Gastrointestinal stromal tumors (GISTs) are the most common mesenchymal neoplasms of the tubular gut and mesentery. Fine-needle aspiration biopsy (FNAB) currently is a useful tool in the diagnosis of GIST because of various mutations of the KIT protooncogene that are recognized as characteristic of these tumors. Despite such advances, the malignant potential of GIST remains variable, and few studies have reported their findings in patients with malignant GIST. Therefore, in the current study, the authors have reported their experience with FNAB as a diagnostic tool in one of the largest series of malignant GISTs and have analyzed the cytomorphologic features of the tumors relative to their clinical behavior to determine which, if any, cytologic features are indicators of malignancy.
All patients with histologically confirmed GIST who were diagnosed by image-guided FNAB and confirmed with positive CD117 staining from 1998 to 2003 were included in the study. All tumors were reviewed for various cytomorphologic features. For study purposes, the patients were divided into two groups: Group A included all patients with malignant tumors, defined as those with metastatic disease or recurrent disease after adequate surgery with or without chemotherapy; Group B included all other patients.
In total, 26 tumors from 23 patients (8 males and 15 females) with a mean age of 60.9 years were available for review. There were 14 primary tumors, 7 metastases (to the liver), and 5 recurrences. Twenty-one tumors were unequivocally malignant and were placed into Group A; the remaining 5 tumors were placed into Group B. The tumors in both groups were characterized by spindled or epithelioid cells with minimal nuclear atypia or pleomorphism and a moderate amount of cytoplasm. None of the cytologic features that were evaluated could distinguish reliably between benign tumors and malignant tumors, as expected. However, on cytologic examination, all tumors that demonstrated mitoses (n = 7 tumors) and/or pretreatment necrosis (n = 3 tumors) were identified as malignant.
FNAB remains a reliable method for the diagnosis of GIST. Immunohistochemical staining of cytologic material with CD117 has been reliable in establishing this diagnosis by FNAB, provided adequate tissue is procured. In the current study, the presence of necrosis or mitoses in cytologic specimens was correlated with a diagnosis of malignant GIST.