Applied Radiology

Prepared by Shirish P. Patel, MD; Thomas B.Kinney MD; Anne C. Roberts, MD; of the Department of Radiology; and Anna Dreilenger, MD of the Department of Pathology; University of California at San Diego Medical Center, San Diego, CA.

CASE SUMMARY

A 47-year-old college professor with no significant past medical history developed the acute onset of pain in the left upper quadrant. He had no associated fever, nausea, vomiting, GI symptoms, or weight loss. Physical examination revealed mild tenderness to palpation in the left upper quadrant. Laboratory values showed mild anemia with a hemoglobin of 12.3 and a hematocrit of 35.8. The patient's serum chemistries, liver function tests, and platelet levels were all within normal limits. An abdominal ultrasound and abdominal CT revealed a splenic mass (figures 1 and 2). A CT-guided needle biopsy of the splenic lesion was non-diagnostic and the patient underwent splenectomy.

DIAGNOSIS

Primary splenic hemangiopericytoma

IMAGING FINDINGS

The sonographic images demonstrate a complex lesion measuring 5.6 cm in greatest dimension within the superior portion of the spleen adjacent to the left hemidiaphragm. CT imaging shows the mass to have an enhancing rim with areas of lower density internally. The mass lies above the splenic hilum, and is posterior and lateral to the stomach, causing compression along the greater curvature. No other significant intra-abdominal pathology was identified.

At the time of surgery, the splenic mass was noted to be adherent to the stomach and diaphragm. Additionally, multiple dense adhesions of the omentum were observed in the left upper quadrant. The adhesions were lysed and a splenectomy performed. Pathologic examination revealed a spleen weighing 240 grams and measuring 13 x 11 x 5 cm. A multinodular mass at an area of surface disruption of the spleen was found and measured 4.5 x 4.0 x 2.0 cm. Microscopic evaluation revealed diffuse proliferation of spindle cells. The tumor cells showed mild to moderate nuclear atypia and high mitotic activity. Large areas of necrosis and hemorrhage were also identified. One lymph node was recovered and showed no evidence of neoplasm. The tumor cells were positive for vimentin and negative for keratin, S-100 protein, HMB-45, actin, and desmin. Additional immunostains also revealed that the tumor cells were positive for CD34. Given this constellation of findings, a diagnosis of hemangiopericytoma was made.

DISCUSSION

This case provides many interesting points of discussion. Initially, the choice was made to proceed with fine-needle aspiration to provide a diagnosis. Percutaneous needle biopsy of the spleen has been shown to be a safe method for obtaining tissue samples. In the past, clinicians were reluctant to perform this procedure due to the splenic vascularity and difficulty in safe access because of the adjacent colon and lung pleura. Quinn et al ¹ reported a series of 35 percutaneous interventions of the spleen including percutaneous biopsy, therapeutic and diagnostic fluid aspiration, and catheter drainage of abscesses in which they encountered no complications. Biopsies in their series were performed using 22g or 20g needles. In our case, a 20g needle was used and three passes were made under CT guidance. No complications were encountered. The potential benefit of needle aspiration biopsy is spleen preservation in certain instances. In this case, the aspirate consisted of necrotic material, and tumor could not be ruled out. Because of these considerations, the decision was made to proceed with splenectomy.

Due the rarity of focal splenic lesions, it may be useful to review the differential diagnosis of this important entity. The differential diagnosis is wide and includes posttraumatic, infectious, vascular, and neoplastic categories.

Different imaging methods are available to assess focal splenic lesions, although there is not one clearly superior imaging modality. Scintigraphy will show intrasplenic masses as areas of decreased uptake, but no distinction is possible between solid and cystic structures. CT can image lesions accurately and reveal their relationship to surrounding organs. However, splenic masses with Hounsfield unit measurements similar to that of normal splenic parenchyma may be missed. Additionally, the sinusoidal circulation of blood through the spleen causes heterogeneity to its appearance with IV contrast administration. Sonography allows differentiation of solid from cystic masses, but no sonographic criteria yet exist to reliably distinguish benign from malignant solid lesions. Also, cyst-like lesions can result from multiple different causes that often cannot be differentiated with ultrasonography.

Splenic cysts belong in a distinct subcategory with a more limited differential diagnosis. Splenic cysts are relatively rare compared with solid lesions. They may be classified as true cysts, in which the cyst is lined with specific secreting membrane or has a parasitic lining. On a worldwide basis, hydatid cysts from echinococcal infection are thought to represent two thirds of all splenic cysts. ² In North America, however, this entity is exceedingly rare. Patients are not usually febrile unless secondary infection of the cyst has developed. Treatment of these hydatid cysts is by surgical removal. The other type of true cyst is a congenital epithelial cyst. Histologically, these cysts have an epithelial lining and one theory is that they originate from peritoneal mesothelial cells that have become trapped within the splenic parenchyma during development. Another general type of splenic cyst is the pseudocyst, which is the most common type of splenic cyst in the United States and occurs most commonly after trauma. These pseudocysts are thought to represent the final stage of evolution of splenic hematomas. Histologically, they are not lined by epithelium. When encountering a primarily cystic splenic mass, other causes to consider are abscess, cystic neoplasm, and intrasplenic pancreatic pseudocyst.

In a significant number of patients with blunt abdominal trauma, splenic rupture and hematoma formation occur. The echogenicity of splenic hematomas varies depending on the stage of evolution. Soon after trauma, these lesions tend to appear as hypoechoic intraparenchymal lesions. A "double contour" sign may indicate a subcapsular component of the hematoma. ³ When the hematomas become chronic, they tend to become more cystic.

Another possibility in the differential diagnosis of focal splenic lesions is infection. The main predisposing factor for splenic infection is immunosuppression. Because of the increasing number of immunocompromised patients secondary to AIDS, posttransplant immunosuppression drugs, and cancer chemotherapy treatments, splenic infections and abscess formation have become more prevalent over the past few decades. Other predisposing factors for infection include diabetes, sickle cell anemia, and parenchymal disruption from trauma or infarction. Infections in the spleen are most commonly caused by staphylococcus and streptococcus, but fungal infections also constitute a good percentage of cases. Although it is difficult to determine bacterial versus fungal etiologies based on imaging characteristics alone, one study has shown that unilocular abscesses have a bacterial etiology in 94% of cases. ⁴ This same study also found that if the abscess is multilocular, a fungal etiology is the cause 64% of the time. Splenic infection usually occurs by hematogenous seeding from a primary source elsewhere in the body. Another, more rare route of infection is by direct spread from adjacent organs.

Splenic infarcts can occur secondary to thromboembolic disease, arteritis, myeloproliferative disease, pancreatitis, and sickle cell anemia. Not all splenic infarcts appear as the classic wedge-shaped sharply marginated lesions described in the literature. They can also appear as multiple, poorly marginated lesions indistinguishable from the other multiple etiologies of focal splenic lesions. Patients with sickle cell anemia usually have repeated episodes of splenic infarction, leading to the so-called auto-splenectomy syndrome. These same patients can also develop acute splenic sequestration crisis, which is characterized by sudden, massive splenic enlargement accompanied by an abrupt decline in hematocrit levels.

The spleen is not an organ prone to the development of tumors and this is thought to be mainly due to immunologic factors and its characteristic blood circulation. Neoplastic lesions of the spleen can be divided into three main categories: lymphoma, metastases, and non-lymphomatous primary splenic neoplasms. The spleen usually is involved secondarily in lymphoma of both the Hodgkin's and non-Hodgkin's subtypes. Primary lymphomas of the spleen are rare, making up 1% to 2% of cases of lymphomatous involvement of the spleen, but the overall prevalence of lymphoma makes this the most common primary splenic neoplasm. Splenic involvement in lymphoma can take several forms, including homogenous enlargement, miliary nodules, multifocal lesions, and a solitary mass. The lesions can also differ widely in their CT and ultrasound appearances, so the diagnosis of splenic lymphoma is difficult to make by imaging alone. Although splenic lymphoma is most often confined by the splenic capsule, local extension with invasion into adjoining structures has been reported.

Metastatic lesions in the spleen occur most commonly by hematogenous spread and are almost always seen only in patients with widespread carcinoma involving three or more organs. The most common primary sites of splenic metastases are breast and lung, although on a percentage basis, melanoma has the highest frequency of splenic involvement with one series showing 34% of melanoma patients have splenic metastasis at autopsy. ⁵ Splenic metastases most frequently appear as multiple nodules, although diffuse infiltration can occur, making the imaging diagnosis difficult.

Neoplasms arising from the mesenchymal components of the spleen are even more rare. Vascular neoplastic proliferations account for the majority of these lesions. Included in the spectrum of vascular proliferations are localized and diffuse hemangiomas, lymphangiomas and lymphangiomatosis, hemangioendothelioma, Kaposi's sarcoma, angiosarcoma, and hemangiopericytoma. Fewer than 100 of all these types of neoplasms of the spleen have been reported in the literature since Langhans first described a case of primary splenic angiosarcoma in 1879. ⁶ Hemangiopericytomas are even more rare with only a handful of cases reported since Jurado first described this tumor in the spleen. ⁷ The tumor arises in the pericytes, cells whose function is not well understood, but seems to be related to the regulation of vessel caliber. Most hemangiopericytomas occur in the soft tissues of the lower extremities and pelvis. Approximately 25% occur within the abdomen, in which case the symptoms depend on the tissue of origin. In summary, although there are no radiologic findings specific for primary splenic hemangiopericytoma, it is important to keep in mind the diversity of pathologic conditions involving the spleen when encountering a splenic lesion on an imaging study.

REFERENCES

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2. Fowler RH: Collective review: Hydatid cysts of the spleen. Int Abstr Surg 96:105-116, 1953.

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4. Nelken N, Ignathius J, Skinner M, et al: Changing spectrum of splenic abscess: Multicenter study and review of the literature. Am J Surg 154:27-34, 1987.

5. Berge T: Splenic metastasis: Frequencies and patterns. Acta Pathol Microbiol Scand 82:499-506, 1974.

6. Langhans T: Pulsating cavernous neoplasm of spleen with metastatic nodules to the liver. Virchows Arch [Pathol Anat] 75:273-291, 1879.

7. Jurado JG, Fuentes FT, Menendez CG, et al: Hemangiopericytorna of the spleen. Surgery 106:575-577, 1989.