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Journal of Clinical Oncology, Vol 25, No 36 (December 20), 2007: pp. 5827-5828 © 2007 American Society of Clinical Oncology. DOI: 10.1200/JCO.2007.13.7174
Rectal GI Stromal Tumor Mimicking a Prostate MassDepartment of Medicine, University of Texas Southwestern Medical School, Dallas, TX
Department of Pathology, University of Texas Southwestern Medical School, Dallas, TX
University of Texas Southwestern Medical School, Hematology/Oncology, Dallas VA Medical Center, Dallas, TX A 55-year-old man presented with a 1-year history of episodic bright red blood per rectum. His review of systems and medical history were otherwise unremarkable. Digital rectal examination revealed a firm rectal mass palpated 4 to 8 cm from the anal verge, located predominantly at the left anterior rectal wall. Prostate-specific antigen was 0.7 ng/mL. Computed tomography (CT) showed an irregularly shaped mass in the region of the prostate measuring 6.5 x 8.6 x 6.5 cm, which also involved the left seminal vesicle and anterior wall of rectum (Fig 1). Colonoscopy showed a nonobstructing, large fungating and friable 3 x 5 cm mass along the anterior wall of rectum. More proximally, the mass was submucosal with normal overlying mucosa. Transrectal ultrasonography showed a 2.6 x 4.4 cm mass that appeared to be arising from the prostate and was invading the anterior wall of the rectum. The mass had the same echogenicity as the prostate gland and was directly contiguous with the gland with no perirectal lymphadenopathy. A transrectal biopsy was performed by urology. Light microscopy examination showed the tumor consisted of spindle-shaped cells (Figs 2A and 2B). Immunohistochemistry showed tumor expression of CD-117 (Fig 3), CD-34 (Fig 4), and vimentin, but no S-100, smooth muscle actin, prostate-specific antigen, and anticytokeratin (CAM 5.2; Becton Dickinson, Mountain View, CA) immunoreactivity. The diagnosis of a GI stromal tumor (GIST) was made. Additional staging showed no evidence of metastatic disease. Given that resection of the mass was felt to require extensive surgery, neoadjuvant therapy was started with imatinib mesylate (Gleevec; Novartis Pharma Stein AG, Stein, Switzerland). After 3 months of treatment, there was no significant change in the size of the mass. The patient underwent surgical pelvic exenteration. Longitudinal section through the center of the specimen parallel to the long axis of the colon, bladder, and prostate revealed a submucosal mass measuring 7 x 6.8 x 6 cm growing through the rectal wall anteriorly in the soft tissue between rectum and prostate with the major bulk of the tumor in the perirectal soft tissue adherent to posterior-lateral aspect of the left half of the prostate with no microscopic invasion of the prostate.
The term "GIST " was introduced in 1983 to refer to the large group of mesenchymal tumors of the GI tract that could not be classified as either smooth muscle or neurogenic in origin.1 GISTs are characterized by the expression of the kit (CD-117) tyrosine kinase as well as by the presence of activating mutations in either kit or the platelet-derived growth factor receptor  . In addition, these tumors have characteristic histologic features including spindle cell, epitheloid, and rare pleomorphic morphology.2 Since most GISTs arise within the muscularis propria, they commonly have an exophytic growth pattern and manifest as dominant masses outside the organ of origin. Anorectal GISTs are most commonly mural masses and manifest as a focal mural mass on CT imaging.3 GISTs are the most common GI mesenchymal tumor. The annual incidence of GISTs has been estimated as 3,300 to 4,350 new cases in the United States. GIST has a predilection for adults older than 50 years, with a median age at diagnosis of approximately 60 years. There is no clear sex predilection.2 GI stromal tumors occur throughout the GI tract from the lower esophagus to the anus. The most common sites are stomach (60%), jejunum and ileum (30%), duodenum (5%), and colorectum (5%). Only small numbers of cases (1%) have been reported in the esophagus and appendix.2 Approximately one of every 1,000 rectal malignancies is a GIST.3 Extraintestinal GI tumors (EGIST) have been reported in many organs including urinary bladder, gallbladder, omentum, mesentry, urinary tract, uterus, fallopian tube, and prostate.4,5 GISTs are currently thought to originate from interstitial cells of Cajal through a gain-of-function mutation of the c-kit proto-oncogene. The presence of interstitial Cajal-like cells in the extraintestinal sites has been reported in many organs including urinary bladder, gallbladder, omentum, mesentery, urinary tract, uterus, fallopian tube, atrial and ventricular myocardium, and prostate.4,5 The presence of interstitial Cajal-like cells in other organs could explain the development of EGIST.5,6 Some authors have suggested the origin of GISTs from intestinal mesenchymal precursor cells with differentiation toward Cajal cells. This could explain why GISTs show marked variability in their morphological, ultrastructural, and immunohistochemical features. This could also explain why GISTs can arise outside the GI tract.7 GISTs are resistant to conventional cancer chemotherapy. Imatinib mesylate, a tyrosine kinase inhibitor, has shown promising results in the management of patients with GIST.7 Tumor responses to imatinib are seen in 80% of patients. However, kinase inhibition by imatinib is not uniformly successful. Ten percent to 20% of patients with GIST exhibit primary resistance to imatinib. Notably, viable tumor cells can be found in most patients who undergo GIST resections during imatinib therapy, indicating that some degree of imatinib resistance is inherent in most GISTs.8 To our knowledge, one case of GIST of the prostate has been reported.7 Our case illustrates that anorectal GIST can mimic the presentation of prostate cancer. Herawi and colleagues recently reported eight cases of GIST diagnosed on transrectal needle biopsy of the prostate. One of those cases was shown to extensively involve the prostate, but the epicenter of the tumor was in the rectal muscularis mucosa.9 Similarly, in the present patient case, the endoscopic ultrasound and CT were compatible with a tumor arising in the prostate with subsequent invasion into the rectum. However, gross and microscopic pathologic examination after pelvic exenteration revealed the tumor to be a lower rectum/upper anal canal GIST. Clinicians should be aware that GIST is in the differential for patients presenting with an apparent prostate mass. Making a distinction between GISTs and other primary rectal and prostatic sarcomas is important because the use of imatinib in conjunction with surgery may considerably influence patients' outcomes.7 AUTHORS' DISCLOSURES OF POTENTIAL CONFLICTS OF INTEREST The author(s) indicated no potential conflicts of interest.
REFERENCES 1. Van der Aa F, Sciot R, Blyweert W, et al: Gastrointestinal stromal tumor of the prostate. Urology 65:388, 2005[Medline] 2. Miettinen M, Lasota J: Gastrointestinal stromal tumors: Review on morphology, molecular pathology, prognosis, and differential diagnosis. Arch Pathol Lab Med 130:1466-1478, 2006[Medline] 3. Levy AD, Remotti HE, Thompson WM, et al: Gastrointestinal stromal tumors: Radiologic features with pathologic correlation. Radiographics 23:283-304, 2003 4. Van der Aa F, Roskams T, Blyweert W, et al: Interstitial cells in the human prostate: A new therapeutic target? The Prostate 56:250-255, 2003[CrossRef][Medline] 5. Min KW, Leabu M: Interstitial cells of Cajal (ICC) and gastrointestinal stromal tumor (GIST): Facts, speculation, and myths. J Cell Mol Med 10:995-1013, 2006[Medline] 6. Gun BD, Gun MO, Karamanoglu Z: Primary stromal tumor of the omentum: Report of a case. Surg Today 36:994-996, 2006[CrossRef][Medline] 7. Lee C-H, Lin Y-H, Lin H-Y, et al: Gastrointestinal stromal tumor of the prostate: A case report and literature review. Hum Pathol 37:1361-1365, 2006[CrossRef][Medline] 8. Fletcher JA, Rubin B: KIT mutation in GIST. Curr Opin Genet Dev 17:3-7, 2007[CrossRef][Medline] 9. Herawi M, Montgomery EA, Epstein JI, et al: Gastrointestinal stromal tumors (GISTs) on prostate needle biopsy. Am J Surg Pathol 30:1389-1395, 2006[CrossRef][Medline]
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Copyright © 2007 by the American Society of Clinical Oncology, Online ISSN: 1527-7755. Print ISSN: 0732-183X
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