An approach frequently used in imaging to an undetermined splenic mass is to first determine if they are cystic or solid and then characterizing them as solitary or multiple lesions [4]. Besides imaging findings, clinical features must be taken into account like left upper quadrant pain, symptoms of infection, immune status, history of known malignancy and white blood cell count [7].

Solitary cystic lesions are almost always benign and can be classified as parasitic or nonparasitic. Nonparasitic cysts are classified as primary true or secondary false cysts (determined by the presence or absence of an epithelial lining). Some examples are: epithelial cysts, pseudocysts and parasitic (echinococcal) cysts [3, 4].

Epithelial cysts are benign, congenital and rare. They are often asymptomatic but can cause symptoms if they grow large enough. They are solitary, unilocular, fluid-filled cysts at all imaging modalities [1, 3, 4] (Figure 1).

Pseudocysts, typically, arise from trauma or inflammation. Multimodality imaging shows their complex pathologic appearance with a thick fibrous wall (frequently containing calcification), internal debris and septa [1, 3, 4].

The most common parasitic splenic cyst is caused by Echinococcus granulosus. Hydatid cysts multimodality imaging appearances depending on their complexity: type 1 (a simple cyst), type 2 (daughter cysts), type 3 (mural calcifications) and type 4 (features of complications - rupture or superinfection) [1, 3, 4].

Multiple cystic lesions are also almost always benign and they can often cause symptoms. Examples of that are abscesses and lymphangiomas [3, 4].

Infectious lesions include bacterial, mycobacterial and fungal abscesses, especially in immunocompromised patients. They can also be found in trauma and emboli. In imaging, abscesses have ill-defined irregular margins with a central fluid component (presence of pus, hemorrhage or debris. Splenic pyogenic abscesses can be solitary or multiple and gas is often present and it is best documented at CT. Fungal (Figure 2) and mycobacterial abscesses are usually multiple tiny fluid lesions. Mycobacterial splenic involvement also occurs with splenomegaly and it is seen in the context of widespread disease [3, 4].

Lymphangiomas are rare benign malformations of lymphatic channels, often presenting as multiple cystic lesions and usually diagnosed in childhood. Multimodality imaging shows well-defined thin-walled unilocular or multilocular cysts with or without septa and calcifications. They are usually located close to the spleen capsule and range is variable [3, 4].

Solitary solid lesions include vascular and nonvascular lesions. The definitive diagnosis is difficult with just imaging but imaging can contribute to differentiate the benign lesions from the malignant ones. Benign lesions are almost always asymptomatic and malignant lesions are rarely isolated findings. Some examples are: hemangiomas, hamartomas, lipomas (Figure 3), infarcts, SANT (Sclerosing Angiomatoid Nodular Transformation) and angiosarcomas [3, 4, 6]. 

Hemangiomas are the most common benign tumors of the spleen, typically asymptomatic, and often discovered incidentally. Imaging characteristics of splenic hemangiomas are typically similar to hepatic hemangiomas (Figure 4). However, complicated splenic hemangiomas (presence of fibrosis, haemorrhage or cystic degeneration) can have variable multimodality imaging characteristics. Thus, dynamic contrast-enhanced CT or MRI are the best methods to evaluate them. Hemangiomas with atypical appearance on contrast imaging may mimic metastasis. FDG PET CT can be performed for lesions with atypical features, for patients at high risk for malignancy or metastatic disease [1, 3, 4, 6].    

Hamartomas are rare, benign nodular lesions composed exclusively of disorganized red pulp. At multimodality imaging, hamartomas are large and well-circumscribed solid vascular lesions with early arterial and persistent delayed enhancement [3, 4, 6] (Figure 5).

Splenic infarct is usually caused by thromboembolic conditions and global infarction may be caused by occlusion of the splenic artery. They usually appear as hypoechoic / hypoattenuating, wedge-shaped defects, extending to the capsule (Figure 6). They do not exhibit mass effect, Doppler signal or contrast enhancement [3, 5].

Angiosarcomas are the most common primary nonhematolymphoid malignant splenic tumor, but it is rare. Most occur after 40 years of age and the clinical features are innespecific with abdominal pain and other constitutional symptoms. Imaging features are heterogenous and non-specific and splenomegaly is frequent. Hemorrhage and cystic necrosis are common [3, 4, 6].

Lymphoma is the most common malignant tumor of the spleen. Primary splenic lymphoma (confined to the spleen and perisplenic lymph nodes) is rare (less than 1% of all lymphomas). Disseminated lymphomatous disease with spleen involvement is more common than primary splenic lymphoma [2] (Figure 7).  The clinical  features include constitutional symptoms of malignancy and patients can present with left upper quadrant pain and splenomegaly. The imaging methods show variable appearance depending on subtype: splenomegaly, tiny diffuse miliary nodules, small nodules or large solitary mass. The nodules are usually homogeneous and hypovascular [3, 4, 6]. 

Multiple solid lesions rarely are found incidentally because most patients will have symptoms and the splenic lesions will be part of systemic disease. Lymphoma, inflamatory lesions (sarcoidosis), Littoral Cell Angioma (LCA) and metastases are some examples [4, 6].

Metastases are rare in the spleen, usually part of widespread metastatic disease from primary sites like the breast, lung, ovarian, melanoma. and colon. Metastases can be cystic or solid and solitary or multiple [4, 6] (Figures 8 – 10).