Extrathoracic Mesothelial Proliferations and Their Mimics
Hospital del Mar-IMAS-IMM
Pompeu Fabra University, Barcelona, Spain
Mesothelial proliferations occurring in extrathoracic sites share many epidemiological and
morphological features with their thoracic counterparts. However, they also have important clinical and
pathological differences, relating to their anatomical location, the organs and tissues they may involve,
and the spectrum of non-mesothelial tumors that may arise in these regions. The application of electron
microscopy (EM) and immunohistochemistry (IHC) to their differential diagnosis must, therefore, take
these peculiarities into consideration. This review is focused on mesothelial neoplasms arising in the
peritoneum and in the testis and paratesticular structures, as well as on the main tumors that may be
confused with them.
EM and IHC in the Identification of Mesothelial Differentiation: Advantages, Pitfalls, and Limitations
EM has been the classic, gold standard tool used to confirm or identify mesothelial differentiation.
Highly sensitive and specific diagnostic clues include the characteristically long and tortuous,
occasionally dichotomized microvilli, with scanty filamentous cores, well-developed desmosomes, and
. In contrast, adenocarcinomas have much shorter, slender microvilli, with
denser filamentous cores, forming core rootlets in the apical cytoplasm. These features are
characteristic of gastrointestinal and other mucinous adenocarcinomas, but they can be less prominent in
other sites, such as the breast, thyroid, urinary bladder, prostate or endometrium. Two well known
disclaimers of EM are that it does not allow the identification of cells as neoplastic, nor does it
distinguish between benign and malignant tumor cells. In addition, careful correlation with light
microscopy is essential, because reactive mesothelial cells, admixed with metastatic tumor cells, may be
erroneously identified as the neoplastic population. Keeping all this in mind, distinguishing between
mesothelioma and adenocarcinoma by EM is a relatively easy task. However, prospective studies, which
address the diagnostic accuracy of EM through a long series of unselected, clinically well-documented
mesotheliomas, are lacking. Specific diagnostic, ultrastructural features are gradually lost in
less-differentiated, solid, or sarcomatoid varieties; thus, they require careful sampling and an
extensive search. Nevertheless, and in spite of the lack of quantitative data, it can be stated that EM
allows the identification of mesothelial differentiation, either focal or widespread, in most cases
There is a plethora of reports dealing with the application of IHC to the differential diagnosis of
mesothelial proliferations, however the ideal antibody or combination of antibodies has not yet been
. Due to the great variation from one study to another, it is difficult to obtain precise
figures on the sensitivity and specificity of the antibodies used in the differential diagnosis between
mesothelioma and adenocarcinoma. A recently developed website, contains information on the application
and sensitivity of many antibodies, pooled and averaged from many references (www.immunoquery.com). Table 1 is the result of a search on this
website, regarding the overall sensitivity of the antibodies most commonly used in the diagnosis of
mesothelioma. Some antibodies show a high sensitivity, but then their specificity tends to be less
optimal. Thus, while AMAD-2 has 100% sensitivity, according to the laboratory where this antibody was
originally synthesized, it also stains around 10% of the pleural metastases . Currently, calretinin,
thrombomodulin, WT1 gene product, and keratin 5/6 are considered the best antibodies for the
identification of mesothelial differentiation . It is generally advised that, in addition to
positive "markers", the panel should also include antibodies which should be negative in mesothelial
cells and which may be positive in adenocarcinoma, particularly carcinoembryonic antigen (CEA), Ber EP4,
Leu M1, MOC31 or B72.3
. Many other antibodies have been used in the past, and new antibodies are
being tested, but the definitive optimal markers are yet to be identified. Thus, D2-40, a recently
developed antibody for germ cell neoplasia and lymphatic endothelium, has been shown to be positive in
around 96% of the mesotheliomas and reactive pleural lesions, but it is also positive in 65% of the
tested ovarian serous carcinomas . Most studies, dealing with the diagnostic value of putative
mesothelial markers, fail to include EM as their gold standard . Thus, the diagnostic accuracy of
new antibodies is compared with that of the old ones without an external control method to confirm the
diagnosis. EM is not a generally available technique but it should be included, whenever possible, in
the work-up of peritoneal and testicular tumors, particularly in uncommon tumors or in cases with
negative or paradoxical immunohistochemical results. As a general approach for the diagnosis of most
peritoneal mesotheliomas, Ordˇ˝ez has suggested a panel of two positive (calretinin and cytokeratin 5/6)
and two negative (CA19-9 and MOC-31) antibodies. When this panel fails to solve the case, he prefers EM,
rather than a second panel of antibodies, to reach the correct diagnosis .
These general remarks apply to all mesothelial proliferations, but the situation is further
complicated in peritoneal and testicular sites, due to the presence of embryologically related, but
distinct, epithelial components. Thus, the so-called secondary MŘllerian system and the epithelium
covering the testicular appendages form a continuum with the respective mesothelial linings. This
explains the occurrence of tumors with non-mesothelial features in these serosal surfaces. Some of the
main antibodies used to identify mesothelial differentiation in the pleura may be positive in tumors
derived from abdominal organs or the testis (Table 2), and therefore specific immunohistochemical
strategies should be designed to address the differential diagnosis of extrathoracic mesotheliomas. In
many of these cases, contribution of EM may be crucial to attain the correct diagnosis.
Table 1. Sensitivity of Antibodies Commonly Used for Identifying Mesothelial Differentiation
|CD44H || 91%|
|Mesothelin || 90%|
|Cytokeratin 5/6 || 85%|
|Calretinin || 84%|
|N-Cadherin || 81%|
|HBME-1 || 80%|
|WT1 || 67%|
|Thrombomodulin || 64%|
|EMA || 56%|
(Retrieved from www.immunoquery.com)
Table 2. Mesothelial Antibodies that may be Positive in Non-mesothelial Abdominal and Testicular Tumors.
|Calretinin || Steroid secreting tumors (adrenal, sex cord)|
|WT1 || Serous carcinoma (ovary)|
|Thrombomodulin || Urothelial tumors|
|HBME-1 ||Serous tumors (ovary, peritoneum, testis)|
|CD10 ||Renal cell carcinoma. Endometrial tumors|
|D2-40 ||Serous carcinoma (ovary)|
Normal Mesothelium and Other Peritoneal and Testicular Lining Cells
During embryonic development, cells of mesodermal origin line the celomic cavity. Similar to
epithelial cells, they cover surfaces, contain keratins, and are joined by desmosomes and tight
junctions. However, these are truly hybrid cells because, in addition, they secrete hyaluronic acid, an
important component of the intercellular substance of connective tissue also produced by fibroblasts, and
their cytoskeleton contains vimentin. The celomic cavity will later be divided into a pleural and
pericardial cavity, a peritoneal cavity, and, in the male, a derivative of the latter that will give rise
to the tunica vaginalis, upon migration of the testis to the scrotum.
In all of these serosal areas, there are two different populations: mesothelial and sub-mesothelial
cells. The former show all of the characteristic features, the long, tortuous, often branching
microvilli, well-developed desmosomes and tight junctions, tonofilaments, abundant cytoplasmic glycogen,
and basal lamina. They express several keratin antibodies (5/6, 7, AE1-AE3), vimentin, HBME-1,
calretinin, and thrombomodulin, among many others. Interestingly, normal and reactive mesothelial cells
may also express desmin . The sub-mesothelial connective tissue contains a mixture of fibroblasts
(vimentin, CD34) and mesothelial spindle cells (keratin and vimentin).
Secondary MŘllerian System
The female reproductive system arises from the mŘllerian or paramesonephric duct. Under the influence
of the wolffian or mesonephric duct, the paramesonephric duct develops in both sexes from a placode-like
thickening and deepening of the celomic epithelium. This celomic epithelium gives rise to the surface
epithelium of the ovary and to the so-called extraovarian peritoneum
. Thus, the concept of the
secondary mŘllerian system implies that a certain potential towards mŘllerian differentiation is retained
by the peritoneum in the adult. This potential may be revealed under endogenous or exogenous hormonal
stimuli, mostly in women, but also, exceptionally, in men
. This notion has been considered a
histogenetic link between the normal peritoneum and the peritoneal surface tumors, showing clinical and
morphological features similar to their ovarian counterparts.
The appendix testis or hydatid of Morgagni, the appendix epididymis, the vas aberrans, and the
paradidymis constitute the so-called testicular appendages
. The appendix testis is a remnant of
the mŘllerian duct found in more than 90% of normal individuals, and as such, it is covered by a
cylindrical or cubic epithelium that may contain some ciliated cells. The appendix epididymis is present
in around 25% of all testes, and it almost always has a cystic structure and a columnar epithelial
lining. The paradidymis and the vas aberrans are related structures, also with a columnar lining around
a cystic space. Out of all of these structures, the hydatid of Morgagni represents the normal
counterpart that explains the occurrence of mŘllerian tumors in this area.
Reactive Mesothelial Hyperplasia
Irritation of serosal surfaces, with or without a subsequent effusion, induces the proliferation of
mesothelial, and often sub-mesothelial cells, giving rise to a variety of hyperplastic appearances. In
some instances, the degree of hyperplasia may be so severe as to suggest a malignant process,
particularly when examining small biopsies . Criteria for this differential diagnosis have been
dealt with extensively in the literature, and are mostly based on light microscopic features . The
invasion of intraperitoneal organs or fat is the most reliable indicator for malignancy, but this may be
mimicked by mesothelial cells entrapped in organizing granulation tissue or between fat lobules. These
benign cells usually arrange themselves in a more orderly fashion and parallel to the peritoneal surface,
and they are associated with perpendicular blood vessels and abundant histiocytes, but all manner of
exceptions can occur. The clustering of mesothelial cells on the peritoneal surface is usually observed
in mesothelial hyperplasia, but when it is organized in papillary structures or cohesive, grossly
apparent nodules, malignancy should be suspected. On the other hand, mesothelial cell clusters within
the stroma, particularly if arranged in cords, nodules, papillae or gland-like structures, are highly
suspicious for malignancy, but reactive, entrapped mesothelial cells may also focally present with this
appearance. Necrosis and nuclear atypia are more often seen in mesothelioma, but atypical cells are
quite common in reactive mesothelium, and necrosis of these benign cells may occasionally occur
IHC or EM is of little help in this differential diagnosis. Although desmin expression is more often
observed in reactive mesothelial cells, it may also, very rarely, be found in mesothelioma
admixture of mesothelial cells with CD68-positive macrophages is characteristic of reactive mesothelial
hyperplasia, but the presence of macrophages cannot be taken as evidence for the benign nature of a
mesothelial proliferation . p53 overexpression and EMA positivity are more often found in malignant,
rather than in reactive mesothelium, but they are not helpful in individual cases . As stated
above, EM is an excellent tool for the study of cell differentiation, but it cannot be used to determine
the neoplastic, either benign or malignant, nature of cells .
Mesothelial Tumors and Tumor-like Conditions
Nodular Mesothelial Hyperplasia
This is a reactive condition presenting mostly, but not exclusively, in male children, as one or
several nodules that may be grossly identified in hernia sacs . The nodules are composed of
polygonal cells, which may be moderately pleomorphic, showing low mitotic activity and accompanied by
hyperplasia of the surface mesothelium. These lesions have been shown to be mostly made up of
histiocytes, with a minor component of entrapped mesothelial cells. Due to this fact, it has been
suggested that the term nodular histiocytic hyperplasia is more adequate . The occasional presence
of multinucleated and occasionally elongated cells may lead to confusion with embryonal rhabdomyosarcoma
in pediatric cases. On the other hand, these nodules may contain a variable proportion of vacuolated
histiocytic cells, which may be wrongly interpreted as signet-ring cell carcinoma.
Adenomatoid tumor is a benign, usually well-circumscribed proliferation of mesothelial cells, arranged
in tubules, cords and gland-like spaces. It is the most common neoplasm in the epididymis
; it is
also relatively common in the Fallopian tube and uterus , and has been reported in other locations,
including the spermatic cord, ejaculatory duct, ovary, adrenal gland , pancreas , pleura , and
even the heart .
Initially considered to be of mesothelial origin by Pierre Masson, it was later the subject of some
histogenetic controversy. The presence of gland-like spaces with very thin linings, mimicking lymphatic
channels or early vascular structures, lead to the suggestion of a possible endothelial nature. EM and
later IHC confirmed the mesothelial phenotype . Considered a reactive lesion by some, there is now a
general consensus on its benign, neoplastic nature.
Adenomatoid tumors present as relatively small, well-demarcated, non-encapsulated nodules with a mean
diameter of 2 cm . The cut surface is firm and solid with variable cystic spaces. Microscopically,
cords, channels, and microcystic spaces are lined by cuboidal or flattened cells, with vesicular nuclei
and occasional nucleoli. Intracellular lumina may predominate, imparting a signet-ring appearance to the
tumor, or may coalesce into larger spaces. Quite often, tumor stroma contains a prominent smooth muscle
component and abundant elastic fibers
. This smooth muscle component may be particularly abundant
in tubaric and uterine adenomatoid tumors . In the uterus, they may be easily mistaken for
leiomyomas with a peculiar, prominent blood vessel component. Although most uterine adenomatoid tumors
are subserosal, some of them are totally intramural and even submucosal, and they often coexist with
Ultrastructurally, the typical mesothelial features are recognized, with abundant, irregular
microvilli projecting into the gland-like spaces, well-developed cell junctions, and tonofilaments
. As the correlate of these EM features, careful light microscopic examination of the luminal
aspect of these cells reveals a prominent fuzzy border, often associated with the accumulation of a faint
basophilic material. This may be shown to be hyaluronic acid by Alcian Blue stain with hyaluronidase
digestion. The cells are positive with antibodies for cytokeratin 5/6, calretinin, EMA, AE1-AE3, and for
most of the antibodies used for mesothelial cells. Antibodies for CEA, MOC-31, Ber-EP4, B72.3, and Leu
M1 are usually negative. The differential diagnosis between adenomatoid tumors and endothelial cell
neoplasms, particularly epithelioid hemangioendothelioma, is easily accomplished with EM, and also by
showing its negative staining with antibodies for Factor VIII, CD31, and CD34
Both in the peritoneum and in paratesticular locations, isolated papillary or multicystic
proliferations of mesothelial cells are usually classified as benign mesotheliomas. The cystic cases
must be distinguished from hydrocele in the testis or from cystic lymphangiomas in both the testis and
the peritoneal cavity
. There is question regarding their true neoplastic nature. On
the other hand, the benign papillary mesotheliomas must be single and relatively small lesions to be
confidently classified as benign. Multifocality, the combination with solid areas, and atypia must raise
the suspicion of a potentially malignant tumor. Actually, even some apparently benign lesions have
evolved towards a more aggressive course
It is defined as a proliferation of mesothelial cells with malignant morphological features and
aggressive growth, arising in the peritoneal cavity, or in the tunica vaginalis and the tunica albuginea
. Patients with mesothelioma originating in one of these two sites may subsequently develop
a tumor in the other. The latter are usually considered an extension of the primary lesion, rather than
synchronous or metachronous tumors. When malignant mesothelioma simultaneously involves the scrotum and
the peritoneal cavity, it is impossible to determine the origin of the process, although it is usually
assumed to most probably be a primary peritoneal tumor. Association with asbestos exposure has been
estimated in around 50% of the reported peritoneal tumors, and it is considered to be similar for the
Peritoneal mesotheliomas occur most often in men, usually over 40 years in
age. Fibrous plaques are found in cases of peritoneal mesothelioma more often than in pleural
mesothelioma. Clinical presentation depends on the areas and organs involved, and it is usually
accompanied by ascites. Grossly, peritoneal mesotheliomas form multiple nodules or plaques, and they
only occasionally present as isolated lesions. Again, microscopically, there is a wide range of
appearances. The usual epithelioid mesotheliomamay grow in tubules, papillae with variable
psammomatous components, and solid areas. Nuclei may be pleomorphic but are more often monotonous,
either with dispersed or condensed chromatin
. Cell cytoplasms may be vacuolated, due to
degenerative changes. They may also contain numerous lipid vacuoles, sometimes mimicking liposarcoma
, or large glycogen pools, resulting in the clear-cell
variety that may be confused with renal cell carcinoma . Sarcomatoid mesothelioma, made
up of diffusely growing spindle cells with variable degrees of pleomorphism, may predominate or occur as
a component in a biphasic tumor
Several clinicopathological varieties of peritoneal mesothelioma have been recognized.
Well-differentiated papillary mesothelioma is mostly a superficial, multifocal tumor, most often
found in women . Thus, the main differential diagnosis will be made with peritoneal and ovarian
serous carcinomas. A morphological variant of peritoneal mesothelioma that may also occur in the testis
is the so-called deciduoid mesothelioma
. As the name implies, it is made up of cells with
a decidualized appearance. In women, this may result in confusion with benign, sub-mesothelial
decidualized nodules, and thus, the malignant nature of the lesion may be overlooked. In contrast with
true decidualized cells that contain large amounts of glycogen, the appearance of these cells has been
shown by EM to be the result of the prominent accumulation of a variety of organelles, including
intermediate filaments, either dispersed or arranged in bundles, mitochondria, and rough and focally
smooth endoplasmic reticulum
. Another variety, linfohistiocytoid mesothelioma, is
characterized by the combination of tumor cells along with a variable component of lymphocytes and
histiocytes . A rare variety, leiomioid mesothelioma, shows the co-expression of mesothelial
markers with desmin and actin . All of these varieties share the same ultrastructural and
immunophenotypical profile as conventional mesotheliomas.
Testicular mesotheliomas present in a wide age range, from children to
elderly men, with a mean age around 50 years. Although testicular mesothelioma is rare, it is the second
most common malignancy in this location, after soft tissue tumors. It usually presents either as an
incidental finding during hernia repair, in association with hydrocele, with or without a clinically
detectable mass, or primarily as a palpable tumor. Grossly numerous, small papillary lesions, multiple
nodules or diffuse thickening of the vaginalis or albuginea are most often found. There may be obvious
signs of infiltration into adjacent structures. Microscopically, 75% of the testicular mesotheliomas are
epithelial and show combinations of tubular, papillary and solid areas. In the remaining cases, a
variable proportion of sarcomatoid elements are found, resulting either in a biphasic or even purely
Clinically, peritoneal and testicular mesotheliomas are aggressive tumors. For those arising in the
peritoneum, prognosis is extremely poor, and the majority of patients die two years after diagnosis.
Progression is associated with extensive infiltration to adjacent organs, and with metastases to pelvic
and retroperitoneal lymph nodes
. The exception to this rule would be the well-differentiated
papillary mesotheliomas arising in women, which tend to show a more protracted course. Testicular
mesotheliomas tend to recur after two years of diagnosis, infiltrating other testicular and
paratesticular structures, the spermatic cord, and disseminating into inguinal and retroperitoneal lymph
nodes, the peritoneum, mediastinum, lungs and pleura, bones, and the brain.
Differential Diagnosis of Peritoneal and Testicular Mesothelioma
Common Epithelial Tumors of the Ovary and Uterus and Their Testicular Counterpart
This is an important differential diagnosis, with remarkable therapeutic and prognostic implications.
Out of all the varieties, the main difficulties may be encountered with serous papillary carcinomas of
the ovary. These tumors present with variable combinations of papillary and solid areas and may extend
along the peritoneal surface, forming nodules and plaques, and making it difficult to establish their
. Similarly, serous carcinomas of the endometrium often disseminate to the
peritoneum, with relatively little or no ovarian involvement . Another finding that can be
misleading is the presence of mesothelial hyperplasia, in association with some serous borderline tumors
. To further complicate the situation, mesotheliomas may preferentially involve the ovarian surface
. When topographic data are not contributory, the differential diagnosis must be based on a
combination of light microscopic, immunohistochemical, and ultrastructural features. Histologically,
cells in mesothelioma tend to have more monomorphous and less atypical nuclei than those of carcinomas,
and they may display a more prominent tubulopapillary pattern. Mucin stains such as mucicarmin are of
little use in serous carcinomas, and, on the other hand, mucicarmin-positive mesotheliomas have been
reported . In the rare instances in which the differential diagnosis is a mucinous ovarian neoplasm,
Alcian Blue stain, with and without hyaluronidase digestion, may be a more useful technique.
The immunohistochemical study of these tumors is even more problematic than the study of those in
other sites (Table 2). Out of all of the mesothelial antibodies, calretinin shows the best sensitivity
and specificity for mesothelioma in this setting. However, sex cord-stromal tumors (i.e., retiform
Sertoli-Leydig cell tumor) may present with peculiar papillary areas, and calretinin is typically
positive in most of these cases
. This may lead to an erroneous diagnosis, if calretinin is not
used in combination with other antibodies. WT1 gene product is of little practical value, as it is
typically expressed by most ovarian and surface serous carcinomas, and also by over 50% of the
endometrial serous carcinomas
. This finding is related to the fact that the Wilms tumor gene
plays complex roles in the development of the genitourinary tract and mesothelium, thus providing
additional evidence for the histogenetic relationship between mŘllerian epithelia and mesothelial cells.
Other antibodies used for mesothelioma or carcinoma are of limited value here: thrombomodulin is
reported to be positive in 56% of the mesotheliomas and 30% of the serous carcinomas, cytokeratin 5/6 in
53% of the mesotheliomas and 25% of the carcinomas, and CD44H in 47% of the mesotheliomas and 25% of
carcinomas . HBME-1 is also expressed in serous carcinomas, although the membranous pattern of
positivity is apical, compared to the more extensive apical and lateral pattern seen in mesothelioma. In
addition, only 35% of the serous carcinomas express Leu M1, 10% polyclonal CEA and 5% monoclonal CEA
. In fact, according to Ordˇ˝ez, CA19-9 is preferable to CEA in this setting .
In serous carcinomas, EM reveals the presence of shorter and straight microvilli, frequent ciliated
cells, and junctional complexes, instead of the isolated, large desmosomes or tight junctions seen in
. Junctions between mesothelioma cells are most often located closer to the basal
domain, resulting in a wide circumferential area covered by the characteristic microvilli . In
contrast, junctional complexes tend to be located closer to the apical domain in serous carcinomas,
except in cells with a prominent hobnail appearance. This explains the different patterns of staining
with antibodies against membranous or glycocaliceal components of microvilli, such as HBME-1. Although
EM may easily lead to the diagnosis, it is crucial to adequately select the areas of interest, in which
all of the diagnostic features may be found, avoiding to erroneoulsy identify entrapped or hyperplastic
mesothelial cells as being part of the tumor. Both ovarian and mesothelial tumors tend to be large, and,
in poorly differentiated cases, the diagnostic areas may be focal and, therefore, missed in the initial
sampling for EM. However, they may be selected and retrieved from the paraffin block, without losing the
essential information, provided that the previous fixation and paraffin embedding were adequate. In
summary, the differential diagnosis between peritoneal mesothelioma and ovarian tumors is clinically
relevant, and it may be achieved, in most cases, by paying close attention to morphology. Difficult
cases will benefit from IHC and, if available, EM confirmation.
Tumors identical to those arising on the ovarian surface, benign, borderline or malignant, may arise
in the testis and paratesticular structures. These are very uncommon neoplasms with a mŘllerian
phenotype, mostly serous, but also of mucinous, endometrioid or Brenner types, and show an
immunohistochemical profile and ultrastructural features similar to their ovarian counterparts. It is
assumed that these tumors have a better prognosis than mesotheliomas in this location, but this is only
based on single case studies or small series
Peritoneal Serous Tumors
There is a group of serous papillary neoplasms, benign, borderline or malignant, that present in the
absence of ovarian involvement. They can be considered the equivalent of serous ovarian tumors in an
. Their histological features and ultrastructural appearance are identical,
including cilia, short microvilli and junctional complexes . The vast majority of cases occur in
women, although there are isolated reports in men. It has been hypothesized that their origin could be
traced through serous metaplasia of the mesothelium or endosalpingiosis, in the context of the so-called
secondary mŘllerian system
. It is important to distinguish between a primary peritoneal
serous tumor and an implant or metastasis from an ovarian primary lesion, because of obvious differences
in staging. Apparently these tumors behave in a similar fashion and are susceptible to similar treatment
strategies with their ovarian equivalents, and therefore, it is even more important to differentiate
between a primary serous peritoneal lesion and a malignant mesothelioma. Again, a judicious combination
of clinical and histological features, IHC and EM will allow the diagnosis. Also, a special note of
caution applies to these tumors, as many of the antibodies that show moderate to low positivity in the
ovarian tumors tend to be completely negative, or only focally and faintly positive, in many of the
peritoneal ones. This is the case with CEA, either mono or polyclonal, Leu M1, or B72.3
In spite of their serous phenotype, many of these tumors are reported to be positive with stains for
epithelial mucin, but, in the absence of other supporting features, this is not enough evidence for the
diagnosis, since mucin stains may be positive in mesothelioma .
Rare Epithelial Testicular and Epididymal Tumors
Testicular mesotheliomas must be distinguished from a group of very uncommon neoplasms, arising in the
epididymis or rete testis. Some of them are benign, and others tend to be more aggressive than
conventional testicular mesothelioma. Because of their rarity, the diagnosis may be easily missed.
Papillary cystadenoma of the epididymis
This is a benign cystic tumor, arising in the epididymis and showing variable, aborizing papillary
structures, that are made up of glycogen-rich, clear cells, in which light or EM shows occasional cilia.
These tumors are bilateral in about half of the cases, and particularly in patients with the Von
Hippel-Lindau syndrome, in which they are much more prevalent. The main differential diagnoses for these
tumors are the malignant counterpart, clear-cell carcinoma of the epididymis, and clear-cell renal cell
carcinoma, which may rarely metastasize to the testis and paratesticular structures. Some areas in
papillary cystadenoma of the epididymis may show features strongly reminiscent of mesothelioma, and, on
the other hand, mesotheliomas may on occasion show a prominent accumulation of glycogen . Therefore,
all of the previously discussed immunohistochemical and ultrastructural features may be helpful in this
differential diagnosis, but it is obviously essential to be aware of the entity in order to recognize it
Adenocarcinoma of the epididymis
This is an uncommon, malignant glandular proliferation, arising in the epididymis, which presents as
an occasionally painful mass, associated with hydrocele in about half of the cases. It occurs between
the third and ninth decade, and may attain a variable size. It shows a tubular, tubulopapillary, or
cystic growth pattern, and is made up of cuboidal or columnar cells, often containing cytoplasmic
glycogen. Usually, there is no associated, desmoplastic stromal response. The main differential
diagnoses include, in addition to the benign epididymal variety already discussed, adenomatoid tumor,
mesothelioma, serous carcinoma, and tumors of the rete testis
. For rete testis and serous
tumors, topography is important, as rete testis tumors must be seated in this structure to assume that
they originate within it, and serous tumors tend to arise in the testiculo-epididymal groove. However,
when the tumors are large, topography may be difficult to assess. Relatively, the easiest differential
is for mesothelial neoplasms, as they may be excluded or confirmed by histological features, plus the
usual ancillary techniques. Again, IHC must be used with care. Epididymal adenocarcinoma shows strong
luminal staining with antibodies for EMA, and CEA has shown contradictory results, according to different
authors; also, Leu M1, B72.3, Ber-EP4, as well as alpha-fetoprotein, prostatic acid phosphatase,
prostatic specific antigen, and vimentin are reported to be negative in most cases
. EM reveals
the classic features of adenocarcinoma, with occasional cilia . The long, branching microvilli of
normal epididymis, also known as stereocilia, have not been reported in these tumors. In addition to
anatomical location, the distinction between epididymal carcinoma and serous and rete testis tumors will
mainly rely upon subtle histological features. There is little information on prognosis, due to the
small number of published cases, but around 50% of the patients with epididymal carcinoma are reported to
die with disseminated tumors, in spite of several methods of treatment.
Adenoma of the rete testis
This is also a rare tumor, with a solid, cystic or mixed macroscopic appearance, arising in the hylum
of the testis. Characteristically, nodular aggregates of tumor cells project into cystic spaces. The
cells form papillae, slit-like spaces, and tubules reminiscent of Sertoli cell tumors. The term
sertoliform cystadenoma has been applied to those cases in which the latter component predominates. In
addition, when there is stromal proliferation, the term adenofibroma is preferred
Adenocarcinoma of the rete testis
This is a tumor reported exclusively in older Caucasian men, and, mainly due to its location in the
posterior aspect of the testis, it is often missed in its initial stages. Related in part to this fact,
adenocarcinoma of the rete testis has a poor prognosis, with frequent local and lymphatic spread, and
with an average survival of 8 months after diagnosis
. Strict requirements for the diagnosis of
adenocarcinoma of the rete testis, proposed by Nochomovitz and Orenstein, include the absence of an
extrascrotal tumor with a similar morphology, a location centered in the testicular hylus, a morphology
different from any other testicular or paratesticular tumor, microscopic evidence of transition between
the rete testis and the tumor, and a predominantly solid growth, although focal cystic change is allowed
. Tumor cells arrange themselves in nodules and form slit-like spaces, combined with
papillary, tubular, and solid areas. Cells are relatively small, lack overt pleomorphism, and have
molded or grooved nuclei. Serous tumors and mesothelioma are the main differential diagnostic
EMA is characteristically positive in rete testis adenocarcinoma, Leu M1 is negative, and CEA gives
contradictory results. Positivity with HBME-1 and thrombomodulin antibodies has been reported in one
case that was classified as an adenocarcinoma of the rete testis . As stated above, HBME-1 is known
to be positive in many adenocarcinomas, and it is the pattern of positivity that helps in its distinction
from mesothelioma. On the other hand, thrombomodulin is not exclusive for mesothelioma. Similar to the
ovary, calretinin must be used with great caution in the differential diagnosis between mesothelioma and
these testicular neoplasms, which may mimic one of the many histological patterns of sex cord tumors, in
which calretinin is also positive. Ultrastructurally, rete testis adenocarcinoma is characterized by
variable proportions of short microvilli, devoid of core rootlets, complex lateral interdigitations with
abundant desmosomes, and characteristically indented nuclei. The cytoplasm contains variable amounts of
lipid and glycogen, but lacks secretory granules. These features are similar to those of normal rete
testis epithelium .
In summary, the use of IHC in the diagnosis of mesothelioma is a difficult and unsettled issue. In
both the peritoneum and the testis and paratestis, it is further complicated by the sometimes paradoxical
or unexpressive immunohistochemical phenotypes of the many tumors which enter the differential diagnosis.
There is no single clue or magic marker, and therefore EM may be particularly helpful in this setting.
As in many other areas of Pathology, the careful evaluation of clinical and histopathological data, along
with the judicious application of IHC and EM, are required to reach the correct diagnosis.
To Dr. Patricia Troncoso, Dr. Anais Malpica and Dr. Pheroze Tamboli, from the
Department of Pathology at the University of Texas M.D. Anderson Cancer Center, for their help with the
preparation of this presentation.
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