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Tomografía por Emisión de Positrones

La PET  con 18-fluorodesoxiglucosa (18F-FDG-PET) es una técnica cuantitativa con la esperanza sustancial puesta en el estadiaje y evaluación de sarcomas de los tejidos blandos. La PET, que mide la actividad metabólica de la glucosa, proporciona una información funcional de la  lesión. Con esta técnica se puede descubrir extensión local insospechada o diseminación a distancia y suele preceder y complementar a las técnicas morfológicas de imagen (Rx, TAC, y RNM). Permite hacer cortes sagitales y transversales y con ello un mejor estudio loco-regional. En un informe, la sensibilidad de la PET Scan para el descubrimiento de sarcomas del tejido blando era del 100 por ciento.

En las lesiones malignas hay una acumulación de fluorodeoxigluosa por aumento en la glucólisis anaeróbica. Hay correlación entre el grado de malignidad y la tasa de consumo de FDG, por tanto la PET también  puede distinguir las lesiones benignas y las lesiones de alto grado; sin embargo, su capacidad para diferenciar lesiones benignas y de bajo grado a intermedio está limitada. La sensibilidad para los sarcomas de grado bajo parece ser menor que para intermedio o los tumores de grado altos. Además, la magnitud de la reducción en la actividad metabólica de la glucosa puede usarse para estimar el grado de necrosis inducido por la radiación y quimioterapia, proporcionando un método no invasivo de evaluar la respuesta. También se ha mostrado útil para predecir los tumores que van a responder bien o mal a la quimioterapia, así como para la detección de las metástasis y la distinción entre recidiva tumoral y fibrosis posquirúrgica.

Indicaciones de la PET

Establecimiento el grado histológico del tumor. A mayor grado histológico corresponde mayor captación de 18F-FDG. Sin embargo  las lesiones de bajo grado son difíciles de distinguir de las lesiones benignas.

Bibliografía

Comparison and discrepancy of 18F-2-deoxyglucose positron emission tomography and Tc-99m MDP bone scan to detect bone metastases. Anticancer Res 2000 May-Jun;20(3B):2189-92   (ISSN: 0250-7005)

Kao CH; Hsieh JF; Tsai SC; Ho YJ; Yen RF
Department of Nuclear Medicine, Taichung Veterans General Hospital, Taiwan.

The purpose of this study was to evaluate the usefulness of positron emission tomography with 18F-2-deoxyglucose (FDG-PET) for the detection of malignant bone metastases, and to compare FDG-PET results with conventional technetium-99m methylene diophosphate (Tc-99m MDP) bone scan findings. Twenty-four patients (10 females, 14 males, ages: 39-71 years) with biopsy-proven malignancy and suspected bone metastases, underwent whole body FDG-PET and bone scan to detect bone metastases. Bone metastases were established in 39 discordant bone lesions by histopathological examination of biopsy samples, MRI/CT, and follow-up bone scan/FDG-PET findings showing progressively and extensively widespread bone lesions. A total of 98 bone lesions found on either FDG-PET or bone scan were evaluated For 39 bone lesions with discordant findings between FDG-PET and bone scan, histopathological examination, MRI/CT and follow-up bone scan/FDG-PET findings revealed 8 metastatic and 0 benign bone lesions with positive FDG-PET findings, not detected on bone scan. Eleven metastatic and 20 benign bone lesions with positive bone scan findings were not detected on FDG-PET. FDG-PET has a better specificity, but a lower sensitivity for detecting malignant bone metastases when compared with bone scan.

Differentiating recurrent tumor from radiation necrosis: time for re-evaluation of positron emission tomography? [see comments]

AJNR Am J Neuroradiol 1998 Mar;19(3):407-13   (ISSN: 0195-6108)

Ricci PE; Karis JP; Heiserman JE; Fram EK; Bice AN; Drayer BP [Find other articles with these Authors]
Department of Neuroradiology, Barrow Neurological Institute, Phoenix, AZ 85001, USA.

Our purpose was to evaluate the ability of FDG PET to differentiate recurrent tumor from posttherapy radiation necrosis. METHODS: MR images, PET scans, and medical records of 84 consecutive patients with a history of a treated intracranial neoplasm were evaluated retrospectively. In all patients, recurrent tumor or radiation necrosis was suggested by clinical or MR findings. Metabolic activity of the PET abnormality was compared qualitatively with normal contralateral gray and white matter. RESULTS: PET findings were confirmed histologically in 31 patients. With contralateral white matter as the standard of comparison, the PET scan sensitivity and specificity were found to be 86% and 22%, respectively. With contralateral gray matter as the reference standard, the sensitivity and specificity became 73% and 56%, respectively. Overall, nearly one third of the patients would have been treated inappropriately in either scheme had the PET scan been the sole determinant of therapy. CONCLUSION: Our data suggest that the ability of FDG PET to differentiate recurrent tumor from radiation necrosis is limited. Both false-positive and false-negative PET scan results contributed to unacceptably low sensitivity and specificity values