Diffuse WT1 Positivity in Smooth Muscle Component Supports Dual Differentiation of Mesothelial Cells in the Histogenesis of Leiomyoadenomatoid Tumors—A Report of two Cases

Introduction

Epidydimal tumors are rare and almost exclusively benign. ¹ The most common epididymal tumors are adenomatoid tumors, which are encountered in middle-aged and elderly men and are mesothelial in origin.^2,3 Epididymal leiomyomas, which arise from smooth muscle, are the second most common tumor of the epididymis in adults.^4–6 Leiomyoadenomatoid tumors of the epididymis, previously described as a “mixed tumor of the epididymis” by Sworn et al ⁷ and “adenomatoid leiomyoma” by Wilson, ⁸ were defined by Epstein ⁹ in 1992 to describe an adenomatoid tumor with a prominent smooth muscle component. Only 7 cases of leiomyoadenomatoid tumors have been reported to date in English literature. ¹⁰ Therefore, leiomyoadenomatoid tumors are exceedingly rare tumors that contain 2 components: an adenomatoid component in the form of gland-like structures that are lined by single flat or cuboidal cells admixed with smooth muscle. Radiological and gross findings cannot distinguish leiomyoadenomatoid tumors from the more common classic adenomatoid tumors or leiomyomas, and a careful microscopic examination is critical in the identification of this esoteric variant. ¹¹

The histogenesis of this entity remains ambiguous. Opinions are divided and evidence is sparse; however, the general hypotheses include a collision tumor, a variant of an adenomatoid tumor with a smooth muscle component, or an adenomatoid tumor arising in the background of reactive smooth muscle hyperplasia. ⁹ We present 2 leiomyoadenomatoid tumors with findings that may help delineate the pathogenesis of this entity.

Case Report

Pathological Findings

Gross examination of both resections revealed a round, firm, well-circumscribed mass in the epididymis measuring 1.2 and 2 cm, respectively. The cut surface was tan-white and whorled, without any hemorrhage or calcifications.

Microscopic examination of both lesions revealed a biphasic tumor composed predominantly of spindle cells arranged in fascicles interspersed with scant cords, nests, and tubules lined with flat to cuboidal cells (Figure 1 and 3). No nuclear atypia, necrosis, or mitoses were identified in either tumor.

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Figure 1.

Representative images of leiomyoadenomatoid tumor from 49-year-old transgender female (A and B) hematoxylin and eosin (H and E) at 5×; (C and D) H and E at 10×.

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Figure 2.

IHC images of leiomyoadenomatoid tumor from a 49-year-old transgender female: (A) desmin (10×) and (B) SMA (10×) are positive in the smooth muscle component; (C) calretinin (10×) is positive in the adenomatoid component; (D) WT1 (10×) staining both components.

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Figure 3.

Representative images of leiomyoadenomatoid tumor from a 66-year-old man (A & B) hematoxylin and eosin (H and E) at 5×; (C & D) H and E at 10×.

Immunohistochemical staining was performed on 4-μm-thick unstained sections cut from paraffin-embedded blocks using the Dako PT Link and Dako Autostainer Link 48 systems (Agilent Technologies). The cuboidal cells forming the nests, cords, and tubules were positive for calretinin and D240, which was consistent with an adenomatoid tumor. The spindle cell component was positive for desmin and smooth muscle actin (SMA), which was consistent with smooth muscle differentiation. The morphology and immunoprofile were diagnostics of a leiomyoadenomatoid tumor. Interestingly, WT1 exhibited diffuse nuclear expression in both the adenomatoid and smooth muscle components of the tumor. The staining pattern in both cases was identical (Figure 2 and 4).

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Figure 4.

IHC images of leiomyoadenomatoid tumor from a 66-year-old man: (A) desmin (10×) and (B) SMA (10×) are positive in the smooth muscle component; (C) calretinin (10×) is positive in the adenomatoid component; (D) WT1 (10×) staining both components.

Discussion

Adenomatoid tumors, first described by Sakaguchi in 1916, ¹² are benign tumors that are usually observed in the male or female genital tract and rarely in extragenital sites in the vicinity of serosal membranes.^13,14 Evans ¹⁵ and Mason et al ¹⁶ described the mesothelial origin of these tumors in 1943.

In the male genital tract, the most common location of the tumor is the distal part of the epididymis, more frequently on the right side, as seen in the 2 tumors included in this report. ¹⁷ They comprise 65% of all benign neoplasms that occur in these zones, followed by leiomyomas, which represent the second most common tumor type. ² Epididymal adenomatoid tumors usually present as slow-growing, painless intrascrotal masses in middle-aged individuals and are discovered incidentally during physical examination or other procedures. ² These tumors appear as heterogeneous hypoechoic or hyperechoic lesions on sonography. ²

Histologically, the tumor may exhibit many architectural patterns, such as adenoid, angiomatoid, solid, cystic, or a combination of multiple types, evoking an extensive list of differential diagnoses.^18,19

The term “leio­myoadenomatoid tumor” was first used by Epstein to describe a variant of an adenomatoid tumor with a prominent smooth muscle component. ⁹ The extent of the smooth muscle component varies and can be so extensive that it overtakes and obscures the adenomatoid tumor, resulting in its misdiagnosis as either a leiomyoma or a malignant tumor infiltrating smooth muscle bundles. ²⁰ First described in 1946, ⁷ the tumor remains a rarely diagnosed entity, with only 7 cases reported to date. This may reflect the truly infrequent nature of this entity or its frequent misdiagnosis as a leiomyoma due to the scant adenomatoid component being overrun by the smooth muscle component.

The uncertainty surrounding the histogenesis of leiomyoadenomatoid tumors persists. ¹⁰ Authors continue to debate whether it represents a collision neoplasm, a variant of adenomatoid tumor associated with smooth muscle proliferation, or an adenomatoid tumor arising in the background of smooth muscle hyperplasia. Some degree of smooth muscle cell hyperplasia has been reported in association with adenomatoid tumors of the uterus and has been interpreted as a reactive phenomenon because the hyperplastic muscle cells exhibit a higher proliferative index than the adjacent normal myometrium. ²¹ However, unlike the uterus, the epididymis has limited native muscular tissue.

WT1 is used as a marker of mesothelial differentiation. WT1 positivity has been described in the vascular smooth muscle of the lung, suggesting that mesothelial cells serve as a source of vascular smooth muscle cells in the developing lung. ²² The diffuse nuclear WT1 positivity in both the adenomatoid and smooth muscle components in our study provides analogical evidence supporting the hypothesis that precursor cells with dual differentiation (mesothelial and muscular) give rise to the 2 different components. ²³ Focal WT1 positivity in adenomatoid tumors with myofibroblastic and myoblastic proliferation has been previously reported. ¹¹ However, the diffuse WT1 positivity observed in our study strongly suggests tunica vaginalis/paratesticular tissue origin via peculiar mesothelial differentiation.

Nuclear WT1 staining has been reported in up to 8% of uterine leiomyosarcomas and leiomyomas.^24,25 However, the staining in these cases is weak to moderate and more focal than the strong diffuse nuclear positivity seen in leiomyoadenomatoid tumors. Furthermore, WT1 positivity is observed in multiple gynecological neoplasms. As such, the WT1 expression seen in uterine smooth muscle tumors is likely attributable to their Mullerian origin as compared with the mesothelial origin indicated in leiomyoadenomatoid tumors. The diffuse nuclear WT1 positivity in these 2 tumors indicates the mesothelial origin of both components, clarifying this entity as a variant of the adenomatoid tumor rather than a collision neoplasm or aberrant hyperplastic smooth muscle.

The lack of calretinin and D240 staining is unusual for mesothelial origin. Due to the scarcity of leiomyoadenomatoid tumors, not much can be ascertained about the D240 and calretinin expression pattern in these tumors. However, a tissue microarray analysis of D240 and calretinin expression in malignant mesotheliomas decreased the sensitivity of both markers in sarcomatoid areas. ²⁶ In a study of sarcomatoid mesothelioma by Chirieac et al, ²⁷ WT1 showed a higher rate of positivity in the sarcomatoid variant as compared to D240 and calretinin. The expression of these three mesothelial markers clearly varies with the extent and pattern of differentiation. WT1-expressing cells have the capacity to switch between a mesenchymal and epithelial state. ²⁸ WT1 has been shown to be integral in mesothelial cell plasticity and the transition of mesothelial cells to a myofibroblast-like phenotype. ²⁹ It can be inferred, WT1 is expressed earlier as compared to D240 and calretinin in the mesothelial maturation pathway, which would explain the calretinin and D240 negativity seen in the smooth muscle component of leiomyoadenomatoid tumors. However, this phenomenon would require deeper study in a larger cohort and is outside the scope of this report.