Case 3 -

Florid Reactive Mesothelial Hyperplasia of Tunica Vaginalis John R. Srigley McMaster University/Credit Valley Hospital Mississauga, ON, Canada

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Clinical History 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. 2.5X Case 3 - Figure 2 - mesothelial proliferation involving inner surface of hydrocele sac with tubular elements extending into underlying connective tissue. 5X Case 3 - Figure 3 - proliferating mesothelial tubules involve wall of hydrocele sac. 5X Case 3 - Figure 4 - surface mesothelial proliferation with multi-layering and papillae. 5X Case 3 - Figure 5 - high power micrograph showing proliferating mesothelium with tubular formation. 10X Case 3 - Figure 6 - inner surface of hydrocele sac showing a papillary proliferation of mesothelium. 10X Case 3 - Figure 7 - high power photomicrograph showing tubular mesothelial elements involving wall of hydrocele. 10X Case 3 - Figure 8 - high power photomicrograph showing mesothelial tubules within hydrocele sac. Note the relative architectural and cytologic uniformity. 10X 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. 40X Case 3 - Figure 10 - cytokeratin 5/6 immunohistochemical preparation showing presence of mesothelial tubules relatively deep within the wall of the hydrocele sac. CK5/6 Case 3 - Figure 11 - calretinin immunostain showing strong positivity in the mesothelial tubules. CALRET Pathologic Findings 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 hyperplasia. Diagnosis 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. [1] A spectrum of mesothelial lesions similar to that seen in peritoneum and pleura may affect the paratesticular compartment (table 1). [2, 3, 4] 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 proper. [2, 3, 4] 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. [5] Rare examples of benign cystic mesothelioma and well differentiated papillary mesotheliomas have also been described in the paratestis and will not be discussed further. [6, 7] The most ominous of the mesothelial lesions involving tunica vaginalis is malignant mesothelioma. [2, 3, 4, 8, 9, 10] 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. [4] 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. [4] 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. [2, 3, 4, 8, 9, 10] 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. [10] Additionally, MMTVs show negativity for carcinoembryonic antigen and CK20. [10] The ultrastructural features of MMTVs are similar to mesotheliomas in other sites. [3, 10] The differential diagnosis of MMTVs is wide (see table 2). Table 2 - Malignant Mesothelioma of Tunica Vaginalis Differential Diagnosis benign reactive mesothelial proliferations adenomatoid tumor well-differentiated papillary mesothelioma borderline serous borderline neoplasm malignant 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 neoplasms. [2, 3, 4, 5, 8, 9, 10, 11] There is less literature on the features separating MMTV and atypical reactive mesothelial proliferations. [3, 4, 12, 13] 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. [4] 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. [4, 12, 13] 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 3. 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. [4] 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. [14, 15, 16] 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. [12, 13] The differential diagnosis of MMTV with other benign and malignant neoplasms is complex and will not be covered in any great detail here. [2, 3, 4, 5, 8, 9, 10, 11] Occasionally benign mesothelial neoplasms such as adenomatoid tumour with atypical features and well differentiated papillary mesothelioma enter the differential diagnosis with MMTV. [3, 4, 5] 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. [5, 11, 17, 18, 19] 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 considered. [3, 8, 10, 11] 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. References 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. 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