Although FDG is by far the most important radiophar-maceutical at present others such as 11C-labelled methionine and choline and fluorine labelled DNA proliferation markers such as fluoro-L-tyrosine (FLT) will have an increasing role in the years ahead. The applications can be classified according to the generic use for which the PET scan is applied, that is detection, staging tumour response, etc or by tumour types. Both are important to understand although the tumour type approach will be the method chosen for agencies responsible for agreeing reimbursements.
• Diagnosis of malignancy: examples will include differentiating malignant from benign pulmonary nodules, and differentiating brain scarring after treatment (surgery, chemotherapy and radiation therapy) from tumour recurrence.
• Grading Malignancy: as the uptake of FDG and other metabolic tracers is related to the degree of malignancy (the principle established by Warburg in the early part of the 20th century) the PET scan can be used to grade tumours and therefore indirectly provide information on prognosis (the so-called "metabolic biopsy").
• Staging disease: staging is documenting how widespread the cancer is in the patient. The PET scan has been show to be superior to anatomical methods of staging disease and therefore planning therapy. Examples include non-small cell lung cancer, lymphoma and oesophageal tumours.
• Residual disease: because purely anatomical methods for deciding on the viability of residual masses after treatment has been poor, metabolic imaging is proving extremely useful e.g., post-treatment mediastinal lymphoma masses and testic-ular abdominal masses.
• Detection of recurrences: good examples include the confirmation and site of recurrent colo-rectal cancer after surveillance blood testing has detected a rise in circulating tumour (CEA) markers.
• Measuring the response to therapy: it is often important to know how effective initial treatment has been in order to plan future therapeutic strategies. The best example is assessing response following the initial course of treatment of Hodgkin's lymphoma, when poor early response indicates that supplemen tary neo-adjuvant therapy may be necessary for the desired effect.
• To identify the site of disease: identifying the site of disease may be important to plan surgery e.g., for squamous cell cancers of the head and neck, to direct biopsy when the disease is heterogeneous, in soft tissue sarcomas, and to find the site of disease when the only sign may be a raised circulating tumour marker such as in thyroid cancer or ter-atomas.
• To identify the primary tumour when secondary cancers are present: it may be critical to discover the primary cancer when a patient presents with an enlarged lymph node, as in head and neck cancers where the primary tumour may be small, or alternatively when the presentation raises suspicion of a para-neoplastic syndrome.
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