Original article
Anterior rectal wall gastrointestinal stromal tumor presenting clinically as prostatic mass

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Abstract

Gastrointestinal stromal tumors (GIST) of the anterior rectal wall can present as a prostatic mass, with concomitant obstructive symptoms. Transrectal biopsies of GIST may be misdiagnosed as primary prostatic sarcomas. We report 3 cases of GIST that were initially characterized as prostatic leiomyosarcomas and treated definitively with pelvic exenteration. The correct diagnosis was possible only after immunohistochemical staining for CD117 and was made retrospectively in 2 of 3 cases. Additional therapy with imatinib (Gleevec™, Novartis Pharmaceuticals Corporation, East Hanover, NJ), an inhibitor of CD117 tyrosine kinase activity, treated recurrence in one patient and effected complete remission.

Introduction

Malignant gastrointestinal stromal tumors (GIST) of the rectum account for 0.1% to 0.5% of all rectal malignancies [1]. Some show smooth muscle differentiation and were classified as leiomyosarcoma in the older literature [2]. Recent investigations have shown that immunoreactivity for CD117 (c-kit) is universal in these lesions [3], just as in GIST originating at other sites. Anterior extension of rectal GIST can lead to obstructive voiding symptoms, [2] including urinary retention, but bladder or prostate involvement has rarely been reported [4]. The advent of effective, specific adjuvant treatment of GIST with an inhibitor (imatinib, Gleevec™, Novartis Pharmaceuticals Corporation, East Hanover, NJ) of the tyrosine kinase activity of the CD117 antigen (the c-kit protooncogene product) has revolutionized treatment of disseminated or recurrent GIST.

Primary stromal malignancies of the prostate are also rare neoplasms [5], and are typically highly aggressive [6]. Classification of these tumors is undergoing reevaluation, [5], [7] but most of the aggressive stromal neoplasms of the adult prostate have traditionally been considered leiomyosarcomas. Immunoreactivity for CD117 is rare in sarcomas other than GIST, including leiomyosarcoma [8], and immunohistochemically detectable expression of CD117 is absent in benign and malignant prostate epithelium or myoepithelium [9].

This report describes 3 cases in which a high stage pelvic sarcoma was initially considered clinically to represent a primary prostatic sarcoma, whereas subsequent histopathologic reevaluation revealed a rectal GIST intruding into the prostate. In one patient, after progressive, recurrent disease refractory to traditional salvage chemotherapy had developed, the tumor was in complete remission after 3 months of imatinib therapy.

Section snippets

Case 1

A 45-year-old white male with voiding difficulties was found to have a large prostatic mass. Computerized tomography showed a 15-cm mass filling the pelvis and displacing the bladder. At surgery, the tumor appeared adherent to the rectal wall, and a radical cystoprostatectomy was performed with resection of a portion of rectum. Surgical margins and pelvic lymph nodes were free of tumor. Postoperative radiation of the prostate bed (total dose 45 Gy) was administered in 5 fractions. Twenty-three

Methods

Resection specimens were examined at the time of operation, and representative sections were obtained. Blocks were fixed in neutral buffered formalin and processed in the usual fashion. Between 6 and 25 hematoxylin & eosin stained tumor sections of tumor were examined in each case. Nuclear variation was rated moderate in each case, defined as noticeable with a 10× objective. Mitoses were counted from 50 adjacent HPF using a 40× objective and 10× ocular (field area of 19 mm2).

Results

The resection specimen in each case included en bloc rectum, bladder, and prostate. The size of the tumor evaluated grossly averaged 11 cm (Table 1). In Case 1, tumor infiltrated the seminal vesicles, and in multiple slides impinged on prostate parenchyma with a “pushing” type border. In Case 2, tumor deeply indented the right prostatic lobe but did not grossly invade into the prostatic parenchyma. It filled the right perirectal fossa and extended within 3 cm of the anal verge; surgical margins

Discussion

GIST are soft-tissue neoplasms that occur in the muscular wall of the bowel, and characteristically express the CD117 (c-kit) antigen as well as CD34. Approximately 1 of every 1000 rectal malignancies is GIST [10]. Although 8% of cases reported as “rectal leiomyosarcoma” present with dysuria [2], to our knowledge, only one case has been reported pathologically of rectal GIST directly invading the prostate and mimicking primary prostatic malignancy [4].

Anorectal GIST usually has an indolent

Acknowledgments

The authors thank Dr. James Hathorn for providing clinical information, and Drs. Stanley Robboy and David Paulson for continuing intellectual support.

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