Therapy-Induced Splenic Activation (“Physiologic Activation of the Spleen”)
As previously discussed, the normal spleen typically demonstrates mild diffuse metabolic activity – less than or equal to the uptake within the liver.
A significant percentage of patients presenting for PET/CT will have recently received chemotherapy and/or colony stimulating factors (CSF), both of which can cause significant diffuse increased metabolic activity of the spleen (and bone marrow, discussed here).
Associated splenomegaly is not uncommon.
The effects of such therapy on the spleen typically resolve within one month, but can occasionally persist for up to 12 weeks.
Mild diffuse splenic uptake can also be seen as the result of anemia and infection.
Interpretation:
- Before calling splenic uptake “physiologic activation”, one must confirm a history of recent chemotherapy and/or colony stimulating factors.
- The uptake must be diffuse and not accompanied by lesions on the CT images.
- If uptake is focal, or if the uptake is accompanied by identifiable lesions on the co-registered CT images, malignancy must be strongly suspected.
- As splenic activation and reactive marrow typically occur together, the presence of co-existing reactive marrow lends great confidence to the radiologist’s decision to call an enlarged and intensely avid spleen normal physiologic activation (especially in a lymphoma patient).
Case 1
Case 1 (Image 2)
Normal MIP appearance of the spleen. The metabolic activity of the spleen (white circles) should be less than or equal to liver uptake (black circles).
Case 1 (Image 3)
Normal fused coronal PET/CT appearance of the spleen. In each case, the metabolic activity of the spleen is less than or equal to liver uptake.
Case 2
Case 2 (Image 2)
Axial images pre- and post-therapy demonstrate splenic activation secondary to recent chemotherapy.
Case 2 (Image 3)
Coronal images pre- and post-therapy demonstrate splenic activation secondary to recent chemotherapy. Such activation can result in splenomegaly.
Case 3
Case 3 (Image 2)
Axial image post-therapy demonstrates diffuse intense splenic uptake consistent with physiologic activation of the spleen.
Case 3 (Image 3)
Coronal image post-therapy demonstrates diffuse intense splenic uptake consistent with physiologic activation of the spleen. Before reporting this diagnosis, however, it is imperative to carefully confirm a history of recent chemotherapy and/or colony stimulating factor administration.
Case 4
Case 4 (Image 2)
Diffuse splenic and bone marrow uptake in a lymphoma patient who has not yet received treatment with chemotherapy or CSF is consistent with active disease.
Case 5
Case 5 (Image 2)
MIP and axial images in this patient presenting for initial staging for lymphoma clearly demonstrate splenomegaly and diffuse increased metabolic activity of the spleen (uptake greater than liver uptake). As there is no history of recent chemotherapy and/or colony stimulating factors, the appearance is consistent with active lymphomatous involvement of the spleen.