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Lymphoid 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 predilection to disseminate to extra-nodal sites.
Extranodal lymphoma describes that there is neoplastic proliferation at sites other than the expected native lymph nodes or lymphoid tissue. The observed rising incidence of NHL and Hodgkin disease (HD) in the past two decades has been characterized by a marked increase in the occurrence of extranodal lymphoma. Lymphomas that initially appear to have the bulk of the disease at extranodal sites are described in primary extranodal lymphoma and categorized as Stage I or II. In secondary extranodal lymphoma, there is secondary involvement of the extranodal sites from primary nodal disease, which is categorized as Stage III or IV.
18F labeled fluoro-deoxy-glucose (18F-FDG) is the most commonly used PET radiotracer for oncologic imaging. 18F-FDG PET provides valuable metabolic information based on the fact that malignant cells exhibit high uptake of FDG, as an analogue of glucose, due to increase of their metabolic rate. But FDG, by the same mechanism, will accumulate in other tissues with increased metabolic rate secondary to a wide range of benign etiologies (e.g. infectious and inflammatory processes). In addition, normal tissues may also accumulate FDG.
The normal distribution and physiologic variants of 18F FDG uptake in is important that they be recognized so as to avoid misinterpretation.
PET/CT, the most common form of hybrid imaging, has transformed oncologic imaging and is increasingly being used also for non-oncologic applications.
Combined PET/CT facilitates the separation of normal physiologic uptake from pathologic uptake, provides accurate localization of functional abnormalities, and reduces 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.PET/CT has proved invaluable as an imaging tool for the staging, restaging, and response to therapy.
PET/CT is accurate for baseline staging and yields important prognostic information for determining the most appropriate initial treatment. Used for evaluation of treatment response, PET/CT can depict residual viable malignant lesions with greater accuracy than can other imaging techniques.
Our data and in agreement with the previous studies has demonstrated that PET/CT is the technique of choice and indispensable 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 tends to upstage 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 often do not regress completely after adequate (curative) treatment because of fibrosis and necrotic debris. It has been demonstrated that 18F-FDG PET/CT is extremely useful 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.
In conclusion, Combined PET/CT using 18F-FDG is the best oncologic imaging modality at present time with indispensable role and valuable application in monitoring and management of the extranodal lymphoma. It can detect metabolically active lesions without CT structural changes and identify a viable tumor in normal size lymph nodes. PET/CT is effective than CT with enhanced contrast in evaluating extranodal Lymphomatous infiltration, especially in spleen, bone, and bone marrow.