrent CT – a PET / CT scan. A novel metabolic–anatomic imaging technique recently has been introduced into clinical practice – PET / MRI. It has a potential benefit in T-staging of primary bone tumors and soft-tissue sarcomas and M-staging of brain, liver, and bone metastases. PET / MRI would be a strong candidate for melanoma staging.
Recent developments in PET instrumentation is promising to deliver a 1mm detection resolution. It can be achieved by bringing the detectors closer to the patient as a locoregional (organ-specific) PET system. This is currently under development at the Molecular Imaging Program at Stanford Department of Radiology. A new design for a submillimeter-resolution small-animal PET system is also being developed.
Such high resolution metabolic imaging would revolutionize cancer management. With submillimeter PET resolution, sentinel lymph node biopsies and staging regional lymphadenectomies can become obsolete. With better tumor assessment, primary tumor surgery can be more precise and more organ-sparing. Science may be finally catching up to the Star Trek Tricorder.
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Lau, et al. Analog signal multiplexing for PSAPD-based PET detectors: simulation and experimental validation. Phys Med Biol 2010; 55:23: 7149-74.