Protocol for the Examination of Specimens From Patients With Wilms Tumor (Nephroblastoma) or Other Renal Tumors of Childhood

The College of American Pathologists offers these protocols to assist pathologists in providing clinically useful and relevant information when reporting results of surgical specimen examinations. The College regards the reporting elements in the “Surgical Pathology Cancer Case Summary” section of the protocols as essential elements of the pathology report. However, the manner in which these elements are reported is at the discretion of each specific pathologist, taking into account clinician preferences, institutional policies, and individual practice.The College developed these protocols as an educational tool to assist pathologists in the useful reporting of relevant information. It did not issue the protocols for use in litigation, reimbursement, or other contexts. Nevertheless, the College recognizes that the protocols might be used by hospitals, attorneys, payers, and others. Indeed, effective January 1, 2004, the Commission on Cancer of the American College of Surgeons has mandated the use of the checklist elements of the protocols as part of its Cancer Program Standards for Approved Cancer Programs. Therefore, it becomes even more important for pathologists to familiarize themselves with the document. At the same time, the College cautions that use of the protocols other than for their intended educational purpose may involve additional considerations that are beyond the scope of this document.This protocol applies to Wilms tumor (nephroblastoma) or other renal tumors of childhood only. There is no American Joint Committee on Cancer/International Union Against Cancer TNM classification system for the staging of nephroblastoma. The National Wilms Tumor Study Group staging system is recommended.First priority should always be given to formalin-fixed tissues for morphologic evaluation. The second priority for tissue processing may include snap-freezing up to 1 g (minimum of 100 mg) of tumor for molecular studies (note A).For more information, contact The Children's Oncology Group Biopathology Center, Columbus, Ohio; telephone: (614) 722-2890 or (800) 347-2486.Because of the high number of false positives, intraoperative frozen sections should be avoided unless the operative procedure will be altered by the result. Biopsies of pediatric renal tumors present significant potential for diagnostic error even on permanent section. However, frozen section from the bivalved nephrectomy specimen to ensure tumor viability or to prompt other differential diagnostic studies may be of value.For future potential molecular studies, viable tumor (up to 1 g or more) should be snap-frozen (liquid nitrogen or cold isopentane) in 2 or more vials, along with a separate portion of nonneoplastic kidney (at least 1 vial).3 The latter serves as a useful control in molecular genetic studies and helps determine whether any detected genomic abnormalities are germline or intratumoral mutations. Nephrogenic rests may also be sampled and frozen for the same reasons.Fine-needle aspirations of Wilms tumor specimens are of limited utility and are not encouraged, as the detection of anaplasia (note E) may be obviated by sampling error. The distinction between a nephrogenic rest and Wilms tumor can seldom be made on the basis of needle biopsies. Moreover, in blastema-rich tumors, the characteristic triphasic histology of the tumor also may be absent due to sampling. In a tumor in which histology remains the gold standard of diagnosis (note C), fine-needle aspiration may not give one an accurate diagnostic classification. An otherwise stage I tumor is upstaged by performing a biopsy prior to nephrectomy.The diagnosis of primary renal tumors in children remains largely based on the examination of hematoxylin-eosin–stained sections. While some studies1 suggest that p53 immunostaining may be a more sensitive predictor of poor outcome than histologic assessment of anaplasia, such studies are fraught with difficulties in interpreting the outside limits of “positivity,” as well as with interinstitutional variability in immunostaining techniques. Confirmation in larger studies using multiple techniques is needed.No single cytogenetic or molecular abnormality has been consistently abnormal in Wilms tumor or its host, but constitutional deletions of the WT1 tumor suppressor gene at 11p13 often predispose the patient to development of Wilms tumors. WAGR syndrome (Wilms tumor, aniridia, genitourinary anomalies, and mental retardation) and Denys-Drash syndrome are characterized by the deletion or mutation of this gene. Intralobar nephrogenic rests are associated with WAGR and Denys-Drash syndrome. Perilobar nephrogenic rests are associated with Beckwith-Wiedemann syndrome, Perlman syndrome, and hemihypertrophy.6–8Genetic tests are often quite useful in the evaluation of several pediatric tumors arising in the kidney, which mimic Wilms tumor. These include the characteristic translocation of cellular mesoblastic nephroma, t(12;15), and peripheral primitive neuroectodermal tumor, t(11;22). Fluorescence in situ hybridization studies to detect the 22q11.2 deletion of malignant rhabdoid tumor of the kidney may also be diagnostically useful. Neuroblastomas not infrequently present as renal primaries, and N-myc amplification detected by fluorescence in situ hybridization may be important in such cases.With pediatric renal tumors, there are many issues that can interfere with making accurate diagnostic and staging decisions. The following guidelines are recommended to ensure the necessary diagnostic features are preserved and properly examined.1. Nephrectomy specimens should be submitted intact by the surgeon. The surface of the specimen should be photographed and inked prior to bivalving to facilitate the recognition of displacement artifacts from the smearing of tumor cells over the specimen surface during sectioning, as well as to evaluate margins. Bivalving will cause the capsule in a fresh kidney to retract, possibly altering the relationship between the tumor and the capsule or surgical margin.2. The capsule from nephrectomy specimens must never be stripped. Invasion of the tumor into the capsule is a criterion in staging. In addition, nephrogenic rests are often subcapsular in location. The medial sinus margin is defined as the medial end of soft tissues surrounding the renal artery and vein.3. Inspect the renal vein for tumor thrombus, as this is a common route by which Wilms tumor exits the kidney. Caution should be used in the evaluation of the margin of the renal vein that contains a thrombus. The vein often retracts after the surgeon sections it, leaving a protruding tumor thrombus, which may erroneously be considered a positive margin. If the thrombus itself is not transected, and if the margin of the vascular wall itself does not contain tumor, this surgical margin is interpreted as being negative.4. The exact site from which each section or paraffin block is obtained may be documented by photograph, photocopy, or drawing. Often, this documentation is critical for recognizing staging problems and for the evaluation of focal versus diffuse anaplasia.5. Take at least 1 microscopic section per centimeter of maximal tumor diameter, with additional sampling of any suspicious lesions.3 The majority of random tumor sections should be taken from the periphery of the tumor, as this is where the invasive pattern of the tumor can be identified and its interface with the capsule and native kidney can be evaluated. Peripheral sections also demonstrate invasion of vessels within the intrarenal extension of the renal sinus. The renal sinus is that area in the hilum of the kidney occupied by the renal pelvis, as well as hilar vessels and fat (Figure). The renal cortex at the sinus lacks a capsule.3 The most important sections are those taken from regions of the sinus adjacent to the tumor to demonstrate involvement (or lack of involvement) of sinus vessels.6. For Wilms tumors that are multicentric, sample each nodule. More than 30% of Wilms nephrectomy specimens contain nephrogenic rests. Samples of native kidney with areas more pale than the usual parenchyma may reveal nephrogenic rests. Nephrogenic rests have important implications concerning the risk of contralateral Wilms tumor development and may have other syndromatic implications. At least 1 random section of normal kidney and possibly more may be taken to detect nephrogenic rests microscopically. The presence of multiple or diffusely distributed nephrogenic rests is termed nephroblastomatosis. Two fundamental categories of nephrogenic rests are recognized: intralobar nephrogenic rests and perilobar nephrogenic rests. In addition to increased risk of tumor arising in the contralateral kidney, intralobar and perilobar nephrogenic rests have specific genetic implications (note C). The topographic and microscopic distinction of perilobar and intralobar nephrogenic rests have been well described elsewhere.67. Nephrectomy weight may be an eligibility factor for some clinical trial protocols. Hence, this measurement is critical.8. In addition to the capsular, vascular, and sinus sampling already described, routine sections taken for margins should include sampling of the distal ureter.4Once a tumor has been diagnosed as Wilms tumor, it is necessary to determine whether it is of favorable histology or if anaplasia is present. Although anaplasia is present in only 4% of all cases,3 it is the major prognostic indicator and will place a tumor in an unfavorable histologic category.Classic Wilms tumors present with a mixture of blastemic, stromal, and epithelial cell types. A common difficulty faced by pathologists interpreting a pediatric renal mass is the distinction between a hyperplastic perilobar nephrogenic rest and a Wilms tumor. These may be cytologically identical. The most helpful histologic feature is the absence of a peritumoral fibrous capsule in perilobar nephrogenic rests.Many other neoplasms may have a histologic appearance similar to blastemal-predominate Wilms tumor. The most common tumors misdiagnosed as Wilms tumors are undifferentiated neuroblastoma, primitive neuroectodermal tumor, and synovial sarcoma. The most helpful feature that favors the diagnosis of Wilms tumor is the presence of overlapping nuclei with finely dispersed chromatin. Similarly, epithelial-predominate Wilms tumors show considerable histologic overlap with papillary renal cell carcinoma and metanephric adenoma (note G).The presence of anaplasia is a significant prognostic factor in Wilms tumor and places the tumor in an unfavorable category. Although the mechanism for unfavorable prognosis is unclear, anaplasia may be a marker of chemotherapy resistance. A diagnosis of anaplasia requires both (1) gigantic polyploid nuclei with increased chromatin content and major diameters at least 3 times those of adjacent cells, and (2) the presence of multipolar or otherwise recognizably polyploid mitotic figures. On a small biopsy, a single multipolar mitotic figure or an unequivocally gigantic tumor cell nucleus may be sufficient criteria for diagnosis. Severe nuclear unrest is defined as nuclear pleomorphism or atypia approaching the criteria of anaplasia.Anaplasia correlates with responsiveness to therapy rather than with aggressiveness. Therefore, tumors with anaplasia confined to the kidney, which are entirely excised, would be expected to demonstrate the same prognosis as favorable histology Wilms tumor of the same stage. Because of this distinction, criteria for focal versus diffuse anaplasia have been defined topographically and are rigorous. This topographic definition of focal anaplasia makes it mandatory that pathologists carefully document the exact site from which every section is obtained (eg, on a diagram, specimen photocopy, and/or photograph of the gross specimen).Focal Anaplasia.—Diagnosis of focal anaplasia is warranted if all of the following descriptions are true.Diffuse Anaplasia.—Diagnosis of diffuse anaplasia is warranted if any of the following are true.True tumor invasion is easy to confirm when the tumor fills the lumen or invades the vascular wall. Displacement artifact is also readily identified when it is present in arterial lumina, when it is accompanied by abundant displacement artifact elsewhere, or when ink is present within the aggregates. More difficult are foci of unattached tumor intermingling with fibrin and red cells, or free-floating rounded tumor fragments that are not associated with other displacement artifact. The presence of these foci in children with small, otherwise stage I tumors not treated with adjuvant chemotherapy are biologically significant and should upstage the patient. Intrarenal vascular invasion does not upstage a renal tumor.The American Joint Committee on Cancer (AJCC)/International Union Against Cancer (UICC) TNM staging systems currently do not apply to Wilms tumor. The National Wilms Tumor Study Group staging system for Wilms tumors is recommended and shown below.5Stage IStage IIStage IIIStage IVStage VIncluding Wilms tumor and its anaplastic variant, 3 other renal tumors—mesoblastic nephroma, clear cell sarcoma, and rhabdoid tumor—account for 95% of all primary renal tumors of childhood.3 A more detailed differential diagnosis of pediatric renal tumors is provided elsewhere.36There is a growing appreciation that congenital mesoblastic nephroma (CMN), a tumor of infancy, represents 2 genetically distinct tumors: the “classic” CMN (24% of cases), which corresponds to infantile fibromatosis, and “cellular” CMN (66% of cases), which corresponds to infantile fibrosarcoma and contains the characteristic t(12;15), resulting in a fusion product detectable by reverse-transcriptase polymerase chain reaction.9 Occasional cases (10%) are classified as “mixed” CMN owing to the presence of both histologic types. There is currently no consensus regarding the pathways through which mixed CMN may arise.Approximately 10% of CMNs relapse. The substantial majority of CMNs that relapse are of the cellular subtype. Recurrences occur very rapidly, often within the first month of diagnosis. Virtually all relapses occur by 1 year of age. More than half are local recurrences; however, pulmonary metastases have been identified in 20% of those that relapse. Preliminary evidence suggests that renal sinus vascular involvement may be closely associated with lung metastasis. However, the primary determinant of outcome is the completeness of excision. Surgeons should be educated and encouraged to secure wide margins when resecting renal tumors in infants, particularly medial margins. Nonetheless, one can rarely be sure that the medial margin is clear; therefore, all cases should be monitored closely. Monthly abdominal ultrasounds should be performed for 1 year, with the hope of catching recurrences early enough to surgically excise them. Adjuvant chemotherapy is required when there is gross residual tumor. Radiation has no demonstrable effect.Clear cell sarcoma of the kidney (CCSK) is capable of mimicking, or being mimicked by, every other major neoplastic entity in the pediatric kidney. A genetic or histochemical feature specific to CCSK has been elusive. Immunohistochemical stains other than vimentin are inconsistent, but these negative results can help rule out other neoplasia in the differential diagnosis.The histologic spectrum and clinical outcome of patients with CCSK has recently been reported by the National Wilms Tumor Study Group.10 Nearly all patients with stage I CCSK survive. Conversely, patients with more advanced disease have a propensity for local recurrence and metastasis. Recurrences can occur from years to decades after initial presentation, sometimes demonstrating a bland histology that differs from the primary tumor. The metastatic pattern tends to be more widespread than that of Wilms tumor and includes bone, brain, and soft tissue. There is a high recurrence rate and death rate even when treated by combination chemotherapy, but survival can be greatly improved following treatment with doxorubicin.10 This situation emphasizes the importance of identifying this neoplasia to facilitate early administration of more effective chemotherapy regimens.Several variants of CCSK have been identified, among which the following are most important.Classic Pattern.—The classic pattern of CCSK presents as an evenly dispersed network of fine, arborizing vessels accompanied by a variable amount of spindle-cell stroma, subdividing the tumor into nests or cords of regular size, usually about 8 to 12 cells in width. The tumor cells are of regular size, usually with stellate cytoplasm, which often surrounds clear vacuoles. The nuclei are notably regular in size, with finely dispersed chromatin and usually inconspicuous nucleoli. Mitotic activity may be sparse. Scattered preexistent tubules or glomeruli often are dispersed throughout the peripheral regions of the tumor. This pattern of growth, which isolates and separates individual nephronic units or collecting tubules, is an important clue that one is not dealing with a Wilms tumor. The latter almost always has a sharply defined, “pushing” border.Hyalinizing Pattern.—The hyalinizing pattern of CCSK often has an osteoid-like, nonbirefringent matrix that separates tumor cells, giving an appearance reminiscent of osteosarcoma. A similar change can be seen in rhabdoid tumor of the kidney.Epithelioid Pattern.—The epithelioid pattern is the most deceptive of the patterns of CCSK, in which the tumor cells align themselves along vessels in a manner mimicking the tubules of Wilms tumor. Often these cells form filigree-like strands.This distinctive renal neoplasm most commonly is encountered in infants younger than 1 year of age and is extremely uncommon in patients older than 5 years. It is extremely aggressive and is the most prognostically unfavorable neoplasm of the kidney in early life. Rhabdoid tumors continue to present significant diagnostic challenges, particularly when they do not show overt rhabdoid features. However, the growing appreciation that this tumor arises in sites other than the kidney and the central nervous system, and the increased appreciation of the wide histologic spectrum of rhabdoid tumors have contributed to a marked increase in their correct diagnosis. Rhabdoid tumor of the kidney should not be confused with the true myogenic cells, which are often found in Wilms tumors.The most distinctive features of rhabdoid tumor of the kidney are rather large cells with large vesicular nuclei, a prominent single nucleolus, and the presence in at least some cells of globular eosinophilic cytoplasmic inclusions composed of whorled masses of intermediate filaments. Another distinctive feature is the extremely aggressive, invasive pattern of this lesion. Rhabdoid tumor of the kidney has a diverse immunohistochemical profile. Tumors may be positive for many supposedly incompatible epitopes for epithelial, myogenous, neural, and mesenchymal cell types. Epithelial membrane antigen should be included in the routine panel applied to small blue cell tumors, largely because of the typical focal strong positivity for epithelial membrane antigen (as well as a multitude of other markers) that rhabdoid tumors demonstrate.Rapid advances in our understanding of the genetic events leading to the development of rhabdoid tumors have been made recently. It now is clear that both renal and extrarenal rhabdoid tumors carry homozygous deletions and/or mutations of the hSNF5/INI1 gene located at 22q11.2.11 Furthermore, germline mutations have been identified in individuals with both renal and central nervous system rhabdoid tumors. The INI1 gene causes conformational changes in the nucleosome, thereby altering and factor The INI1 deletion can be with fluorescence in situ but no other diagnostically useful molecular tests are currently

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