الفهرس 
Physical Background and Technical Aspects of PET/CTOnly 14 pages are availabe for public view
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Abstract
L
ymphoid neoplasms are broadly divided into Hodgkin disease (HD) and non-Hodgkin’s lymphoma (NHL). Non-Hodgkin lymphoma accounts for about 5% of all cases of cancer with greater tendency to extend to extra-nodal sites.
Aggressive NHL usually detected by nodal and or extra-nodal involvement and although it is an aggressive lymphoma, many cases show complete remission explained that it is a chemo-sensitive tumor.
Most radiologic procedures map the anatomy and morphology of tumors with little or no information about their metabolism. In recent years, imaging with positron emission tomography (PET) for tumor staging and therapy control has been introduced. Rather than anatomical information, it provides physiologic information on glucose uptake and metabolism. The main limitation of PET in tumor imaging is mainly complete absence of anatomical landmarks, which is obscured the localization of lesions.
The major advantage of PET/CT is that both metabolic and anatomic information for the same body region, which are obtained at the same time. The PET and CT devices are physically aligned and the PET/CT viewer software allows close correlation of the images.
Combined PET/CT facilitates the separation of normal physiologic uptake from pathologic uptake, provides precise detection of functional abnormalities, and decreased the incidence of false-positive and false-negative imaging studies. The imaging time for a whole-body scan is also markedly reduced, enhancing patient comfort and convenience.
Our data are in agreement with the previous studies has demonstrated that PET/CT is the technique of choice and irreplaceable tool for patients with lymphoma used in:
• Initial diagnosis and staging: as pretreatment staging determines the extent of disease and helps direct therapy. PET/CT is highly sensitive in detecting nodal and extra-nodal involvement, it play an important role in staging of lymphoma and this is attributed to the detection of FDG-avid normal-sized lymph nodes (usually <1 cm), and of extra-nodal sites that were previously missed at CT (most commonly the liver, spleen, cortical bone and bone marrow).
• Follow up and detection of recurrence: positive PET-CT findings of residual lesion activity suggest aggressive lymphoma. A long period of disease-free survival can be expected in NHL patients with negative PET-CT findings, whereas early relapse will occur if positive PET-CT findings are seen after standard chemotherapy. Early detection of disease with frequent follow-up is believed to have an important effect on outcome as early therapy for recurrent disease is more effective than delayed therapy.
• Response assessment after end of treatment: masses mostly don’t diminished completely in size after adequate (curative) treatment because of fibrosis and necrotic debris. It has been demonstrated that 18F-FDG PET/CT has greater role for therapy response assessment due to its capacity to help distinguish between residual metabolically active tumor and areas of necrosis and fibrosis, thus identifying which of these patients have achieved satisfactory functional remission and which that needs further treatment.
The use of diagnostic contrast enhanced CT with PET/CT although it expose the patients for more radiation risks, it helps in better anatomical localization, sometimes reveals some important but unrelated (medical or surgical) issues to the patients, in addition it constitute a base line exam for further response assessment and follow up if PET/CT is not affordable and only CT will be used.
Combined PET/CT using 18F-FDG is the best oncologic imaging modality at present time with indispensable role and valuable application in management of lymphoma. It is very efficient with least possible pitfalls and false results compared
to either of its component alone. It is taking its place as the standard imaging modality for lymphoma assessment providing a new vision to management and treatment plan.
In response to developments involving PET-CT, recommendations from the ICML imaging group have been made to update practice. These include guidance on reporting of PET-CT for staging and response assessment using the Deauville criteria. PET-CT is recommended for interim assessment in place of CT alone, if imaging is clinically indicated, and for remission assessment.
Although Deauville criteria has specific qualitative criteria for response based on a visual activity of the lesion, collection of additional data as SUV is recommended so as to develop a database suitable for additional information to refine the response metrics for a given tumor and therapy and as potential prognosticators. However, several studies recommend total lesion glycolysis for later analysis.
Response assessment according to the Deauville criteria classification appears in discordance with IHP classification especially in response assessment and intended to represent a practical framework used for interim PET CT follow up of NHL.
• PET/CT is the technique of choice for patients with Non Hodgkin lymphoma as It has been demonstrated that 18F-FDG PET/CT is extremely useful tool for therapy response assessment especially for those patients with residual disease.
• Response assessment according to the Deauville criteria classification appears to represent a practical frame work used for proper assessment of response after treatment of NHL patients and PET/CT as a tool of assessment of response can discriminates between active lesion and non-active necrotic or fibrotic lesions.
The present study had some limitations and challenges:
• Pathological confirmation was not available for most of the residual lesions, for which diagnosis was based on imaging criteria, Clinical /laboratory data and follow-up.
• Small sample size of post treatment patients so our results in this group need to be further evaluated on a larger sample size.
• The proposed five-point scale is a visual measure of FDG PET/CT response assessment; the risk of a degree of subjectivity in the interpretation of FDG avidity has not been eliminated.