Extrauterine mixed endometrial stromal-smooth muscle tumor: Report of an unusual entity.

Dear Editor,

The mesenchymal tumors of the uterine corpus prove to be of great diagnostic difficulty to most histopathologists due to many overlapping features on light microscopy. Pointing at the origin of these tumors, whether they arise from uterine smooth muscle or endometrial stroma, may be truly challenging. A rare group of these neoplasms contain both smooth muscle and stromal cells and were designated as “stromomyomas” in the older literature. Currently, they have been renamed as mixed endometrial stromal smooth muscle tumors (MSST) and are diagnosed when a minimum of 30% of the minor component is present in an otherwise typical stromal neoplasm or leiomyoma.[1] These tumors are sparsely reported and are said to occur within the uterine corpus. Extrauterine location as seen in this case has not been reported till date.

A 52 year old lady presented with abdominal distension and early satiety over a period of 1 year, with a significant weight loss of about 7-8 kg within 6 months. Abdominal examination revealed a smooth, nontender mass extending upto the epigastrium. On pervaginal and per speculum examination, the vagina was healthy and a smooth mass was felt occupying the whole of pelvis with restricted mobility.

The abdominal ultrasound revealed a huge heteroechoic pelvic mass extending from epigastrium to hypogastrium, with possible retrovesicle extension. The mass measured 45 cm in greatest dimension and showed a few cystic areas within. But no calcific foci were detected. The possibilities of a giant fibroid arising from the right broad ligament and gastrointestinal stromal tumor were given on clinical and radiological grounds. Surgical procedure comprising of total abdominal hysterectomy with bilateral salpingo-oophorectomy and excision of the broad ligament fibroid was performed.

On gross examination, the mass was completely extrauterine, nodular, with whorled cut surface and weighed approximately 7 kg. The uterus was distorted, although no other gross abnormalities were identified in the uterus, cervix or adnexa. Multiple sampling of the nodular mass was performed and microscopy showed a benign neoplastic proliferation of spindle shaped cells with hyalinization, ischemic necrosis and fat infiltration. Also noted were adjoining areas showing small groups of cells interspersed between the smooth muscle cells. These cells had a vesicular nucleus, scant cytoplasm and were seen whorling around small arterioles [Figure 1]. There was no nuclear pleomorphism, atypia or increased mitotic activity noted. A provisional diagnosis of mixed endometrial stromal smooth muscle tumor was made on light microscopy.

Figure 1

Photomicrograph showing mixed endometrial stromalsmooth muscle tumor with both components (×100 original magnification, [H and E]; inset showing endometrial stromal cells (×400, original magnification, HE)

Immunohistochemical markers like CD10, Desmin, SMA, vimentin and h-caldesmon were performed for demonstrating the histogenesis. The smooth muscle cells were positive for Desmin [Figure 2] and vimentin and the small group of cells interspersed between them, which were likely of endometrial stromal origin, were positive for CD10 [Figure 3]. The stromal component accounted for more than 30% of the tumor. The patient is disease free for the past 8 months.

Figure 2

Immunohistochemical stain for Desmin showing cytoplasmic staining in smooth muscle component (×400, original magnification, immunohistochemistry)

Figure 3

Imunohistochemical stain for CD10 showing membranous staining in endometrial stromal component with negative staining in adjacent smooth muscle cells (×400, original magnification, immunohistochemistry)

MSSTs are rare entities described in literature. Originally they were thought to be endometrial stromal neoplasms with smooth muscle differentiation, with minor component being identified only on immunohistochemistry. The proportion of each component was not specified.[23] In tumors, where both the components were distinguished on histology, were called stromomyomas.[4]

Some studies have showed that distinguishing the deep endometrial stromal cells and myometrial cells on histology is difficult.[2] Some authors have also suspected the role of a multipotent stem cell in the myometrium that can differentiate in lines of stromal, smooth muscle or other elements. Scully et al. proposed that a part of female genital tract, the peritoneum, particularly its pelvic component including the mesothelium and subjacent connective tissue has the capacity to differentiate into endometrial, endosalpingeal and rarely endocervical type epithelia. The underlying connective tissue is a multipotential layer that can give rise to endometrial stroma type cells, decidua and proliferating smooth muscle cells, the entity termed as leiomyomatosis peritonalis disseminate.[5]

The clinical presentation of these tumors is very similar to that of a pure endometrial stromal or smooth muscle tumor. Enlarged uterus or mass per abdomen being the most commonly reported presenting complaints in one of the largest series reported.[6] Our patient had slowly enlarging abdominal mass. The location of these tumors in usually intramural and sometimes submucosal or subserosal. But this case was unusual in that the tumor was located in the broad ligament and came with a diagnosis of giant fibroid of the broad ligament. A review of the existing literature on MSST had not shown any such report till date. The gross appearance of the tumor in the present case was like a leiomyoma which is similar to that reported in literature.

The microscopic examination revealed both the components with a predominance of smooth muscle component and nodular small aggregates of the stromal component among the interlacing smooth muscle fascicles. Immunohistochemistry is a very useful adjunct in diagnostically difficult scenarios[7] which has helped in the present case as well.

Although these tumors are reported to be benign and have indolent course, larger studies and longer follow-up are required in these patients to clearly establish the behavior of such tumors. The present reports adds to the existing sparse knowledge on these rare neoplasms, most of which were reported within the uterine corpus.