Retroperitoneal fibrosis

* We report the first 2 cases, to our knowledge, of retroperitoneal cysts with features of mesothelial differentiation that clinically mimic renal masses. The first lesion occurred in a 71-year-old man who presented with flank pain. Ultrasound and magnetic resonance imaging studies showed a unilocular cystic structure arising from the upper pole of the left kidney. The second lesion was in a 44-year-old woman who presented with left flank pain. Imaging studies revealed an 8-cm hemorrhagic cyst at the lower pole of the left kidney. Histologic examination of the nephrectomy specimens in each case revealed a unilocular cyst with intracystic and pericystic hemorrhage. In each case, the cyst was lined by a single layer of cells with ample eosinophilic cytoplasm and benign nuclear features without mucinous or mullerian differentiation. Histochemical staining showed Alcian blue positivity on the cell surface, which was sensitive to hyaluronidase digestion. Intracytoplasmic mucin, however, was not detected. Immunostaining showed that the cyst lining cells were positive for keratin, vimentin, HBME-1, WT1, and thrombomodulin but negative for carcinoembryonic antigen, B72.3, teu-M1, and BerEP4. The first case was positive for calretinin, whereas the second was negative. These findings support the mesothetial nature of the cysts.

(Arch Pathol tab Med. 2000;124:766-769)

Primary retroperitoneal cysts are rare lesions. Handfield-Jones defined them as "those cysts lying in the retroperitoneal tatty tissue which have no apparent connections with any adult anatomical structures save by areolar tissue."1 Several pathogenetic hypotheses for these cysts have been proposed, including lymphatic, traumatic,

parasitic, and muullerian origin.1-10 A mesothelial origin has been proposed as well, but no reports appear in the literature that supports thisl postulate.

We report 2 cases of primary retroperitoneal cysts, which were attached to the renal capsules and presented clinically as renal masses. Histologic and immunohistochemical analysis suggested a mesothelial origin of the cyst lining cells. We believe these 2 cases are the first such cases reported in the literature.

REPORT OF CASES

Case 1

A 71-year-old man presented with left flank pain of recent onset. Urinalysis revealed microscopic hematuria, ultrasound evaluation suggested the presence of a cystic mass that involved the upper pole of the kidney, and a magnetic.resonance imaging study revealed a 4-cm mass that involved the upper pole of the left kidney (Figure 1, A). Results of cystoscopy and bilateral retrograde pyelograms and a metastatic workup were all negative. A left radical nephrectomy was performed, with the mass removed intact. The patient is alive and well several months after the operation; however, the microscopic hematuria persists.

Gross examination of the specimen revealed a 6.5 x 4.0 X 3.5-- cm mass, which protruded from the upper pole of the left kidney and was independent from the adrenal gland. This mass was attached to the renal capsule but did not involve the kidney parenchyma. The portion of renal capsule attached to the mass could easily be separated from the underlying renal cortex. Serial sections of the mass showed a unilocular cystic structure with intracystic and focally pericystic hemorrhage (Figure 2, A). The kidney and adrenal gland were grossly unremarkable.

Microscopic examination of the entire lesion showed that the cyst wall was lined with a single layer of cuboidal cells characterized by ample eosinophilic cytoplasm but without atypia or mitoses (Figure 3). In some areas, the lining cells invaginated into the underlying fibrous tissue and appeared as glandlike structures. There was extensive intracystic hemorrhage, which was focally associated with destruction of the cyst lining cells and fibrosis of the cyst wall. Mullerian differentiation or intracytoplasmic mutin was not present in the cyst lining cells. The cyst wall was focally attached to the renal capsule but did not involve the renal parenchyma. The kidney showed mild arterionephrosclerosis and arteriolonephrosclerosis, whereas the adrenal gland was histologically normal.

Mucin stains, including periodic acid-Schiff with and without diastase digestion, Alcian blue with and without hyaluronidase digestion, and mucicarmine, showed that mucin was not identified within the cell cytoplasm or the lumen of the glandlike structures. However, the surface of the cyst lining cells expressed hyaluronic acid, as evidenced by a focal positive Alcian blue stain, which disappeared after digestion.

Immunostaining showed that the cyst lining cells were positive for keratin, vimentin, HBME-1, WTl, thrombomodulin, and calretinin but were negative for carcinoembryonic antigen, B72.3, Leu-Ml, and BerEP4.

Case 2

The patient was a 44-year-old woman with sudden onset of left flank pain. Imaging studies revealed an 8-cm hemorrhagic cystic mass that emanated from the lower pole of the left kidney (Figure 1, B). A renal cell carcinoma was considered in the differential diagnosis. A left partial nephrectomy was performed, with the mass removed intact. The patient is doing well 1 month after the operation.

Gross examination of the specimen revealed a 7.5 x 5.5 x 4-- cm well-circumscribed cystic mass directly adjacent to but not involving the lower pole of the left kidney. Sectioning revealed a unilocular, smooth-walled cyst filled with blood clot (Figure 2, B). The lower pole of the kidney was easily separated from the cyst and was grossly unremarkable.

Microscopic examination of the lesion revealed that the cyst wall was lined by a single layer of cuboidal epithelium and adjacent blood clot. The microscopic appearance, along with the histochemical and immunostaining profile, was identical to the first case, except that no calretinin reactivity could be demonstrated in this second cyst wall.

To test whether this immunoprofile implies mesothelial differentiation, similar immunostaining was also applied to 3 cases of hyperplastic mesothelium found in hernia sacs. The light microscopic features of hyperplastic mesothelial cells in the control cases were similar to those of the cyst lining cells of the current tumors. The obtained immunoprofile of the hyperplastic mesothelium was practically identical to that of the retroperitoneal systs (Figure 4 and Table). These finding suggest a mesotheial origin of the cyst lining cells.

COMMENT

Primary retroperitoneal cysts, defined as retroperitoneal cystic structures not deriving from any retroperitoneal organs, are rare. Although the most frequent type is mucinous cystadenoma of mullerian origin, some examples of cysts with mesonephric differentiation or lymphangioma have been described.1,2,4,5,7-11 On the other hand, various types of cysts of mesothelial origin have been described in the abdominopelvic region. Most frequent among them is the multicystic mesothelioma, which typically appears as a multicystic mass occupying the peritoneum in a young woman.12-20 A unilocular variant of this tumor has been described in the peritoneal cavity but not in the retroperitoneal space.17 In addition, isolated examples of a unilocular cyst of possible mesothelial origin have been described in the spleen and uterine adnexa.b-21 Lastly, a case of primary retroperitoneal malignant mesothelioma has also been reported.22

We present these cases for 2 reasons. First, we believe these are the first examples of primary retroperitoneal cysts of mesothelial origin. Second, these tumors clinically presented as renal masses. The mesothelial differentiation of the cyst lining cells is suggested by its light microscopic appearance, that is, ample eosinophilic cytoplasm, a lack of significant nuclear atypia, morphologic uniformity of the component cells, and formation of a single cell layer. These features are reminiscent of the typical mesothelial cells seen in hyperplastic mesothelium regardless of location and that of the epithelioid-type mesothelial cells frequently observed in malignant mesothelioma. Indeed, the presence of small glandlike spaces, observed in our cases, is a feature not infrequently encountered in cases of hyperplastic mesothelium. The mesothelial differentiation of the cyst lining cells in these cases is also supported by the presence of hyaluronic acid on the cell surface and the absence of intracytoplasmic mucin. Finally, immunostaining incorporating not only the traditional markers for determining epithelial versus mesothelial differentiation but also more recently described ones, such as HBME-1, BerEP4, WT1, thrombomodulin, and calretinin, also reveals a mesothelial profile for the cyst lining cells.23-27 This profile is also seen in the cases of hyperplastic mesothelium included as controls.

These tumors are also interesting, since they both clinically appeared as renal masses. Primary retroperitoneal tumors may attach to the renal capsule or invade the kidney and, therefore, appear clinically as a primary renal tumor. Tumor types with such a propensity have included mutinous cystadenoma, solitary fibrous tumors, and a variety of retroperitoneal sarcomas.28 However, to the best of our knowledge, this type of presentation is not reported for a mesothelial cyst.

Primary retroperitoneal cyst of mesothelial origin needs to be differentiated from other predominantly cystic retroperitoneal tumors, including lymphangioma, mutinous cystadenoma, and cystadenoma of mesonephric (wolffian) origin. Lymphangioma is generally multicystic and contains smooth muscle and lymphocytes in the cyst wall and lumens. Immunostains are helpful, since keratin is expected to be negative in lymphangioma and positive in mesothelial lesions.20 Mullerian cyst of the mutinous type displays intracytoplasmic mucin but does not express hyaluronic acid. Mutinous cystic lesions in the retroperitoneum share the immunostaining pattern of their ovarian counterparts and, thus, show carcinoembryonic antigen positivity, contrasting with an absence of carcinoembryonic antigen staining in most lesions of mesothelial origin.15 A mesonephric remnant may become cystic, but this lesion contains well-developed smooth muscle in its wall and occurs in the female adnexal region.6 Lastly a pseudocyst may form in the retroperitoneum and become hemorrhagic, similar to our case. However, the wall of the pseudocyst, by definition, should be composed of connective tissue devoid of epithelial lining.1

The fact that these 2 lesions were removed in only a 6-- month period at our hospital raises the possibility that they are not rare. Both were hemorrhagic, making it easy to have diagnosed them as merely hematomas or simple hemorrhagic cysts. Does the mesothelial cell lining develop in response to a preexisting retroperitoneal hematoma? Since hematomas adjacent to mesothelial linings are common, one would expect this to be seen more often. A more likely possibility is that the hemorrhage is secondary to a rich vascularity of the pericystic stroma adjacent to a preexisting mesothelial cyst. These questions will only be answered by further exploration of this entity.

References

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Vincent C. Smith, MD; Robert A. Edwards, MD, PhD; Jeffrey L. Jorgensen, MD, PhD; Richard A. Goldfarb, MD; Dov Kadmon, MD; Philip Cagle, MD; Luan D. Truong, MD

Accepted for publication September 30, 1999.

From the Departments of Pathology (Drs Smith, Edwards, Jorgensen, Cagle, and Truong), Medicine (Drs Cagle and Truong), and Urology (Drs Goldfarb and Kadmon), Baylor College of Medicine and The Methodist Hospital, Houston, Tex.

Reprints: Luan D. Truong, MD, Department of Pathology, MS-205, The Methodist Hospital, 6565 Fannin, Houston, TX 77030 (e-mail: (truong@bcm.tmc.edu).

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