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Spleen on FDG PET/CT
When confronted with a focal splenic lesion, knowledge of the differential diagnosis for splenic masses is valuable ( Fig. 13.1 ). For malignancies of the spleen, lymphoma would be most common, whereas metastases are relatively uncommon and primary splenic malignancies are rare. Benign splenic lesions encountered on fluorodeoxyglucose positron emission tomography/computed tomography (FDG PET/CT) are common and include infections, sarcoidosis, and splenic repopulation.
Differential Diagnosis for Splenic Lesions.
Axial PET, noncontrast CT, and fused PET/CT through the spleen demonstrate a focus of FDG avidity within the spleen (arrows) , without CT correlate. Differential diagnosis of FDG-avid splenic lesions on FDG PET/CT includes malignancies (lymphoma, metastasis, angiosarcoma) and benign etiologies (infections, sarcoidosis, splenic repopulation). This patient has a diagnosis of lymphoma, and the splenic focus is presumed lymphoma.
Splenic Lymphoma
Lymphoma is by far the most common malignancy to involve the spleen. Splenic lymphoma may manifest as splenic enlargement (splenomegaly) or solitary or multifocal lesions. FDG avidity of lymphoma ranges from no apparent FDG avidity to markedly FDG avid. The extent of FDG avidity in lymphoma tends to correlate with pathologic rate. Lymphomas with maximum standarized uptake values (SUV) greater than 20 are most often high-grade lymphomas ( Fig. 13.2 ), whereas lymphomas with maximum SUV values less than 10 are most often low grade. This can be valuable in screening patients with low-grade lymphoma for transformation to higher-grade malignancy. Highly FDG-avid lesions would be the lesions to target for evaluation of possible high-grade malignancy. Low-grade lymphoma made be incidentally discovered during FDG PET/CT performed for another malignancy ( Fig. 13.3 ). If the FDG avidity of low-grade lymphoma nodes is low enough, they may not be readily apparent on the FDG PET images and must be appreciated by the enlarged lymph nodes on the corresponding CT images.
As with other organ systems, FDG PET provides valuable information about response to therapy in splenic lymphoma, if the initial lymphoma was adequately FDG avid. For example, the patient in Fig. 13.3 with incidentally discovered low-grade lymphoma demonstrates only minimally FDG-avid malignancy and would not be appropriate for using FDG PET to track treatment response. If the initial lymphoma is adequately FDG avid, Lugano Criteria can be followed for evaluation of treatment response. In essence, reduction of FDG avidity to less than liver background represents a complete response to treatment, whereas residual FDG avidity greater than liver background is suspicious for residual active lymphoma.
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