DOI: 10.1148/rg.252045153
RadioGraphics 2005;25:481-485
© RSNA, 2005
Best Cases from the AFIP
Intrarenal Teratoma1
David J. Choi, MD, PhD,
E. Christine Wallace, MD,
Armando E. Fraire, MD and
Daniel Baiyee, MD
1 From the Departments of Radiology (D.J.C., E.C.W.) and Pathology (A.E.F., D.B.), University of Massachusetts Medical School, 55 Lake Ave North, Worcester, MA 01655. Received July 30, 2004; revision requested October 19 and received December 8; accepted December 8. All authors have no financial relationships to disclose.
Address correspondence to D.J.C. (e-mail: djc{at}alum.mit.edu).
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History
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A 4-year-old girl who had been delivered at full term was found to have a non-tender, right-sided abdominal mass at physical examination during a well child visit. The patient’s past medical history and the remainder of the physical examination were otherwise unremarkable. The patient had mildly elevated levels of blood urea nitrogen, alkaline phosphatase, and lactic dehydrogenase. The complete blood count and the remainder of the serum chemistry and liver function tests were within normal limits.
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Imaging Findings
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Ultrasonography (US) revealed a 12.8 x 7.6 x 8.7-cm mass arising within the upper pole of the right kidney (Fig 1). The mass consisted of a mantle with intermediate echogenicity and an ovoid, hypoechoic central region. This central component demonstrated posterior acoustic enhancement and contained faint nodular areas with intermediate echogenicity. The inferior vena cava was not well seen.
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Figure 1. Transverse US image of the upper pole of the right kidney shows a large mass (outer cursors) consisting of a mantle with intermediate echogenicity and a hypoechoic central region (inner cursors).
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At helical computed tomography (CT) of the chest, abdomen, and pelvis performed after the oral and intravenous administration of contrast material, the mass was well circumscribed with a fat-attenuation mantle (Fig 2). The central component, measuring 7.0 x 5.2 x 4.0 cm, had fluid attenuation. No calcification was seen.
Magnetic resonance (MR) imaging of the abdomen helped confirm the fatty nature of the mantle. The central component of the mass had mixed intermediate to high signal intensity at T2-weighted imaging (Fig 3a). Gadolinium-enhanced imaging showed no enhancement of any part of the mass and clearly demonstrated the "claw sign," indicating the intrarenal origin of the lesion (Fig 3b).
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Figure 3a. (a) Axial fat-saturated fast spin-echo T2-weighted MR image helps confirm the fatty nature of the mantle of the mass. (b) Coronal gadolinium-enhanced fat-saturated spoiled gradient-echo MR image demonstrates the claw sign with no enhancement of any part of the mass.
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Figure 3b. (a) Axial fat-saturated fast spin-echo T2-weighted MR image helps confirm the fatty nature of the mantle of the mass. (b) Coronal gadolinium-enhanced fat-saturated spoiled gradient-echo MR image demonstrates the claw sign with no enhancement of any part of the mass.
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Pathologic Evaluation
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Nephrectomy revealed a large, fatty mass arising from the upper pole of the right kidney but no enlarged lymph nodes.
Gross examination of the 330-g specimen revealed a 12.0 x 8.0 x 5.0-cm yellow, fatty ovoid mass arising from the upper pole. Sectioning revealed a 7.0 x 5.0 x 2.2-cm central cystic structure containing gray granular material (Fig 4). Microscopic examination helped confirm that the tumor mantle consisted of adipose tissue. The lining of the cystic structure was squamous epithelial in nature with abundant keratinous flakes. Numerous pilosebaceous adnexal elements were found just beneath the squamous epithelium (Fig 5). No musculoskeletal, neural, myeloid, or other heterologous tissues were seen. Cytogenetic studies of the tumor cells showed no chromosomal abnormalities. These findings were most consistent with a mature teratoma consisting of ectodermal (skin) and mesodermal (fat) components. The rest of the kidney consisted of normal renal parenchyma, without evidence of nephroblastomatosis or dysplasia. The rare teratoid variant of Wilms tumor was excluded because the tumor formed an organ other than kidney (ie, skin) (1).
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Figure 5. Photomicrograph (original magnification, x100; hematoxylin-eosin stain) shows adipose tissue (A), along with pilosebaceous elements (B) and keratinizing squamous epithelium (C).
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Discussion
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Thought to develop from pluripotent cells, teratomas are neoplasms that differentiate along one or more embryonic germ lines, with attempts at organ formation (1). They most commonly arise in the gonads but have also been found in the anterior mediastinum, retroperitoneum, sacrococcygeal region, brain, and gastrointestinal tract. This diverse distribution is thought to reflect the migration of primitive germ cells from their origin in the wall of the yolk sac along the dorsal mesentery of the hindgut to the genital ridges during the 3rd to 6th weeks of embryonic life. The proximity of the genital ridge to the nephrogenic anlage may partly explain how germ cells could be displaced into the kidney (2).
Teratomas of the kidney are exceedingly rare. We could find only 20 reports of presumably primary intrarenal teratomas published since 1934 (Table). Twelve of these teratomas (60%) were found in children and eight (40%) in adults (average age, 17 years; median age, 3 years). The female-to-male ratio was about 1.4:1 (1.8:1 in children; gender was not reported in one pediatric case [7]). Fifteen teratomas (75%) were of immature histologic grade, and of these tumors, just under one-half were locally infiltrating or meta-static (similar for children). Primary carcinoid tumors were found to arise within four renal teratomas (all in adult patients) (6,16,19,21). Associated congenital abnormalities (horseshoe kidney [8,10,16], duplicated collecting system [4], prune belly syndrome [3], and oligodactyly-like syndrome [8]) were seen in five cases, suggesting that maldevelopment increases the risk of teratoma. In fact, horseshoe kidney has been associated with increased risk of Wilms tumor, renal pelvis cancer (16,19), and primary renal carcinoid tumor (21). Wilms tumor has also been associated with aniridia and hemihypertrophy (8).
During pathologic evaluation of a teratomatous renal mass, the diagnosis of a teratoid Wilms tumor must be excluded. Classical Wilms tumors are characterized by a triphasic histologic pattern that includes blastemic, stromal, and epithelial cell types. Small foci of heterologous differentiation may be found, including squamous or mucinous epithelium, smooth muscle, adipose tissue, cartilage, and osteoid and neurogenic elements. The rare teratoid variant of Wilms tumor consists predominantly of heterologous tissue. The somewhat more advanced tissue differentiation of this variant may in part explain its favorable prognosis, despite its reduced sensitivity to chemotherapy or radiation therapy (22). The clear intention to form nonrenal organs distinguishes teratomas from nephroblastomas (1). Whereas squamous epithelium and structures resembling hair follicles may occur in Wilms tumors, hair shafts signify a terminal stage of dermogenesis and have not been described in nephroblastomas (18).
At radiography, intrarenal teratomas have manifested as large, unilateral abdominal masses, sometimes with calcifications and sometimes crossing the midline (12,13,20). Intravenous and retrograde urography have shown distortion of the collecting systems, sometimes with dilatation, and have delineated the relationship between the masses and any functioning parenchyma (4–7,10,12–14,16). US has shown cystic (10), heterogeneous (7,13,14,17), mixed cystic-solid (20), and hyperechoic (21) masses with occasional coarse foci of calcification. At arteriography, a renal teratoma containing a primary carcinoid tumor appeared as a hypovascular mass (19).
CT has demonstrated heterogeneous masses, sometimes with cystic areas (2,13), coarse foci of calcification (16,19,21), or necrosis (17). Similar appearances have been reported for extragonadal teratomas in other organs (5,13). Intravenous contrast material was used in only two cases. A mature renal teratoma containing a primary carcinoid tumor appeared hyperattenuating at unenhanced CT (21), with globular calcifications at the periphery and an incomplete, thin, hypoattenuating rim. A minimal increase in attenuation of the mass was reported after contrast material administration (65–72 HU). Yoo et al (19) described minimal enhancement of another mature renal teratoma containing a primary carcinoid tumor, although Hounsfield values were not reported.
The single case in which MR imaging was performed (14) showed heterogeneous, primarily low signal intensity with T1-weighted sequences and high signal intensity with T2-weighted sequences. To our knowledge, the postgadolinium MR imaging appearance of a renal teratoma has not been reported previously.
The imaging differential diagnosis for renal masses containing macroscopic fat includes angiomyolipoma, renal cell carcinoma, Wilms tumor, lipoma, liposarcoma, teratoma, and oncocytoma (23). An important goal of imaging is to refine the pretest probabilities of these possible diagnoses. The presence of intratumoral fat strongly suggests the diagnosis of angiomyolipoma. Radiologically visible intratumoral calcification within angiomyolipomas has not been reported (23). Rarely, renal cell carcinomas may contain fat as a result of osseous metaplasia, production of lipid vacuoles and amalgamated cholesterol from necrosis, or engulfment of perirenal or sinus fat. At CT, Wilms tumors may contain small amounts of fat or dystrophic calcification and enhance less than normal renal parenchyma. At US, these tumors are often heterogeneous, sometimes with cystic components. At MR imaging, Wilms tumors have variable appearances and may or may not enhance. At CT, lipomas can appear indistinguishable from angiomyolipomas with a predominantly fatty component. The diagnosis of liposarcoma is suggested when the tumor originates from the renal capsule or sinus, is hypovascular at arteriography, or receives its blood supply predominantly from capsular arteries. Rare cases of fat-containing oncocytoma have been hypothesized to result from entrapment of perirenal or sinus fat or production of fatty marrow in association with osseous metaplasia (23). Other causes of apparent intraparenchymal renal fat include junctional parenchymal defects and fat-filled postsurgical cortical defects.
In eight cases of intrarenal teratoma, the chief presenting complaint was a large abdominal mass, usually asymptomatic or with minor symptoms such as vague discomfort and constipation (7,10–14,20,21). Other presenting complaints included abdominal pain, anorexia, vomiting, and hematuria (4–6,16–19). Pyuria was present in four cases (2,4,6,9). Teratomas were incidentally discovered in a patient undergoing abdominal US for an unrelated reason (21) and in patients with prune belly syndrome, oligodactyly-like syndrome, or a synchronous malignancy (18). None of the four patients with primary carcinoid tumor arising in an intrarenal teratoma had symptoms of carcinoid syndrome.
The preoperative differential diagnosis for children with an intrarenal teratoma included Wilms tumor, neuroblastoma, hydronephrosis, retroperitoneal teratoma, unspecified tumor, and infected renal cysts. For adults, the list included unspecified tumor and renal cell carcinoma. Intrarenal teratomas were often quite large at presentation, with a diameter of 1.3–15.6 cm in adults (n = 4) and 5.6–11.5 cm in children (n = 7).
Management of an intrarenal teratoma should include a thorough search for an extrarenal source, regardless of the histologic maturity of the elements of the tumor, since metastatic spread from well-differentiated extrarenal teratomas has been observed (6,11). Follow-up data for renal teratomas are limited. Of the nine pediatric patients (seven with immature tumors and two with mature tumors) for whom follow-up information was available (average follow-up, 2.7 years), one patient with an immature teratoma and an oligodactyly-like syndrome died at birth (8) and two died several weeks after presentation. Of these latter two patients, one had a mature renal teratoma and a synchronous malignant neuroepithelial tumor of the ipsilateral adrenal gland (18). The other had prune belly syndrome and tumor extension into the spinal canal (3). Of the five adult patients (all with immature tumors) for whom follow-up information was available (average follow-up, 2.3 years), one died 6 months after nephrectomy with apparent recurrence and nodal metastases (4). No other cases of recurrence or metastasis were reported. The patient described in this article was healthy 7 months after undergoing nephrectomy; no local recurrence, lymphadenopathy, or metastasis was seen at follow-up CT of the abdomen and pelvis.
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Acknowledgments
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We would like to thank Sjirk J. Westra, MD, for his helpful comments on the manuscript.
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Footnotes
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Editor’s Note.—Everyone who has taken the course in radiologic pathology at the Armed Forces Institute of Pathology (AFIP) remembers bringing 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. With each issue of RadioGraphics, one or more of these cases are published, 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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References
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History
Imaging Findings
