Pathology Update

LEIOMYOBLASTOMA OF THE OMENTUM PRESENTING AS HEMOPERITONEUM: REPORT OF A CASE AND REVIEW OF THE ENGLISH LITERATURE

Asligul Umar, MD, and Burton C. West, MD, of the Department of Medicine,
and Leon D. Brown, MD, of the Department of Pathology Meridia Huron Hospital, Cleveland, Ohio.

This regular feature presents special issues in oncologic pathology.

Introduction

Leiomyoblastomas are rare, smooth muscle tumors that were first described in 1962.¹ Ninety-three percent of all leiomyoblastomas are found in the stomach.² The uterus, small intestine, colon, and kidneys are less frequent sites of occurrence. Leiomyoblastomas of the omentum are extremely rare; we could find only 12 cases reported in the English literature since 1969. We present a case of omental leiomyoblastoma that presented with acute hemoperitoneum.

Case Report

A 56-year-old woman was admitted with the chief complaint of acute postprandial abdominal pain located in the right upper quadrant. The pain, which was continuous and increased with deep breathing, was relieved when the patient lay on her right side and was not associated with nausea or vomiting. Her past medical history was significant for hypertension, appendectomy, and hysterectomy. Physical examination revealed tenderness in the right upper quadrant mimicking a positive Murphy’s sign. There was no organomegaly, mass, rebound tenderness, or guarding. Bowel sounds were normal. The rectal examination revealed brown stools, no occult blood, no tenderness, and no mass. A pelvic examination was normal. Abnormal laboratory data upon admission were as follows: hemoglobin, 121 g/L; hematocrit, 34%; white blood cell count, 13.8 x 10⁹/L (neutrophils 82%, lymphocytes 12%, monocytes 5%, basophils 1%); mean corpuscular volume, 81 fL; platelet count, 213 x 10⁹/L; and erythrocyte sedimentation rate, 65 mm/h. Amylase and lipase levels were normal. A chemistry profile was within normal limits. On the second hospital day, the pain shifted to both lower quadrants of the abdomen, and tenderness was observed in the right lower quadrant and epigastrium. There was no guarding and no rebound. Bowel sounds were hypoactive. A computed tomography of the abdomen revealed ascites, bilateral pleural effusions, and a mass located in front of the anterior gastric wall that measured 4 x 4 x 11 cm (Figs 1 and 2). The patient’s hemoglobin level that day was 96 g/L, and her hematocrit level was 29%. She underwent surgery that disclosed a copious amount of defibrinated blood within the peritoneal cavity. On further exploration, a mass measuring 6 x 3 x 2 cm, which appeared to have ruptured, was found attached to the greater omentum, with no connection to the stomach. The surgeon’s intraoperative diagnosis was ruptured ectopic spleen. Intraoperative pathologic evaluation of the specimen by frozen sectioning did not suggest normal splenic tissue, although some large, multinucleated cells raised a question of megakaryocytes as a part of extra-medullary hematopoiesis. Permanent sections excluded extra-medullary hematopoiesis and revealed a tumor with a mixture of spindle and epithelioid cells that was consistent with leiomyoblastoma, with some of the cells being giant and multinucleated (Figs 2 and 3). The tumor exhibited minimal mitotic activity, with no mitoses found in 20 high-power fields. Tumor cells were shown to contain muscle-specific actin by immunostaining, providing further evidence for a smooth muscle origin (Fig 4). In addition to positive immunostaining for muscle-specific actin, focal positive staining was demonstrated with S-100 antigen. At the periphery of the tumor was a region of orderly, normal-appearing, smooth muscle that was more strongly actin-positive, suggesting an origin from some smooth muscle-containing structure. Negative results with immunostains for low-molecular-weight keratins (AE1 and AE3) were obtained, which tend to exclude an anaplastic carcinoma.

The postoperative course was uneventful, and a recent telephone follow-up indicated the patient is fine at 14 months after surgery.

Discussion

Leiomyoblastomas usually arise in the stomach. The most common manifestations of gastric leiomyoblastomas are gastrointestinal bleeding, abdominal mass, and dyspepsia,³ although they also have been reported to present as hemoperitoneum.^4-6 Among the 12 omental leiomyoblastomas that we were able to find reported in the English literature, 11 presented as an abdominal mass, pain, or fullness. Only one previous case presented as hemoperitoneum.² These tumors were quite large, ranging from 6 x 3 x 2 cm to 30 x 20 x 13 cm. The patient population consisted of seven women and six men. No racial information was given. Their mean age was 57 years, ranging from 36 to 76 years at the time of diagnosis (Table).

Leiomyoblastomas belong to the spectrum of smooth muscle tumors that includes leiomyomas, leiomyoblastomas, and leiomyosarcomas. In terms of malignant potential, leiomyoblastomas stand between their benign counterpart leiomyomas and malignant counterpart leiomyosarcomas, with malignant behavior occurring in up to 12% of cases.⁷ Histologically, leiomyoblastomas contain spindle cells like leiomyomas and leiomyosarcomas, but unlike the latter two categories of smooth muscle tumors, leiomyoblastomas contain predominantly epithelioid myocytes, ie, myocytes having polygonal or round form. This is the key histologic feature for these tumors. The epithelioid cells of leiomyoblastoma frequently possess vacuolated or clear cytoplasm. Mitotic activity for leiomyoblastomas is variable, whereas it should be absent in leiomyomas and is characteristic of leiomyosarcomas. Demonstration of actin supports smooth muscle-cell origin for tumor cells. An admixture of S-100 positive cells in smooth muscle tumors of the gastrointestinal tract is not unexpected and has prompted some to refer to such tumors more broadly as stromal tumors.⁸

The concept of leiomyoblastoma, first introduced by Stout in 1962,¹ has been respected and accepted in a number of subsequent reports.^5,9,10 However, Evans⁷ has questioned the validity of such a distinction, claiming round cells in smooth muscle tumors represent merely a clinically unimportant variation in cellular morphology among leiomyomas or leiomyosarcomas rather than the mark of a separate tumor entity.

This case represents the 13th reported patient with a leiomyoblastoma having an omental origin, and it represents the second case to present with hemoperitoneum. This uncommon tumor with a truly rare presentation has had a favorable outcome to date.

References

1. Stout AP. Bizarre smooth muscle tumors of the stomach. Cancer. 1962;5:400-409.

2. Rye BA, Christiansen E, Larsen LG. Acute bleeding from leiomyoblastoma of the greater omentum: a case report. Tumori. 1989;75:296-298.

3. Shiu MH, Farr G, Papachristou DN, et al. Myosarcomas of the stomach: natural history, prognostic factors and management. Cancer. 1982;49:177-187.

4. Kelsey JR. Leiomyoblastoma of the stomach presenting as acute intraperitoneal hemorrhage. Gastroenterology. 1966;51: 539-541.

5. Kokubo T, Ohtomo Y, Iseki J, et al. Gastric leiomyoblastoma: report of a case. Radiat Med. 1986;4:124-126.

6. Zerbi A, Braga M, Sironi A, et al. An uncommon cause of massive intraperitoneal bleeding: leiomyoblastoma of the stomach. Tumori. 1988;74:365-367.

7. Evans HL. Smooth muscle tumors of the gastrointestinal tract: a study of 56 cases followed for a minimum of ten years. Cancer. 1985;56:2242-2250.

8. Miettinen M. Gastrointestinal stromal tumors: an immunohistochemical study of cellular differentiation. Am J Clin Pathol. 1988;89:601-610.

9. Waldrep D, Morgenstern L, Alshak NL. The enigma of leiomyoblastoma: spectrum of a rare gastric tumor. Am Surg. 1993; 59:256-260.

10. Lerner ME, Farman J, Cho K, et al. Leiomyoblastoma: varied CT appearance. Clin Imaging. 1992;16:194-197.

11. Smithwick III W, Biesecker J, Leonol PM. Leiomyoblastoma: behavior and prognosis. Cancer. 1969;24:996-1003.

12. Weinberger HA, Ahmed MS. Mesenchymal solid tumors of the omentum and mesentery: report of four cases. Surgery. 1977;82:754-759.

13. Tsurumi H, Okada S, Koshino Y, et al. A case of leiomyoblastoma (epithelioid leiomyosarcoma) of the greater omentum. Gastroenterol Jpn. 1991;26:3070-3075.

14. Hertzanu Y, Mendelsohn DB, Murray JF. Leiomyoblastoma of the omentum: a case report. S Afr Med J. 1982;62:297-298.

15. Laky D, Halalau F. Leiomyoblastoma of the lesser omentum sac: a case report. Morphologie et Embryologie. 1989;35:45-48.

Presented at the regional meeting of the American College of Physicians, Columbus, Ohio, November 9, 1995.

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