Department of Pathology, Sinai Hospital of Baltimore Pathology, Baltimore, MD, USA
KeywordsRenal cell carcinomaChromophobeOncocytomaAngiomyolipomaClear cellNephroblastomaMetanephric adenomaPapillary
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The kidney is primarily composed of glomeruli, tubules, stroma, and vasculature. However, unlike in some other organs, the neoplasms of the kidney do not faithfully reflect or recapitulate their cells of origin. Therefore, recognizing a lesion is not so much a systematic process as a recognition of patterns. However, there are certain features to notice in evaluating any kidney mass, as they will help narrow the differential diagnosis in tough cases:
Circumscription and/or encapsulation
Presence of stroma within the tumor
Vascular or capillary pattern
Architectural pattern (solid, acinar, trabecular, tubular, papillary, pseudopapillary, cystic)
Cellular pleomorphism (ranging from monotonous to bizarre)
Cytoplasm (clear to granular pink to densely eosinophilic, perinuclear halos)
Nuclear size and contour (shape, and whether the membrane is smooth or wrinkled)
When studying the kidney grossly, many details crucial to staging are identified (or lost) at the bench. Key prognostic factors include the following:
Tumor invading into the renal sinus fat at the hilum
Tumor extending through the kidney capsule and into the perirenal fat
Tumor invading adrenal gland (always note whether the adrenal is even present)
Gross tumor in the renal vein, both at the margin and in the renal pelvis (always open the renal vein)
Tumor growing through Gerota’s fascia (the very delicate membrane surrounding the perirenal fat; this is actually fairly uncommon but indicates stage T4 disease)
Other helpful gross features include the following:
Circumscription and presence of multiple lesions
If cystic, multilocular versus unilocular, the presence of mural nodules, the relationship to renal pelvis
If solid, the homogeneity and the color(s)—yellow gold, mahogany brown, areas of hemorrhage, necrosis, fibrosis (gristle gray), or possible sarcomatoid foci (dense white)
Site of origin (cortex vs. medulla or pelvis), if you can tell
Now that you have the key identifying features of your tumor, let us look at the differential diagnosis for tumors in the adult . In the decade since the first edition of this book was published, a host of new entities has been described, and, unfortunately, the names for renal tumors read like they were generated by pulling kidney-related terms out of a bag (like the world’s smallest magnetic poetry kit). There is a lot of overlap of syllables and words. Take “nephro” as an example. A metanephric adenoma is a benign tumor of the kidney, discussed below. A nephrogenic adenoma is a benign lesion of the bladder (see Chapter 12). Mesonephric refers to the Wolffian duct in embryos and can be used to describe vestigial remnants in the female cervix or vanishingly rare carcinomas of the cervix. Cystic nephroma actually describes two different tumors of the kidney (see below), and nephroblastoma is a synonym for Wilms tumor, but a congenital mesoblastic nephroma is a rare stromal tumor of infants. Some new adjectives would be very helpful in this field.
Simple cysts are a very common finding, even at autopsy. The simple cyst is essentially a dilated tubule and will have a low cuboidal or flattened pink epithelial lining (Figure 13.1). It is usually unilocular. If multilocular, the septa dividing the cysts should be unremarkable stroma with no epithelial islands or nodules. There should be no clear cells.
Simple cyst . The cyst lining (arrowhead) consists of a thin layer of cuboidal cells. Below the cyst, dilated tubules filled with proteinaceous fluid are visible (arrow).
Cystic Nephroma/Multilocular Cyst
The term “cystic nephroma,” when used in the adult, falls in the category of “one of those ectopic-ovarian-stroma-lesions found mainly in women” and is related to the mixed epithelial and stromal tumor (MEST) , discussed below. (In the pediatric population, however, the term indicates a benign tumor on the nephroblastoma spectrum.) This is a multilocular cyst with a background of ovarian-type stroma: fairly blue, cellular, spindly, and estrogen and progesterone receptor positive (Figure 13.2). The cyst lining is cuboidal to hobnailed. There should be no clear cells.
Cystic nephroma . Like the simple cyst, this cyst is lined with bland epithelial cells (arrowhead). However, there is adjacent spindly stroma, similar to ovarian stroma (arrow). Kidney parenchyma is seen at the right.
Clear Cell Renal Cell Carcinoma
Clear cell renal cell carcinoma (RCC) can present as a cyst in several ways. First, it can arise in the wall of a preexisting simple cyst. Second, a solid tumor mass can undergo cystic degeneration. Third, and most sneaky, it can occur purely as a cyst lining, usually in a multilocular cyst: this is called multilocular cystic renal neoplasm of low malignant potential , due to its extremely indolent behavior. The main indicator is the presence of clear cell aggregates and nests in the cyst walls (Figure 13.3). The cyst walls may be denuded of epithelium, though, so careful sampling and hunting are essential.
Multilocular cystic renal neoplasm of low malignant potential. The cyst lining and fibrovascular septa (arrowhead) are composed of clear cells with small dark nuclei (arrow); compare these cells to conventional renal cell carcinoma (see Figure 13.6).
Lesions with Multiple Cell Populations
The angiomyolipoma is, at first, a difficult lesion to recognize, because it looks like just a mishmash of normal soft tissue components. From the name, you know that it must have vessels, smooth muscle, and fat, but then so do most organs of the body. Also working against you is the fact that these lesions can have one or two components predominating, so all you see is a mass of plump spindly cells with a vessel here and there and maybe a couple of fat cells. The key to recognizing an angiomyolipoma is knowing that you have a mass lesion and appreciating the unusual vessels that are the hallmark of this tumor.
This tumor is benign. The usual histologic features include the following:
Large, tangled, tortuous, thick-walled, hyalinized vessels
Smooth muscle cells (pink to clear and spindly) that seem to spin off of, or be continuous with, the vessel walls (Figure 13.4)
Angiomyolipoma . This example does not show the fatty component, but the prominent vessels (V) and smooth muscle components here are classic. In angiomyolipoma, the spindle cells seem to merge with, or spin off from, the thick-walled vessels (arrow).
Mature fat cells without atypia or lipoblasts
Pushing borders but not encapsulated
HMB-45 and Melan-A positive . This tumor is in the perivascular epithelioid cell tumor (PEComa) family, all of which stain for melanoma markers but not for S100.
Mixed Epithelial and Stromal Tumor (MEST)
The MEST, although rare, is simple in concept: it is the renal version of an adenofibroma, or a fibroadenoma, or any other benign mixture of stromal and epithelial elements. Because it can be cystic, it is also included in the differential diagnosis of cystic lesions. The histologic findings include a population of cytologically benign tubules of varying shapes and sizes set in a background of bland spindled stroma, which may consist of smooth muscle, fibroblasts, or myofibroblasts. This may also be in a spectrum with adult cystic nephroma (discussed earlier), because it also has estrogen and progesterone receptor-positive stroma.
Tumors with Clear Cells
The presence of clear cells in a renal tumor immediately puts renal cell carcinoma at the top of the differential. For all practical purposes, there are no clear cell adenomas. A 3 mm clear cell focus is still a clear cell carcinoma, albeit a fairly nonthreatening one. Note: Avoid the big, embarrassing, novice mistake number 1—mistaking the normal adrenal cortex for a clear cell tumor. The adrenal cell should have visible vacuoles that indent the nucleus, giving it a stellate outline (Figure 13.5).
Normal adrenal cortex . Unlike clear cell carcinoma, the cells of the adrenal cortex have discrete cytoplasmic vacuoles that indent the nuclei, creating a stellate outline around the nucleus (arrow).
Clear Cell Renal Cell Carcinoma
Clear cell renal cell carcinoma is a common tumor that is grossly a granular, golden-yellow-orange, well-circumscribed tumor, with a color and texture similar to normal adrenal cortex . It may get quite large and have areas of necrosis, hemorrhage, cystic degeneration, and fibrosis. All different looking areas should be sampled, especially the firm solid white-to-gray areas, which could indicate sarcomatoid transformation .
Histologically, the tumor may be solid with an acinar pattern, pseudopapillary (which is an acinar pattern with central dropout), or cystic. Areas of sheeting, spindly, sarcomatoid growth will bump up the tumor to grade IV. Identifying features include the following:
A netlike array of delicate capillaries, dividing cells into packets (“acinar” pattern)
Clear cytoplasm, at least focally if not diffusely (Figure 13.6)
Clear cell renal cell carcinoma . The tumor is composed of packets of clear cells, divided by delicate fibrovascular septa (arrow). These septa are characteristic of renal cell carcinoma and are seen even in high-grade or metastatic tumors. The nuclei in this example are enlarged, but nucleoli are visible only at high power, consistent with ISUP grade II.
Delicate, distinct cell membranes
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