What causes false positives and negatives in nuclear imaging of thoracic non-Hodgkin lymphoma (NHL)?

Updated: Mar 05, 2019
  • Author: Ali Nawaz Khan, MBBS, FRCS, FRCP, FRCR; Chief Editor: Eugene C Lin, MD  more...
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Sarcoid anterior mediastinal lymphadenopathy may also uptake 67Ga. Ga-67-avid sarcoid disease is reported in over 90% of cases of pulmonary involvement. A lambda pattern of uptake in the parahilar, infrahilar bronchopulmonary, and mediastinal lymph nodes has been described in 72% of patients with intrathoracic sarcoidosis. Accumulation of 67Ga is a sensitive but nonspecific indicator of active inflammation in patients with sarcoidosis. Ga-67 scintigraphy is also useful in identifying extrathoracic sites of involvement, detecting active alveolitis, and assessing response to treatment.

Ga-67 uptake in the thoracic lymph nodes, the lungs, and the salivary and lacrimal glands is particularly suggestive of sarcoidosis. How well the extent of 67Ga uptake in the lung correlates with the degree of alveolitis is controversial. However, 67Ga scans may be useful as a baseline study at the time of diagnosis; if the results from 67Ga scintigraphy are initially positive, negative findings from a subsequent 67Ga scan obtained during the course of treatment suggest that alveolitis has resolved. In such a patient, 67Ga may be a useful marker for disease in activity and response to therapy.

Ga-67 uptake may also occur in infections and granulomatous diseases.

Because of gut activity, thallium imaging is not useful for the evaluation of abdominal or pelvic disease.

Inaccurate staging of lymphomas and a false-positive diagnosis may occur as a result of FDG uptake in association with nonspecific inflammatory lymph nodes. Lack of increased metabolic activity within enlarged nodes has been shown to correlate well with lack of tumor involvement. However, nodes and lesions smaller than 1 cm may lack sufficient metabolic activity and potentially can cause a false-negative result. The same size criteria also apply to cross-sectional imaging of nodes smaller than 1 cm, which can again lead to false-negative findings.

Persistent FDG uptake within a residual mass should prompt strong consideration of additional therapy, although occasionally the uptake may be related to thymic hyperplasia or a histiocytic reaction.

Several sites demonstrate normal physiologic activity on FDG-PET scans, and these may contribute to false findings. FDG-PET scanning can be degraded by physiologic activity that masks lesions, although this appears to be less of a problem with this modality than it is with 67Ga imaging.

Although the normal intense FDG-PET scan brain activity is not relevant to thoracic imaging, it is important overall for staging NHL. This activity may obscure a brain lesion, but it is not a severe limitation in FDG-PET scanning, because standard staging with CT scanning does not usually include the brain. Cardiac uptake may interfere with thoracic FDG-PET scanning, particularly in a postprandial state, when cardiac activity is at its maximum. Therefore, patients should fast to minimize myocardial uptake and maximize thoracic uptake. Urinary excretion of FDG may obscure abnormalities in the abdomen. Faint bowel activity can also be seen on FDG studies, but it is generally not enough to cause confusion with actual disease.

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