Case 3 -
Florid Reactive Mesothelial Hyperplasia of Tunica Vaginalis
John R. Srigley
McMaster University/Credit Valley Hospital
Mississauga, ON, Canada
Click on each slide thumbnail image for an enlarged view
A 53 year old male presented with non-painful left scrotal enlargement of several months duration.
Clinical and ultrasound examinations revealed a hydrocele without any obvious mass lesion and a
hydrocelectomy was performed (case contributed by Dr. Françoise Naud, Quebec City, Canada).
Case 3 - Figure 1 - low power photomicrograph of hydrocele specimen showing fibrosis and chronic information. Note proliferation of surface mesothelium and tubular glands uniformly distributed in a linear fashion within connective tissue.
Case 3 - Figure 2 - mesothelial proliferation involving inner surface of hydrocele sac with tubular elements extending into underlying connective tissue.
Case 3 - Figure 3 - proliferating mesothelial tubules involve wall of hydrocele sac.
Case 3 - Figure 4 - surface mesothelial proliferation with multi-layering and papillae.
Case 3 - Figure 5 - high power micrograph showing proliferating mesothelium with tubular formation.
Case 3 - Figure 6 - inner surface of hydrocele sac showing a papillary proliferation of mesothelium.
Case 3 - Figure 7 - high power photomicrograph showing tubular mesothelial elements involving wall of hydrocele.
Case 3 - Figure 8 - high power photomicrograph showing mesothelial tubules within hydrocele sac. Note the relative architectural and cytologic uniformity.
Case 3 - Figure 9 - high power photomicrograph showing mesothelial cells with mild nuclear enlargement, pleomorphism and prominent nucleoli. Note the presence of a mitotic figure in the lower part of the field.
Case 3 - Figure 10 - cytokeratin 5/6 immunohistochemical preparation showing presence of mesothelial tubules relatively deep within the wall of the hydrocele sac.
Case 3 - Figure 11 - calretinin immunostain showing strong positivity in the mesothelial tubules.
The distributed images representing the hydrocelectomy specimen show a thickened hydrocele sac with
prominent fibrosis and chronic inflammation. The surface mesothelium is focally flat but in areas shows
multi-layering and papillary change. Prominent tubular gland formation is identified and in many areas,
the mesothelial tubules are present relatively deep within the mural fibrous connective tissue where they
are often surrounded by chronic inflammation. The depth of glandular involvement is relatively uniform
throughout the length of the specimen.
In areas, the mesothelial cells display mild atypia with nuclear enlargement and pleomorphism.
Nucleoli are centrally placed and quite prominent in many cells. Occasional mitoses are seen but no
atypical mitoses are identified.
Immunohistochemical studies show that the glandular cells have the following immunophenotype: CAM
5.2+, CK5/6+, CK7+, 34 bE 12+, calretinin+, EMA+, CK20-, CEA-.
Based on the clinical, gross and microscopic features, a diagnosis of "atypical mesothelial
proliferation," was rendered. Since malignant mesothelioma of tunica vaginalis was still a
consideration, the patient underwent a completion radical orchidectomy and left hemi-scrotectomy. A
healed scar was present on the scrotal skin. On sectioning, the testicular and epididymal parenchyma was
unremarkable. The tunica albugenia and paratesticular soft tissue were densely adherent. In the
paratesticular area near the superior pole of testis there was an irregular area of firm tissue involving
tunica albugenia and extending into the scrotal soft tissue. The lesion measured 5.0 cm in greatest
dimension. The cut surface was grey-white and punctuated by small foci of hemorrhage and necrosis.
Extensive sampling of the orchidectomy specimen showed marked fibrosis and chronic inflammation with
focal hemorrhage and foreign body type granulomatous reaction. These changes were especially prominent
in the grossly identified tumor-like area. In many areas, especially those where residual tunica
vaginalis was present, residual tubular and papillary structures similar to those seen in the
hydrocelectomy specimen were noted. These glandular structures were distributed in a linear (zonal)
pattern respecting the anatomic boundaries of the obliterated tunical sac without indiscriminate
infiltration of tunica albuginea, testis, epididymis or scrotal soft tissue. The cytologic features of
the mesothelial cells were similar to those noted in the hydrocele specimen.
Immunohistochemical studies utilizing cytokeratins (CK5/6, CK7) and calretinin confirmed a linear
zonal pattern for the glandular elements, thus supporting a diagnosis of reactive mesothelial
Florid Reactive Mesothelial Hyperplasia of Tunica Vaginalis
Differential Diagnosis and Comment
Mesothelium is a relatively small but important histologic component of the paratesticular region.
Embryologically the tunica vaginalis originates as an outpouching of peritoneum and is lined by typical
mesothelium.  A spectrum of mesothelial lesions similar to that seen in peritoneum and pleura
may affect the paratesticular compartment (table 1).
Table 1 - Paratesticular Mesothelial Tumors and Tumor-Like Lesions
- Mesothelial cysts
- Mesothelial hyperplasia ± atypia
- Adenomatoid tumor
- Benign cystic mesothelioma
- Well-differentiated papillary mesothelioma
- Malignant mesothelioma of tunica vaginalis
Paratesticular mesothelial lesions, especially the true neoplasms, are quite uncommon. Mesothelial
cysts are occasionally encountered and these are usually lined by an attenuated layer but may display
increased cellularity and papillary change in keeping with mesothelial hyperplasia. Reactive mesothelial
proliferations are often present within hydroceles. Simple and papillary-glandular hyperplasia are
sometimes encountered and on rare occasions, florid reactive hyperplasia with cytologic atypia (such as
the current case) is noted.
The most common benign neoplasm of mesothelial origin is the adenomatoid tumor which
characteristically involves epididymis but can also involve tunica vaginalis, spermatic cord or testis
These tumors are generally small, circumscribed, firm, gray nodules which have a
characteristic histology consisting of small vascular-like spaces lined by attenuated or cuboidal cells
separated by variable amounts of fibrous stroma. Adenomatoid tumors rarely cause diagnostic difficulties
unless they have unusual histologic features.  Rare examples of benign cystic mesothelioma and
well differentiated papillary mesotheliomas have also been described in the paratestis and will not be
The most ominous of the mesothelial lesions involving tunica vaginalis is malignant mesothelioma.
Patients with malignant mesothelioma of tunica vaginalis (MMTV) usually present with
scrotal enlargement and often have a hydrocele. In some instances, a history of recurrent hydrocele is
noted. Occasionally, MMTV may present as a scrotal skin nodule or as a distant metastasis.
Some cases of MMTV are associated with asbestos exposure; however, the associated occupations and
other exposure details are often unclear.  It is thought that less than one third of MMTVs are
truly associated with asbestos and the majority appear to be idiopathic in their etiology. 
Macroscopically, MMTV presents as a mass in a hydrocele sac or a solid tumor, often encasing the
testis proper. Multiple nodules are sometimes seen. The tumor may grossly invade testis, epididymis,
scrotal skin and other local structures.
At the histologic level, about three-quarter of MMTVs have an epithelial histology and the remainder
are biphasic with very rare sarcomatoid examples.
The epithelial tumors tend to have
a tubulopapillary morphology, although cords and solid tumor sheets are sometimes seen. The sarcomatoid
elements generally consist of atypical spindle cells, with fascicular, storiform or diffuse growth
patterns. MMTVs are histologically similar to malignant mesotheliomas of pleura and peritoneum. They
also display similar immunophenotypes and characteristically show positivity for cytokeratins such as CAM
5.2, CK5/6, CK7, 34 bE 12. They also stain positively for epithelial membrane antigen, thrombomodulin
and calretinin. Occasional positivity for BerEP4 has been noted but the majority of cases are negative
with this marker.  Additionally, MMTVs show negativity for carcinoembryonic antigen and
CK20.  The ultrastructural features of MMTVs are similar to mesotheliomas in other
The differential diagnosis of MMTVs is wide (see table 2).
Table 2 - Malignant Mesothelioma of Tunica Vaginalis Differential Diagnosis
- reactive mesothelial proliferations
- adenomatoid tumor
- well-differentiated papillary mesothelioma
- serous borderline neoplasm
- serous carcinoma
- adenocarcinoma of rete testis
- adenocarcinoma of epididymis
- metastatic adenocarcinoma
Most of the literature deals with the distinction of MMTV and other benign and malignant
There is less literature on the features separating MMTV and atypical
reactive mesothelial proliferations.
The current case is an excellent example of
this problem and the remainder of this contribution will focus on the issue of reactive versus malignant
mesothelial proliferation in the tunica vaginalis.
Hydrocele sacs generally have thin walls and are lined by a simple layer of flattened
mesothelium.  Sometimes there are minor degrees of surface proliferation with piling up of
mesothelial cells and papillary formations. In general, these processes are not difficult to separate
from true neoplasms.
In some instances however, hydrocele sacs are thickened, fibrotic and may be associated with
mesothelial proliferation that includes both papillary exophytic growth and tubules with a "pseudo
invasive" growth pattern. The latter change when florid can simulate MMTV. When there is mesothelial
proliferation associated with an exudative process, the fibrin may organize and become fibrotic, thus
entrapping proliferating glandular structures. Often there is accompanying chronic inflammation or
lymphoid aggregates. Sometimes a proliferating spindle cell element may also be seen. Often in the
reactive situation, there is a distinct zonation with a band of fibrous connective tissue juxtaposed to
an area of a mesothelial proliferation.
When proliferating mesothelial cells forming
glands are caught up in organizing exudates, they can mimic an invasive front of MMTV. Some worrisome
histologic features that are seen in reactive mesothelial hyperplasia in hernia sacs are shown in table
Table 3 - Reactive Mesothelial Hyperplasia in Hernia Sacs Worrisome Histologic Features
- high cellularity
- cytologic atypia
- mitotic activity
- papillae and tubular gland formation
- pseudo invasion (entrapment)
High cellularity, cytologic atypia, and mitotic activity may be seen in reactive proliferations.
Reactive cells often show cytologic atypia with enlarged nuclei and prominent nucleoli. Mitoses may also
be seen. Atypical mitoses are not a feature of reactive mesothelial proliferations. Cytologic atypia,
except when extreme in degree, or when atypical mitoses are seen, is not a useful criteria in separating
a reactive mesothelial proliferation from MMTV. Additionally, papillae and glandular formations are
noted and pseudo invasion or entrapment of glandular elements is a common phenomenon. The features that
point to a benign diagnosis include the uniform depth of involvement of the "pseudo invasive" glands
along with the zonation of the juxtaposed fibrous connective tissue. Keratin stains can be quite useful
in helping to find this characteristic.  The relatively sharp circumscription of the reactive
process contrasts with the more irregular pattern of infiltration characteristic of MMTV. The
proliferating mesothelial cells in benign reactive processes tend to be present as linear arrays parallel
to the free surface of the hydrocele sac. MMTV typically has a haphazard distribution of glandular
elements. Additionally the tubular glands of mesothelial hyperplasia tend to be simple without the
complex branching architecture which is more characteristic of MMTV.
Generally, immunohistochemistry is not helpful in separating reactive mesothelial proliferation from
mesothelioma. However, there is some data from studies of malignant mesothelioma of pleura suggesting
that mutant p53 protein and epithelial membrane antigen (EMA) may be of use in separating benign from
malignant mesothelial proliferations; however, the literature is somewhat contradictory.
p53 and EMA expression are more commonly seen in malignant mesothelioma than in reactive conditions but
both markers may be present in hyperplastic lesions as well. In the individual case,
immunohistochemistry is not particularly useful. Furthermore, these studies have not been performed in a
comprehensive fashion in the mesothelial proliferations of tunica vaginalis. In most instances using the
above histologic criteria, one can reasonably separate reactive mesothelial proliferation from malignant
mesothelioma. However, in some rare cases where separation is not possible, a term such as "atypical
mesothelial proliferation, see comment" may be used.
The differential diagnosis of MMTV with other benign and malignant neoplasms is complex and will not
be covered in any great detail here.
Occasionally benign mesothelial
neoplasms such as adenomatoid tumour with atypical features and well differentiated papillary
mesothelioma enter the differential diagnosis with MMTV.
On the malignant side, there is an
interesting differential diagnosis which includes the serous borderline tumors and serous carcinomas of
paratestis along with the extremely rare adenocarcinomas derived from rete testis and epididymis.
The gross morphology, including the location of the tumour epicentre and its
distribution, along with the histologic and immunohistochemical features generally allow separation of
these rare entities. Finally, the possibility of secondary adenocarcinoma should always be
Tumors originating in the gastrointestinal tract, lung and kidney may
sometimes present as testicular or paratesticular masses. Clinical history, routine microscopy, mucin
stains and immunohistochemistry are usually able to sort out these differential diagnoses.
In summary, florid reactive mesothelial proliferations of tunica vaginalis may sometimes create
confusion with MMTV. Careful histologic assessment coupled with the use of keratin stains to better
appreciate the pseudo-invasive growth pattern can generally lead to a correct diagnosis.
- Srigley, John R: The paratesticular region: histoanatomic and general considerations. Semin Diagn Pathol 17:258-269, 2000.
- Srigley JR, Hartwick RW: Tumors and cysts of the paratesticular region. Pathol Annual 25 Pt 2:51-108, 1990.
- Perez-Ordonez B, Srigley JR: Mesothelial lesions of the paratesticular region. Semin Diagn Pathol 17:294-306, 2000.
- Churg A: Paratesticular mesothelial proliferations. Semin Diag Pathol 20:272-278, 2003.
- Amin MB: Selected other problematic testicular and paratesticular lesions: rete testis neoplasms and pseudotumors, mesothelial lesions and secondary tumors. Mod Pathol 18, S131-S145, 2005.
- Lane, TM, Wilde M, Schofield, J et al: Benign cystic mesothelioma of the tunica vaginalis. Br J Urol Int 84:533-534, 1999.
- Chetty R: Well differentiated (benign) papillary mesothelioma of the tunica vaginalis. J Clin Pathol 45:1029-1030, 1992.
- Jones MA, Young RH, Scully RE: Malignant mesothelioma of the tunica vaginalis. A clinicopathologic analysis of 11 cases with review of the literature. Am J Surg Pathol 19:815-825, 1995.
- Attanoos RL, Gibbs AR: Primary malignant gonadal mesotheliomas and asbestos. Histopathology 37:150-159-, 2000.
- Winstanley AM, Landon G, Berney D, et al: The immunohistochemical profile of malignant mesotheliomas of the tunica vaginalis. A study of 20 cases. Am J Surg Pathol 30:1-6, 2006
- Jones, EC, Murray, SK, Young, RH: Cysts and epithelial proliferations of the testicular collecting system (including rete testis). Semin Diagn Pathol 17:270-293, 2000.
- Churg A, Colby TV, Cagle P, et al: The separation of benign and malignant mesothelial proliferations. Am J Surg Pathol 24:1183-1200, 2000.
- Cagle PT, Churg A: Differential diagnosis of benign and malignant mesothelial proliferations on pleural biopsies. Arch Pathol Lab Med 129:1421-1427, 2005.
- Mayall FG, Goddard H, Gibbs AR: p53 immunostaining in the distinction between benign and malignant mesothelial proliferations using formalin-fixed paraffin sections. J Pathol 168:377-381. 1992.
- Cagle PT, Brown RW, Lebovitz RM: p53 immunostaining in the differentiation of reactive processes from malignancy in pleural biopsy specimens. Human Pathol 25:443-448, 1994.
- Wolanski KD, Whitaker D, Shilin K B, et al. The use of epithelial membrane antigen and silver-stained nucleolar organizer regions testing in the differential diagnosis of mesothelioma from benign reactive mesothelioses Cancer 82:593-590, 1998.
- Young RH, Scully RG: Testicular and paratesticular tumors and tumor-like lesions of ovarian common epithelial and mullerian types. Am J Clin Pathol 86:146-152, 1986.
- McClure RF, Keeney GL, Sebo TJ, et al: Serous borderline tumor of the paratestis. Amer J Surg Pathol 25:373-378, 2001.
- Jones MA, Young RH, Srigley JR, et al: Paratesticular serous papillary carcinoma. Am J Surg Pathol 19:1359-1365, 1995.