(Radiographics. 2001;21:S255-S260.)
© RSNA, 2001
Best Cases from the AFIP
Hydatid Cyst of the Kidney: Radiologic-Pathologic Correlation1
Wim K. Volders, MD,
Geert Gelin, MD and
Relinde C. Stessens, MD
1 From the Departments of Radiology (W.K.V., G.G.) and Pathology (R.C.S.), Ziekenhuis Oost-Limburg, Genk, Belgium. Received April 30, 2001; revision requested July 2 and received July 31; accepted August 2. Address correspondence to W.K.V., Katteweidelaan 14, 3590 Diepenbeek, Belgium (e-mail: wimvolders@yahoo.com).
Index Terms: Echinococcosis, 81.208 • Kidney, cysts, 81.31
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Editor's Note
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Everyone who has taken the course in radiologic pathology at the Armed Forces Institute of Pathology (AFIP) remembers bringing two beautifully illustrated cases for accession to the Institute. In recent years, the staff of the Department of Radiologic Pathology has judged the "best cases" by organ system, and recognition is given to the winners on the last day of the class. Beginning with the July 2001 issue of RadioGraphics, one of these cases is published with each issue of the Journal, written by the winning resident. Radiologic-pathologic correlation is emphasized, and the causes of the imaging signs of various diseases are illustrated.
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History
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A 21-year-old man was admitted to the emergency department for colicky pain of several hours duration in the left flank and radiating to the left lower quadrant. The pain was associated with nausea and vomiting. The patient’s medical history was unremarkable. He had lived for many years in a sheep-grazing area of the Mediterranean region.
Abdominal examination demonstrated a palpable left kidney that was painful. Small bowel peristalsis was normal, and the liver was not enlarged. The remainder of the systemic examination was normal. Initially, the laboratory findings were normal, except for an increased sedimentation rate of 64 after 1 hour (normal value, 0–30). No fever was present, and urine analysis was negative.
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Imaging Findings
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Conventional abdominal radiography demonstrated normal bowel gas distribution. No abnormal air-fluid levels were visible. Ring-shaped calcifications could be seen in the left upper quadrant. Chest radiographic findings were normal.
Ultrasonography (US) revealed a well-defined, hypoechoic mass anteriorly in the left kidney with multiple hyperechoic septations (Fig 1). The renal pelvis was not clearly visible. The right kidney, liver, spleen, and retroperitoneum were normal.
Intravenous urography with conventional tomography performed 15 minutes after contrast material injection demonstrated normal excretion in the right kidney but a delayed nephrogram in the left kidney (Fig 2). A large, radiolucent tumor in the upper pole causing displacement of the collecting system was also visible. Some inhomogeneities were visible within the tumor, a finding that suggested a multilocular morphology for the lesion. Peripheral curvilinear calcifications were clearly seen superiorly.
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Figure 2. Intravenous urogram (conventional tomography) shows a large, radiolucent tumor in the upper pole of the left kidney with faint curvilinear calcifications superiorly (arrow). The caliceal system is not seen. Findings on the right side are normal.
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Intravenous urography was immediately followed by computed tomography (CT) of the upper abdomen. No more contrast material was administered. In this "late excretion phase," CT demonstrated an expansile, hypoattenuating tumor with a well-defined wall and daughter cysts within the parent cyst anterior to the renal pelvis, expanding to the upper pole of the left kidney (Fig 3). The central cystic part of the lesion between the numerous small cysts had an attenuation of 33 HU, in contrast to the much lower attenuation of the fluid in the surrounding cysts (5–15 HU), giving the mass a wheel-like or rosette appearance. Dorsally, some peripheral calcifications were seen; anteriorly, faint enhancement of the tumor wall could be detected. The tumor caused significant compression of the collecting system, and there was moderate anterior displacement of the splenic flexure of the colon and jejunal loops. Some enlarged retroperitoneal lymphadenopathies were also visible. Taken together, these findings strongly suggested a diagnosis of renal hydatid cyst.
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Figure 3. CT scan demonstrates a large, thick-walled cystic mass anteriorly in the upper pole of the left kidney. The mass consists of multiple small, hypoattenuating thin-walled cystic lesions with a typical peripheral location. The fluid in the central part of the mass (open arrow) is markedly hyperattenuating relative to the fluid in the surrounding cysts. Anteriorly, the wall demonstrates little enhancement (arrowhead); posteriorly, peripheral calcifications that are coarse and irregular can be seen (solid arrow).
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Magnetic resonance (MR) imaging was performed to confirm this hypothesis and visualize the lesion in different planes. Axial T2-weighted imaging revealed a solitary, high-signal-intensity mass consisting of multiple thin-walled lesions and outlined by a thick, hypointense rim (Fig 4). The high signal intensity was due to the characteristic high fluid content of the mass. The signal intensity of the small cysts was similar to that of the central component. T1-weighted MR imaging demonstrated a thick wall that was nearly isointense relative to the septa and the central part of the mass (Fig 5). The small peripheral cysts were clearly hypointense relative to the central component. They were surrounded by a zone of tissue that demonstrated enhancement after the administration of gadolinium-based contrast material, whereas the mass itself did not enhance (Fig 6). No involvement of the surrounding organs was seen.
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Figure 4. Axial turbo spin-echo T2-weighted MR image (repetition time msec/echo time msec = 3,200/138) clearly demonstrates a hydatid cyst with a thick, low-signal-intensity wall as well as hyperintense peripheral lesions attached to the germinal layer and the intervening septa. The central fluid (arrow) is isointense relative to the small cysts.
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Figure 5. Coronal unenhanced turbo spin-echo T1-weighted MR image (147/4.8) demonstrates peripheral cysts with a signal intensity clearly lower than that of the central fluid. The maximal diameter measures 9.7 cm. The intervening septa are also clearly seen.
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Figure 6a. Axial (a) and coronal (b) unenhanced turbo spin-echo T1-weighted MR images (147/4.8) show normal contrast material excretion by the residual compressed kidney parenchyma. Only the hydatid wall demonstrates some contrast enhancement. As on the unenhanced image in Figure 5, the multiple vesicles within the mass have a signal intensity that is clearly lower than that of the central fluid (arrowhead).
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Figure 6b. Axial (a) and coronal (b) unenhanced turbo spin-echo T1-weighted MR images (147/4.8) show normal contrast material excretion by the residual compressed kidney parenchyma. Only the hydatid wall demonstrates some contrast enhancement. As on the unenhanced image in Figure 5, the multiple vesicles within the mass have a signal intensity that is clearly lower than that of the central fluid (arrowhead).
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Pathologic Evaluation
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After completion of imaging, immunologic tests were performed and revealed significantly elevated echinococcus antibody titers.
Because of the patient’s persistent pain, surgery was performed. Resection of the left 12th rib was followed by total nephrectomy because the tumor was quite large and residual kidney function was very poor. The specimen was removed en bloc. During surgery, the liver was normal in appearance, and no other hydatid cysts were seen in the abdomen.
Gross pathologic examination showed a huge white cyst (13 x 8 x 8 cm) containing radiolucent fluid and daughter cysts of varying size (Figs 7–9). The wall of the parent cyst was focally thickened and probably calcified. These findings confirmed the diagnosis of uncomplicated primary hydatid cyst of the left kidney.
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Figure 8. Photograph of the left kidney (crossover incision) shows the cut surface with a spongelike appearance. There is no invasion of the encapsulated mass into the residual normal renal parenchyma.
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Histologic analysis revealed a hydatid cyst with three layers: (a) the outer pericyst, composed of modified host cells forming a dense, fibrous protective zone; (b) the middle laminated membrane (Fig 10a), which is acellular and allows the passage of nutrients; and (c) the inner germinal layer, where the scolices (the larval stage of the parasite) (Fig 10b) and the laminated membrane are produced. The middle laminated membrane and the germinal layer form the true wall of the cyst, usually referred to as the endocyst, although the acellular laminated membrane is occasionally referred to as the ectocyst.
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Figure 10a. (a) Photomicrograph (original magnification, x100; hematoxylin-eosin stain) of a renal hydatid cyst shows a nonnucleated laminated membrane composed of innumerable delicate laminations (open arrows) and a nucleated inner germinative membrane (solid arrows). (b) High-power photomicrograph (original magnification, x400; hematoxylin-eosin stain) of the cyst clearly depicts an Echinococcus granulosus scolex with shark tooth hooklets (arrows) floating in proteinaceous cyst fluid.
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Figure 10b. (a) Photomicrograph (original magnification, x100; hematoxylin-eosin stain) of a renal hydatid cyst shows a nonnucleated laminated membrane composed of innumerable delicate laminations (open arrows) and a nucleated inner germinative membrane (solid arrows). (b) High-power photomicrograph (original magnification, x400; hematoxylin-eosin stain) of the cyst clearly depicts an Echinococcus granulosus scolex with shark tooth hooklets (arrows) floating in proteinaceous cyst fluid.
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Discussion
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Echinococcosis is a worldwide zoonosis produced by the larval stage of the Echinococcus tapeworm. In this case, hydatid disease was caused by one of the main types, E granulosus. The adult worm lives in the proximal small bowel of the definitive host, attached by hooklets to the mucosa. Eggs are released into the host’s intestine and excreted in the feces. Humans may become intermediate hosts through contact with a definitive host (usually a domesticated dog) or ingestion of contaminated water or vegetables. The ovum loses its protective layer as it is digested in the duodenum. Once the parasitic embryo passes through the intestinal wall to reach the portal venous system or lymphatic system, the liver acts as the first line of defense and is therefore the most frequently involved organ. In humans, hydatid disease involves the liver in approximately 75% of cases and the lung in 15%. Secondary involvement due to hematogenous dissemination may be seen in almost any anatomic location (1). Kidney involvement in echinococcosis is extremely rare (2%–3% of cases), even in areas where hydatid disease is endemic (2). Renal hydatid cysts usually remain asymptomatic for many years. There are no pathognomonic clinical signs except cystic rupture into the collecting system, which leads to acute renal colic and hydatiduria (3,4).
Imaging findings in hydatid disease depend on the stage of cyst growth (ie, whether the cyst is unilocular, contains daughter cysts, or is partially or completely calcified [dead]) (1). A difference in attenuation and signal intensity between the fluid in the central portion of the cyst and that in the peripheral cysts is a typical finding in echinococcosis due to a difference in content (5–7). Daughter vesicles (brood capsules) are small spheres that are formed from rests of the germinal layer and appear as cysts within a cyst. They contain the scolices and hooklets, along with sodium chloride, proteins, glucose, ions, lipids, and polysaccharides (1). When daughter cysts are separated by the hydatid matrix, they demonstrate a "wheel spoke" pattern as was seen in this case (8). The matrix represents hydatid sand containing membranes of broken daughter vesicles and scolices (9). Regressive changes occur in the center, whereas peripheral proliferation continues indefinitely. The cytotoxic effects of the vesicular fluid may result in an exuberant granulomatous response by the host’s immune system. This reaction has two main consequences: fibrosis and necrosis. Complete calcification of the wall of a hydatid cyst can be considered an indication of quiescence or perhaps death of the parasite (10).
Renal hydatid disease may mimic other diseases. Detection of a cystic lesion with internal septations and sand, wall calcifications, or the rosette sign in the proper clinical setting allows the correct diagnosis in a majority of cases (10,11). Lesions with a solid appearance may be encountered. Lack of internal contrast enhancement allows one to classify them as hyperattenuating or hyperintense cystic lesions and to avoid misinterpreting them as tumors. Intact cysts may produce a low level of antigenic stimulation. Advanced radiologic techniques such as CT and MR imaging remain the mainstays of diagnosis (12,13).
In general, surgery is the treatment of choice in renal hydatid cyst. Kidney-sparing surgery (cystectomy with pericystectomy) is possible in most cases (75%). Nephrectomy (25% of cases) must be reserved for destroyed kidneys (14). A major concern during surgery for cyst removal is that, if a cyst ruptures, the brood capsules can spread throughout the body, and secondary cysts can grow wherever their contents come to rest. Although puncture of hydatid cyst has been considered as a possible source of anaphylactic reactions and spread of the parasite, studies have shown good long-term results with percutaneous hydatid cyst treatment under US guidance (15,16).
The patient in this case made an uneventful recovery. He continues to do well several years after surgery.
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Acknowledgments
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We especially thank Piet Vanhoenacker, MD, for his photographic assistance.
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References
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