PET/CT: Clinical role in lymphomas.

¹⁸F-FDG PET/CT is the imaging modality of choice for the accurate initial staging of most lymphomas. Hodgkin's, Diffuse Large B-cell and follicular lymphomas show avid FDG uptake, while a minority of Non-Hodgkin lymphoma subtypes namely MALT, marginal and small lymphocytic lymphoma demonstrate low or moderate avidity. As a rule of thumb, indolent lymphomas show lower FDG activity than aggressive ones. PET/CT has increased sensitivity in the detection of nodal involvement even in small or normal-sized nodes. It shows higher sensitivity than CT in the detection of extra-nodal disease, most often in the spleen and bone marrow. PET/CT leads to upstaging in up to 25% of Hodgkin lymphomas, paving the way to intensified therapy. It has excellent Negative Predictive Value (NPV>95%) in the detection of bone marrow involvement in Hodgkin's rendering bone marrow biopsy not absolutely necessary: a negative PET rules out bone marrow disease in Hodgkin's patients, yet this does not universally apply in Non-Hodgkin lymphomas. PET/CT is superior to other imaging modalities in the initial stating of aggressive Non-Hodgkin lymphomas detecting disease in previously not suspected or occult sites. ¹⁸F-FDG PET/CT is applied in the early therapeutic evaluation of Hodgkin's by means of interim PET performed after 2-3 initial cycles of chemotherapy. Patients with negative interim PET and no hypermetabolic disease identified may continue with the same effective treatment or switch to less aggressive therapy. On the other hand, patients who do not show PET response may be subjected to more intensified treatment to eradicate hypermetabolic active disease. Randomized controlled trials have proven that interim PET/CT shows high NPV for final treatment response and for increased progression free survival in Hodgkin's. The accuracy in reporting and interpretating interim and post-treatment PET/CT has increased by applying specific objective criteria: Deauville 5-score scale. Deauville's uptake scores of 4-5, more intense than liver activity, correspond to active lymphomatous disease. ¹⁸F-FDG uptake in lesions, equal or lower than mediastinal blood-pool, is interpreted as negative: Deauville scores of 1-2. Role of interim PET is also investigated in Non-Hodgkin Lymphomas, especially nowadays with more effective treatments being applied. PET/CT is highly recommended for post-treatment assessment of lymphomas with excellent NPV and superior diagnostic accuracy compared with CT. After treatment, a significant proportion of patients show residual anatomic lesions on CT f.e. residual mediastinal soft-tissue; yet, in the minority of cases, these lesions correspond to active disease. PET/CT has high diagnostic accuracy in the assessment of residual tissue and may distinguish between PET-negative fibrotic or necrotic tissue and PET-positive, active residual disease. The modality also has high NPV in the evaluation of megatherapy before stem cell transplantation: a favorable PET response is associated with better progression free survival and overall survival. To sum up, PET/CT has evolved as an established method in lymphoma patients being incorporated into clinical algorithms and guidelines altering therapeutic decisions.