What is the role of FDG-PET scans for thoracic non-Hodgkin lymphoma (NHL) imaging?

Updated: Mar 05, 2019
  • Author: Ali Nawaz Khan, MBBS, FRCS, FRCP, FRCR; Chief Editor: Eugene C Lin, MD  more...
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Several studies have shown the superiority of FDG-PET scanning over 67Ga scanning in the initial diagnosis of HD and NHL, as well as in the diagnosis of recurrent HD and NHL. As many as 100% of patients with a positive PET scan after chemotherapy have an early relapse, while in more than 80% of patients with a negative PET, long-term remission occurs. Most studies indicate that FDG-PET scanning results are significantly correlated with patient outcome, whereas the correlation for CT scanning is poorer.

FDG-PET scanning does not rely entirely on lymph size, as do anatomic imaging methods, but is dependent on levels of metabolic activity, which can be high enough to visualize even some small lesions. CT scanning often cannot be used to differentiate residual disease and posttherapeutic fibrosis. On the basis of a number of clinical results, FDG-PET is gaining increasing importance for staging, restaging, and monitoring treatment responses in malignant lymphomas.

FDG-PET scanning has high sensitivity for the detection of nodal involvement in HD and NHL and generally detects activity in all abnormal lymph nodes identified on CT scans. FDG-PET scanning also has the advantage of detecting additional nodal disease, which might be missed on CT scans because of size criteria; this is particularly true for small mesenteric lymph nodes. The sensitivities reported for FDG-PET scanning in the detection of nodal disease are 62-100%; the variability is related to the fact that FDG-PET scanning's sensitivity differs according to the particular histologic grade of lymphoma that is present. Despite this range of sensitivities, FDG-PET scanning consistently has higher accuracy than CT scanning in the staging of lymphoma.

In a retrospective study of 56 patients (42 with mediastinal Hodgkin lymphoma and 14 with primary mediastinal B-cell lymphoma) to assess differences in clinical, laboratory, and FDG PET/CT metrics, lactate dehydrogenase (LDH) levels and several FDG PET/CT findings (tumor size, presence of necrosis, and degree of FDG uptake) were found to be helpful in discriminating mediastinal Hodgkin lymphoma from primary mediastinal B-cell lymphoma (PMBCL). [31]

However, another reretrospective study of FDG PET to assess response to therapy in 36 consecutive patients treated for PMBCL found that a positive interim FDG-PET/CT did not reflect persistence of active disease in the vast majority of PMBCL cases. The relapse rates were similar regardless of interim FDG-PET/CT results and interpretation criteria. [32]  In NHL, extranodal disease generally worsens the prognosis. FDG-PET scanning detected 57% more extranodal sites of disease than can CT scanning.

A small study at 2 treatment sites found that coregistering FDG-PET and CT scans on treatment planning for lymphoma patients yielded changes in management, volume definition, and normal tissue dosimetry for a significant number of patients. [4]

F18 FDG-PET uptake variability within the liver and mediastinum during chemotherapy should be taken into account when this parameter is used to score the interim PET scans and to make decisions in defining response-adapted therapeutic strategies. The liver uptake increases in the interim period in comparison to a baseline study. Conversely, the stability of mediastinal uptake activity during therapy provides a more reliable benchmark for the response assessment. Finally, the intersubject’s variability of both parameters should be considered when point score models perform the visual evaluation of the interim PET. [16]

A study by Albano et al found that pulmonary MALT lymphoma is 18F-FDG avid in most cases, 18F-FDG avidity is correlated with tumor size, and single or multiple areas of consolidation are the most common pattern of presentation of lung MALT lymphoma on CT. [2]

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