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Abstract
Positron emission tomography, also called PET imaging or a PET scan, is a type of nuclear medicine imaging technique which produces a three-dimensional image or picture of functional processes in the body. PET is a major diagnostic imaging modality used predominantly in determining the presence and severity of cancers.
PET images demonstrate the chemistry of organs and other tissues such as tumors. A radiopharmaceutical, such as FDG (fluorodeoxyglucose), which includes both sugar (glucose) and a radionuclide (a radioactive element) that gives off signals, is injected into the patient, and its emissions are measured by a PET scanner.
Because PET measures metabolism, as opposed to MRI or CT, which ”see” structure, it can be superior to these modalities, particularly in separating tumor from benign lesions, and in differentiating malignant from non-malignant masses such as scar tissue formed from treatments like radiation therapy. PET is often used in conjunction with an MRI or CT scan through ”fusion” to give a full three-dimensional view of an organ and the location of cancer within that organ. The newest PET scanners are a combination of PET and CT devices that provide the important metabolic information from PET superimposed on the high-quality anatomic information from CT.
Because the two scans can be performed in immediate sequence during the same session, with the patient not changing position between the two types of scans, the two sets of images are more-precisely registered, so that areas of abnormality on the PET imaging can be more perfectly correlated with anatomy on the CT images.
PET scanning with the tracer fluorine-18 (F-18) fluorodeoxyglucose (FDG), called FDG-PET, is widely used in clinical oncology. This tracer is a glucose analog that is taken up by glucose-using cells and phosphorylated by hexokinase (whose mitochondrial form is greatly elevated in rapidly-growing malignant tumours).
Positron Emission Tomography with fluorine 18 deoxyglucose (FDG) is gaining importance in the staging and evaluation for recurrence in all cancers of the female reproductive cancers. It has been found to be useful in ovarian, cervical and endometrial cancer in this capacity. Its utility lies in assessing local disease, but more importantly its ability to detect distant disease.
Cervical cancer is the second most common cancer in most (80%) cases occur in resource-poor countries that have no effective screening programmes.
In cervical cancers FDG PET is very useful due to the high accumulation of FDG within the cancers .the role of FDG PET in cervical cancer for both staging and evaluating for recurrence .PET has proven useful in detecting abnormal para-aortic nodes, which has prognostic significance for disease course.
PET is the most useful test for staging or re-staging cervical cancer because it is more accurate than CT or any other test. Before PET, it was extremely difficult to monitor patients to see if the cervical cancer had spread. Other imaging tests might not see the cancer as sensitively as PET, which could result in a delay of further treatment. Imaging with PET to look for recurrence is critical to find it at its earliest stage .PET can also be used to image tumor response to therapy. After surgery and other treatments, PET is extremely important to monitor and see if the cancer cells have returned and help determine if treatment should be re-started. PET gives us another tool to evaluate whether a patient has metastatic disease before we operate, and it can help us avoid surgery if it is not appropriate. It’s still not 100 percent accurate, but it’s the best tool available to help in determining which patients need surgery and which patients need radiation therapy
The integrated PET/CT may be an effective means of evaluating the lymph node status of patients with early-stage cervical cancer. With PET/CT, the capability to differentiate malignant nodes, which manifest as foci with high FDG uptake at PET, is not compromised by the use of morphologic size criteria. Consequently, PET/CT enables one to detect and localize metastatic lymph nodes that are not enlarged (i.e., smaller than 1 cm in short-axis diameter) and thus overcome the limitations of the size-based characterization used with morphologic imaging modalities.
The ovarian carcinoma has the highest mortality rate of any gynecological malignancy, primarily due to late detection. PET has limited value in lesion localization in early stages of ovarian cancer, but plays significant role in identifying recurrent tumour in patients with rising tumour marker. FDG-PET has been shown to be highly sensitive and specific in staging of ovarian cancer. So FDG-PET might be considered in patients with ovarian cancer for the following indications: rising CA-125 and negative CT; preoperative assessment for recurrent ovarian cancer, especially when CT findings are equivocal; and monitoring response to chemotherapy.
One of the problems of a PET study is the false-positive and false-negative results. False-positive results were observed with nonpathologic uptake in the alimentary and urinary tracts. In addition, a number of benign gynecological diseases, such as mucinous cystadenomas, endometrial and follicular cysts, functional corpus luteum cysts, salpingooophoritis, fibromas, cystadenofibromas, teratomas, dermoid cysts, endometriosis, tubo-ovarial abscesses and benign thecoma, may lead to false positive results. Another problem is false-negative results for small lesions, such as the lesions in peritoneal dissemination; in other words, PET can miss poorly localized microscopic spread of disease. Also, well differentiated serous/mucinous cystadenocarcinoma and borderline tumors, are common causes of false negative results because the ovarian tumors that are cystic or mucinous often do not accumulate significant FDG, further reducing detection .
The recent introduction of PET-CT devices should significantly enhance the diagnostic accuracy of metabolic imaging by reducing the number of false positive results as well as by improving the anatomic localization of 18FDG-avid pathological sites.
Endometrial cancer is both the most common type of uterine cancer and the most common cancer of the female reproductive system.
The role of positron emission tomography (PET) in endometrial cancer imaging is still under investigation; FDG PET is particularly useful in the setting of post-radiation and post-surgical changes, which may reduce the sensitivity of conventional imaging such as CT, MRI or ultrasound.In detecting lymph node involvement by tumor, PET performs with accuracy (95%) comparable to that of CT or MRI.
However, because 45% of endometrial cancer is stage I and not FDG-avid, the reported improved sensitivity of PET (60%-86%) is only true for nodes >1 cm. Metabolic imaging was moderately sensitive in predicting lymph-node metastases from endometrial cancer. Therefore, 18FDG PET should not replace lymphadenectomy.
The PET images show focal intense accumulation of 18F-FDG consistent with the uterine mass. Although 18F-FDG uptake by tissue is a sensitive indicator of a malignant tumor, identifiable focal 18F-FDG uptake is sometimes seen in benign uterine tumors. Leiomyoma of the uterus is the most common uterine benign neoplasm and often shows moderately intense 18F-FDG accumulation.
The new positron emission tomography with F18 17β-estradiol (FES-PET) has the potential to provide functional information about the hormone responsiveness of well-differentiated endometrial adenocarcinoma. When serial FES-PET imaging was donning on a woman with well-differentiated adenocarcinoma treated with medroxy progesteron acetate (MPA), a decrease in FES-PET uptake was seen after a therapeutic response. FES-PET is a new way to evaluate ER activity in endometrial adenocarcinoma and it provided more useful information than did FDG-PET.
Combined 18F-FES and 18F-FDG PET scans would be a useful diagnostic tool which provides supplementary information for differential diagnosis of endometrial tumors.