Our knowledge of the pathology of endometrial carcinoma began to accrue, for practical purposes, with the publication of Dr. Thomas S. Cullen's book "Cancer of the Uterus" ¹ early in the last century. That book is one the great trilogy of gynecologic pathology texts of Dr. Cullen, an early giant of the John Hopkins School. His text was accompanied by the beautiful illustrations of the legendary medical illustrator Max Brödel ⁴ and his colleagues. Anyone who has not perused Dr. Cullen's book (and for that matter all his books) has really missed something. Subsequent to Cullen's timeless work there were relatively few advances of great note for many years. Clear cell carcinoma became established as an entity in a series of publications beginning with that of Dr. Saul Kay in 1957 .^5-7  Seventeen years later the first of several papers that culminated in the establishment of serous carcinoma as a distant entity appeared ^8-10 and both clear cell and serous carcinomas take their place in the current World Health Organization Classification of the endometrial carcinomas.¹¹  The year 1980 saw the publication of the outstanding book "Pathology of the Uterine Corpus" ¹² written by Dr. Richard L. Kempson and Dr. Michael R. Hendrickson. Their book, and other original publications that came around that time and since, have taken us into a new era of understanding of the complexity of the pathology of endometrial neoplasia and its mimics. This talk and handout will attempt to present our current stage of understanding of one aspect of this area, the morphology of endometrioid carcinoma.

Endometrioid adenocarcinoma is often considered relatively homogeneous and discussion of histologic variants in standard texts is often limited to issues such as the significance of squamous elements being benign appearing (adenoacanthoma) or malignant appearing (adenosquamous carcinoma), and the presence in some cases of secretory change or mucinous differentiation. I hope to show that what is indeed usually a rather "simple" neoplasm can produce some rather varied and intriguing microscopic appearances.¹³ 

Table 1 lists the various patterns, cell types, and miscellaneous other changes that may be encountered in endometrioid carcinomas of the corpus. The morphology of typical endometrioid carcinomas is well known and will not be elaborated upon here other than to note that many otherwise typical tumors contain luminal or even focal cytoplasmic mucin. Minor foci of mucinous differentiation are present in approximately 40% of tumors on routine staining,¹³  or 50% with the use of mucin stains.¹⁴  Luminal mucin may be particularly prominent in some tumors and the term "mucin-rich" endometrioid carcinoma has been used for such tumors. Ciliated cells may occasionally be seen lining the neoplastic glands in an otherwise typical, almost always well differentiated, neoplasm ¹⁵ and are inconsequential; the term "ciliated carcinoma" is reserved for the distinctive tumor described by Hendrickson and Kempson,¹⁶  as discussed later. Many otherwise typical endometrioid adenocarcinomas contain argyrophilic cells on Grimelius staining,¹³  a finding of only academic interest.

Variants of Endometrioid Carcinoma
I shall begin with the best know variants proceed through to the least well known and finish by considering noteworthy patterns of invasive tumor.

Secretory Carcinoma
This term is applied to rare well or moderately differentiated endometrioid carcinomas in which glycogen vacuoles (subnuclear, supranuclear, or both) are present within the majority of the neoplastic cells .^17-19  About two-thirds of the patients are postmenopausal. In some cases, a secretory pattern is encountered in endometrial adenocarcinomas during the course of treatment with progestins. Most cases, however, have occurred in women with no known source of excess progesterone. In cases encountered in premenopausal women who are not on hormone therapy, a corpus luteum has typically been present when the ovaries have been available for histological examination. The tumors have a behavior similar to that of other well differentiated endometrioid adenocarcinomas.

Secretory carcinomas should be distinguished on histological examination from other endometrial cancers with clear cells (Table 2) such as clear cell adenocarcinoma, which is composed of tumor cells that are polygonal and clear (without subnuclear vacuoles), flattened, or of hobnail type, and that usually contain higher grade nuclei. Clear cell carcinoma also more typically has a diffuse growth of clear cells, and the distinctive tubulocystic and papillary patterns of clear cell carcinoma rule out secretory carcinoma. Rare otherwise typical secretory carcinomas have foci of solid growth that raise the possibility of clear cell carcinoma, but the merging of these foci with typical secretory carcinoma and the presence of grade 1 nuclear features in the solid areas favor the interpretation that these foci represent an unusual growth pattern in a secretory carcinoma.

Endometrioid Carcinomas with Squamous Differentiation
These tumors account for approximately 25% of endometrioid carcinomas. Squamous change, albeit sometimes only abortive, in endometrioid carcinomas can have diverse overlapping appearances. Dr. Philip B. Clement and I ³ recently broke the appearances into the following seven groups: 1. rounded intraluminal aggregates (morules); 2. infiltrating nests resembling conventional squamous cell carcinoma; 3. plaque-like foci; 4. Oval, round, column-like, or wedge-shaped foci of well differentiated, often glycogenated cells; 5. micropapillae; 6. individual cell keratinization; 7. solid foci in which cells are often fusiform to spindle-shaped and which show varying degrees of subtle evidence of squamous differentiation including occasional small whorls or eddies of immature squamous cells. Occasionally the squamous cells may have clear glycogen-rich cytoplasm, a finding that should be distinguished from clear cell carcinoma by the absence of the tubulocystic and papillary patterns and hobnail cells characteristic of the latter tumor and presence of sometimes very subtle, but definite, evidence of squamous differentiation in cases in which the clear cell change is attributable to glycogenated squamous cells. I have a high threshold for diagnosing focal clear cell carcinoma in a tumor that is obviously dominantly endometrioid and shows squamous differentiation. The malignant squamous cells may also be spindled shape resulting occasionally in a sarcomatoid appearance (see below).

Papillary Variants
(i) Villoglandular Endometrioid Carcinoma (VGEC)
This is the most common papillary pattern. These tumors account for about 20% of endometrioid carcinomas .^20-22  VGECs are characterized by often long, slender, villous papillae with thin fibrovascular cores, usually admixed with a variable proportion of endometrioid glands, although rare tumors are purely or almost purely villous. The cells lining the villi and glands resemble those of typical endometrioid carcinoma. Ninety-seven percent of the tumors in one study ²¹ were grade 1 or 2; the corresponding figure for typical endometrioid carcinomas was 74%. The villous pattern may be seen in myoinvasive foci, although it tends to be most conspicuous in superficial tumor.

VGECs should be distinguished from other papillary endometrial lesions. The highly stratified cells, the complex papillae often lacking stromal cores and the epithelial buds of serous papillary carcinoma of the endometrium are usually absent. Also high-grade nuclei are generally lacking in VGECs. Although mucinous, clear cell, and transitional cell carcinomas of the endometrium are often papillary, at least focally, other features of the tumors readily facilitate their distinction from VGECs. The non-neoplastic papillary endometrial lesion described by Lehman and Hart ²³ lacks the cytologic atypia to justify a diagnosis of carcinoma and is less complex architecturally.

The behavior of VGECs was similar to that of typical endometrioid carcinoma of similar grade in one study .²¹  Another, however, found that myoinvasive VGECs had a higher frequency of vascular invasion and nodal involvement and a worse outcome than myoinvasive endometrioid carcinoma of usual type.²⁰  The two types, however, had a similar behavior when confined to the endometrium. The more aggressive behavior of myoinvasive VGECs in that study was independent of grade and stage.²⁰ 

(ii) Endometrioid Carcinomas with Small Nonvillous Papillae
This variant of endometrioid carcinoma, which is particularly likely to be confused with serous papillary carcinoma, has recently been described by Murray et al.²⁴  Those authors found that this subtype accounted for 8% of their endometrioid carcinomas. The mean age of the patients was intermediate between that of patients with endometrioid carcinoma of usual type and that of patients with papillary serous carcinoma. These tumors are characterized by small papillae within glands of otherwise typical endometrioid carcinomas, usually grade 1 or 2, or on the villous projections of villoglandular endometrioid carcinomas. Most of the papillae are in the form of buds of cells with ample eosinophilic cytoplasm and a low nuclear-to-cytoplasmic ratio, but some papillae have a more complex pattern or an exquisite delicate filiform appearance. Overt squamous differentiation is present in half the cases, and the eosinophilic cells in the buds likely reflect abortive squamous differentiation. The tumors in the series of Murray et al ²⁴ had a prognosis identical to that of other endometrioid adenocarcinomas of similar grade, in contrast to the generally poor prognosis of serous adenocarcinomas. The distinction from papillary serous carcinoma rests on the recognition that the small nonvillous papillae occur in the background of an otherwise typical or villoglandular endometrioid carcinoma that lacks the typical cellular buds or classic slit-like spaces of serous carcinoma. The typically low-grade nuclear features of the papillae contrast with the almost invariably high-grade nuclear features of the cells in the papillae of serous carcinomas.

Rare high-grade endometrioid carcinomas can have small nonvillous papillae, but a variety of differences, including lack of both conspicuous budding and slit-like spaces, enable distinction even in these cases.

Small Glandular or Tubular Variants
(i)    Endometrioid Carcinomas with a Microglandular Pattern
In 1992, five endometrial carcinomas that histologically simulated microglandular hyperplasia were described,²⁵  a phenomenon previously noted by others.²⁶  The tumors occurred in postmenopausal patients from 57 to 69 years of age. Two of the patients were or had been receiving Premarin and Provera and two were receiving only Premarin. The clinical presentation was identical to that of patients with typical low-grade endometrioid adenocarcinoma. The neoplasms had conspicuous microglandular patterns with luminal eosinophilic secretion and luminal and stromal neutrophils. In the hysterectomy specimens, two of the tumors were mixed endometrioid-mucinous carcinomas (the others were pure mucinous carcinomas), but we have seen microglandular changes now in tumors that would be classified as endometrioid because they had no mucinous component or contained a mucinous component that accounted for less than 10% of the tumor.

The distinction of microglandular carcinomas from microglandular hyperplasia rests on the merging of the microglandular pattern with that of a typical endometrioid carcinoma and a degree of nuclear atypia and mitotic activity in the microglandular areas that exceed those allowable in microglandular hyperplasia. The age of the patient may be helpful as microglandular hyperplasia is rare in postmenopausal women who are not on hormonal treatment.

(ii) Endometrioid Carcinomas with a Sertoliform Pattern
These are rare endometrioid carcinomas with a focal to predominant pattern resembling that of ovarian Sertoli cell tumors.^27,28  The sertoliform component is composed of small hollow or solid tubules lined by or composed of columnar cells with apical, occasionally clear, cytoplasm, or short slender cords. The reported tumors have been low-grade. The cells in the sertoliform component are immunoreactive for cytokeratin, EMA, and vimentin, but not actin or desmin. These tumors should be distinguished from endometrial stromal sarcomas with sex cord-like elements and uterine tumors resembling ovarian sex cord tumors by the presence of typical endometrioid carcinoma and squamous elements as well as nonimmunoreactivity of the sertoliform component for smooth muscle antigens. Sertoliform endometrioid carcinomas represent the uterine counterpart of the much more common ovarian endometrioid carcinomas that resemble sex-cord tumors, and potentially any sex-cord-like patterns seen in the ovarian tumors could be seen in the endometrial tumors. Tumors with cords that appear to be due to marked stromal hyalinization are considered below.

Endometrioid Carcinomas with Clear Cell Change, Not Otherwise Specified
Most endometrioid carcinomas with clear cells are in the category of a tumor with squamous change in which prominently glycogenated squamous cells appear clear (the most common cause of clear cells in endometrioid adenocarcinoma in our opinion) or the much less common secretory variant of endometrioid carcinoma (Table 2). Rarely, nonspecific clear cell change is seen which cannot be attributed to glycogenated squamous cells or secretory change. In some of these tumors, it may be related to abundant cytoplasmic lipid.

Endometrioid Carcinomas with Surface Metaplastic Changes
A constellation of surface metaplastic changes may be seen in endometrioid carcinoma. Jacques and coauthors ²⁹ encountered them in almost 50% of their endometrioid carcinomas. In 30% of the tumors with surface metaplastic changes, syncytial aggregates of bland, eosinophilic, sometimes squamoid, cells were present, often in a papillary pattern, simulating papillary syncytial change. Some tumors have filiform papillae on the surface and there is overlap between this pattern and that which simulates papillary syncytial change. In 12% of the cases of Jacques et al ²⁹ the surface change resembled microglandular hyperplasia (as described above) and in the remaining 58% of the cases, a mixture of patterns was seen. The degree of nuclear atypia in these surface changes was usually less than that of the underlying tumor. Careful attention to cytologic detail in these cases and awareness that the specimens are from the endometrium rather than the endocervix will facilitate the diagnosis of carcinoma or should prompt a request for additional tissue if the initial specimen is too scanty for a definite diagnosis.

Ciliated Carcinoma
Hendrickson and Kempson ¹⁶ described an unusual type of ciliated carcinoma with a distinctive appearance characterized by sheets of cells punctured by small lumina imparting a cribriform appearance. The tumor cells have grade 1 nuclear features and cilia that project into extracellular and intracellular lumina. The behavior of the tumors in their study was similar to that of typical endometrioid carcinoma.

Oxyphilic (Including Oncocytic) Endometrioid Carcinomas
Pitman and coworkers ³⁰ described a series of otherwise typical endometrioid carcinomas that were composed predominantly or entirely of cells with abundant oxyphilic cytoplasm. Although no distinctive findings were found ultrastructurally in that series, Silver et al ³¹ found that the neoplastic cells in the tumors in their study contained abundant mitochondria, hence their preferred designation of "oncocytic" carcinoma. All of the reported oxyphilic and oncocytic endometrioid carcinomas have been grade 1 or 2,^30-33  but solid nests of cells with striking eosinophilic cytoplasm may be seen in occasional high-grade carcinomas.

Endometrioid Carcinomas with Spindled Epithelial Cells
In some endometrioid carcinomas,³⁴  the neoplastic epithelial cells, like those of some ovarian endometrioid carcinomas,³⁵  are focally spindle-shaped, and rarely such foci are conspicuous. The spindled cells are almost always less atypical than the sarcomatous spindle cells of a malignant mullerian mixed tumor (MMMT) and a biphasic pattern, so typical of the latter tumor, although sometimes seen, is generally not as striking. It is only rarely difficult to distinguish a sarcomatoid carcinoma from a MMMT on routinely stained slides and immunostains are not needed. I object to the trend in some quarters to consider a MMMT a sarcomatoid carcinoma. They are of epithelial histogenesis but tumors are classified on their morphology rather than only on their cell lineage. Although the MMMT is the best known biphasic tumor, pure endometrioid carcinomas of various subtypes (Table 3) may descriptively be biphasic, a feature that can result in a misdiagnosis of MMMT.

Endometrioid Carcinomas With a Poorly Differentiated or Undifferentiated Carcinomatous Component
Rare otherwise typical endometrioid carcinomas may be associated with a poorly differentiated or undifferentiated carcinomatous component that may exhibit signet-ring differentiation,³⁶  trophoblastic differentiation (including choriocarcinoma),³⁷  hepatoid differentiation (resembling hepatocellular carcinoma),^38-41  resemble giant cell carcinoma,⁴²  have the features of small cell undifferentiated carcinoma,⁴³  or be simply undifferentiated carcinoma, not otherwise specified. Distinguishing the last type from the rare large cell lymphoma that presents in the corpus may be difficult occasionally but this is an area where, if needed, immunohistochemistry will be definitive.

Other Rare Patterns and Findings
The solid foci of high-grade endometrioid carcinomas usually have the nonspecific morphological features of pleomorphic epithelial cells. Occasional grade 1 or 2 tumors may also have solid foci but with less atypical cytologic features. The solid foci in these tumors may merge with sarcomatoid foci. A sharp distinction of the solid foci from the neoplastic endometrioid glands may impart a biphasic pattern (Table 3) that should not be considered MMMT in the absence of the high-grade cytology of that tumor. In some cases either the solid rounded or spindled foci merge with areas in which the stroma is extensively hyalinized. Epithelial cells in varying arrangements (clusters, cords, trabeculae) may be embedded in the hyaline material.

Psammoma bodies can occur in otherwise typical endometrioid carcinomas; Cullen (1) illustrates such a tumor. Parkash and Carcangiu ⁴⁴ have noted that the presence of psammoma bodies appears to be related to inflammation and necrosis within the tumor. Rare endometrioid carcinomas may contain benign heterologous elements in the stroma (fat, osteoid), the presence of which should not result in a misdiagnosis of an MMMT.⁴⁵ 

Unusual Patterns of Invasion
In some cases invasive tumor takes the form of individual glands sometimes widely scattered through the myometrium. In such cases, especially when the neoplastic glands are well differentiated and associated with little or no stromal response, the process can be misdiagnosed as adenomyosis with atrophy of its stroma. The terms "diffuse infiltrating" ⁴⁶ and "adenoma malignum pattern" ⁴⁷ of invasion have been applied to some such cases.

Occasionally myoinvasive endometrioid glands undergo a peculiar but distinctive change characterized by microcystic or slit-like glands with a lining of flattened epithelial cells that often have appreciable eosinophilic cytoplasm. The lumens may or may not contain detached clusters of carcinoma cells. The flat linings of the slit-like glands have been referred to as "endothelial-like" in one of the rare comments on that pattern in the literature.⁴⁸  The cysts seem to evolve and become "pinched off" from pointed projections that emanate from the conventional "parent" endometrioid glands. In some cases, the cystic glands break down, becoming represented by rare tumor cells associated with a myxoid, edematous or desmoplastic stromal reaction that may draw attention to the foci in question. Sometimes these foci and the better preserved foci of this phenomenon lie beyond the main margin of the tumor and can potentially be overlooked leading to an underestimation of the depth of invasion. Whether this pattern of invasion has any significance prognostically remains to be proven. In some cases with the peculiar pinched off glands, the striking stromal reaction is an initial clue to the presence of the invasive tumor.

In addition to the desmoplastic reaction just noted, other types of reaction to the invasive tumor may be occasionally seen. In some cases there is a lymphocytic response around myoinvasive tumor nests. Occasionally in such cases, depending on the plane of section, these inflammatory cells are unassociated with tumor but the latter may become apparent on deeper sectioning. In some cases, the lymphocytes surround blood vessels, as discussed later. Finally, in some cases, as with any invasive adenocarcinoma that may contain mucin within gland lumens, gland rupture may result in mucin pools.⁴⁹  This finding, referred to as "free mucin", was associated with an increased risk of vascular invasion in the only study of the phenomenon of which we are aware. The mucin is epithelial mucin in contrast to the acidic mucin of the myxoid stromal response noted above.

Table 1

Endometrioid Carcinomas of the Uterine Corpus

typical secretory with papillae villoglandular small nonvillous papillae with small glands, tubules or cords microglandular sertoliform with metaplastic changes squamous differentiation clear cell change, not otherwise specified surface changes resembling syncytial metaplasia or microglandular hyperplasia ciliated cells oxyphilic (or oncocytic) cells with spindled epithelial cells (sarcomatoid) with a poorly differentiated or undifferentiated carcinomatous component with other patterns diffuse growth with low-grade cytology extensive hyalinization with other rare findings psammoma bodies stromal metaplasia (e.g. osteoid) with <10% component of one or more other cell types of endometrial carcinoma, e.g. serous, mucinous, etc. with unusual patterns of invasion diffusely infiltrating "pinched off" glands that may be cystic, pseudovascular or degenerate with prominent lymphocytic response with mucin pools

Table 2 - Carcinomas with Clear Cells Involving the Endometrium

Carcinomas (i) endometrioid secretory glycogenated squamous cells lipid-rich cells clear cells not otherwise specified (ii) clear cell (iii) metastatic

Table 3 - Primary Endometrial Tumors with Endometrioid Glands and A Biphasic Appearance

Endometrioid Carcinoma (i). With spindled epithelial cells (sarcomatoid carcinoma) (ii). With interglandular diffuse growth a. Small cells with low-grade cytology b. Small cell carcinoma c. Undifferentiated carcinoma, not otherwise specified (iii). With extensive hyalinization (iv). With stromal osteoid Malignant mullerian mixed tumors

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