—  SPECIALTY CONFERENCE  —

Cytopathology

Case 4 - Metastatic Gastrointestinal Stromal Tumor

Chris S. Jensen
Department of Pathology
University of Iowa


Click on each slide thumbnail image for an enlarged view
Clinical History
The patient is a 24-year-old female with no significant past medical history. She presented with sudden onset of upper abdominal pain, which was constant and severe but not associated with nausea or vomiting. A CT scan of her abdomen showed multiple large low-density lesions scattered throughout the liver. The patient had no history of prior liver disease and was not jaundiced. An initial fine needle aspiration under ultrasound guidance was inconclusive due to low cellularity. A subsequent MRI exam again revealed multiple liver lesions as well as a large mass between the liver and the anterior wall of the stomach impinging upon the gastric orifice. A fine needle aspiration was performed of an enlarged celiac lymph node under endoscopic ultrasound (EUS) guidance.


Case 4 - Figure 1 - Papanicolaou stain, low power (100X) - View shows larger tissue fragments which are composed of the mixture of fibrillar stroma and tumor cells. Tumor cells are present as epithelioid clusters as well as dispersed single cells. There is some tendency of small clusters of cells to group together in small gland-like structures.
Case 4 - Figure 2 - Diff-Quik stain, high power (400X) - This high power view shows small cohesive groups of epithelioid cells. The cells have fairly abundant, slightly granular cytoplasm. Nuclei are round to slightly oval. Individual cell borders are not distinct. There are several stripped nuclei noted in the background. A rare intranuclear cytoplasmic inclusion is noted.
Case 4 - Figure 3 - Papanicolaou stain, high power (600X) - This high power field shows a group of loosely cohesive cells. Several cells show prominent intranuclear cytoplasmic inclusion. The nuclei are somewhat ovoid with slight nuclear irregularities, chromatin is granular to slightly clumped. Cytoplasm is granular, with a suggestion of fibrillar cytoplasmic processes.

Cytologic Findings:
Smears are quite cellular and composed of predominantly epithelioid cells in loosely cohesive aggregates. There is, however, significant discohesion and numerous stripped nuclei as well. Larger fragments show fibrillar stroma between nests of cells which stains intensely metachromatic on Diff-Quik stained smears. The cells have variable amounts of cytoplasm with the suggestion of fine granularity in some areas. Cytoplasmic borders are predominantly not distinct. In areas fibrillar cytoplasmic processes are noted extending from many of the cells as are a few spindled cells. Nuclei are round to oval with slightly irregular contours. The chromatin is finely granular to slightly clumped. Small nucleoli are present in many cells and numerous intranuclear cytoplasmic inclusions are noted.

Additional cytologic material was collected at the time of the procedure for a cell block preparation. H&E stained sections have clusters of epithelioid cells with similar nuclear features. These cells are arranged in solid sheets or clusters. There was no additional identifiable architectural arrangement including no trabecular architecture, gland formation or rosettes. A spindled morphology is again not prominent. Additional sections of the cell block for immunohistochemical analysis demonstrate expression of CD117, vimentin and CD34. There was no expression of keratins, smooth muscle actin, neuroendocrine markers (synaptophysin, chromogranin) or melanoma associated antigens (S100, HMB45, MART 1). Given the characteristic immunohistochemical the cytologic final diagnosis was as follows.

Cytologic Diagnosis: Metastatic gastrointestinal stromal tumor

Clinical Follow-Up:
On EUS examination, extrinsic compression of the lesser curvature of the stomach was appreciated. Endosonographic examination showed this compression to be due to a large (10 x 7 cm) mass likely originating in the muscularis propria of the stomach and growing in an extra-gastric fashion. The mass was solid, heterogenous and well defined. It contained areas with an echogencity consistent with necrosis. This mass was noted to abut the liver. In addition, many enlarged lymph nodes were visualized in the celiac region and perigastric region. The diagnostic fine needle aspiration was taken from one of these nodes.

Given the size and extent of the tumor along with hepatic and nodal metastasis, the patient was not a candidate for surgical therapy. She was started on Gleevec (imatinib) for treatment of metastatic gastrointestinal stromal tumor. Follow-up radiologic examination revealed some reduction in size of the gastric and liver lesions without complete resolution.

Discussion:

Gastrointestinal stromal tumors
(GIST's) are mesenchymal tumors arising within the gastrointestinal tract (or attached structures) which are immunohistochemically c-KIT (CD117) positive and characteristically signal driven by the presence of KIT activating mutations. These lesions represent the majority of mesenchymal lesions arising within the gut. Immunohistochemical and ultrastructural features suggest an origin from interstitial cells of Cajal (ICC's) or a related undifferentiated mesenchymal precursor cell with potential to differentiate toward ICC's or smooth muscle.

The classification of stromal neoplasms occurring with the gastrointestinal tract has historically been an area of controversy in pathology resulting in confusing, ambiguous and overlapping terminology. The recent recognition of c-KIT (CD117) expression in most of these tumors by immunohistochemistry along with the identification of associated oncogenic mutations resulting in activation of the KIT receptor tyrosine kinase has clarified the diagnosis of GIST's and as noted above CD 117 positivity now essentially defines these tumors from a diagnostic standpoint. The recent development of a successful therapy (Gleevec, imatinib, STI-571) based on the underlying molecular pathology in these tumors has fueled further interest in these tumors. Prediction of individual tumor behavior remains a challenge.

GIST's are relatively uncommon neoplasms but according to SEER data represent approximately 2.2% of gastric malignancies and 13.9% of small bowel cancers. The tumors can occur throughout the tubular intestine, as well as in the adjacent omentum, mesentery and retroperitoneum. The proportion of tumors at different sites is shown below.

Stomach (50-60%) > Small intestine (20-30%) > Large intestine (10%) > Esophagus (5%) > Extraintestinal- omentum, mesentery, etc. (5%)

A significant proportion, if not the majority, of GIST's are relatively small incidentally noted lesions which are identified at the time of endoscopy, abdominal surgery for other reasons or abdominal imaging studies. The tumors occur in middle aged or older individuals with a median age in the late 50's in most studies. Symptoms are dependent on the location of the tumor. Symptoms most commonly associated with gastric GIST's include vague abdominal pain (50-70%) and gastrointestinal bleeding (20-50%). Small intestinal tumors may result in pain, gastrointestinal bleeding or symptoms of intestinal obstruction. Large (usually malignant) tumors may present with a palpable abdominal mass.

Endoscopic features are variable, but typically demonstrate an intramural tumor which may have ulceration of the overlying mucosa. Polypoid projection of the tumor into the intestinal lumen can occur, but outward extension with associated extrinsic compression or distortion is more common. The most common pattern of dissemination of malignant tumors at presentation is diffuse intraabdominal spread. Liver followed by lung are the next most common metastatic sites following the hematogenous route usually associated with sarcomas. Lymph node metastases as seen in this case are uncommon.

The most common histomorphology associated with GIST's is that of a spindle cell proliferation resembling smooth muscle tumors, although typically fairly cellular, the cellularity can vary significantly within the tumor. A fascicular arrangement may be seen as well as whorls. Nuclei tend to be ovoid with fairly bland vesicular chromatin. Prominent vacuoles adjacent to the nuclei are a feature noted in some cases. The second most common histologic pattern (20-30%) is that of an epithelioid tumor as was prominent in this case. These tumors are composed of rounded cells with variable amounts of eosinophilic or occasionally clear cytoplasm. The nuclei again tend to be round to oval with vesicular chromatin. Epithelioid tumors are most commonly seen in the stomach. Occasional tumor showing a mixture of epithelioid and spindle cell areas are seen. Tumors arising within the small bowel have been associated with a pattern of prominent fibrillar hylan fibers known as skeinoid fibers.

Cytologic features of GIST
The cytologic features of stromal tumors by fine needle aspiration have been well-described. This is especially true with the advent of evaluation and aspiration by endoscopic ultrasound (EUS) as in the current case. In addition, to allowing cytologic sampling of tumors, EUS can accurately gauge tumor size which may be an important predictor of prognosis.

There are two distinct, predominant cytologic patterns which may occasionally occur together. The most common pattern is that of a spindle cell neoplasm. The smears are typically quite cellular and may have large three dimensional tissue fragments with prominent vascularity and varying amounts of fibrillar, collagenous stroma between cells. Individual cells are discohesive or loosely cohesive with indistinct cell borders and fibrillar cytoplasmic processes. Individual stripped nuclei may occasionally be quite numerous. Nuclei are typically ovoid to spindled and often wavy with bland granular chromatin. Nucleoli may be prominent and there may be nuclear grooves or inclusions. Nuclear pleomorphism is generally uncommon in spindled GIST's.

The second cytologic pattern is that of an epithelioid neoplasm, typically with prominent discohesion. The epithelioid cells may be admixed with spindle cells as described above. The smears are again typically quite cellular with loosely cohesive sheets and clusters as well as numerous discohesive single epithelioid cells. Individual cells have round to slightly ovoid nuclei. Cytoplasm is typically granular but may be clear or slightly vacuolated. Wispy or fibrillar cytoplasmic processes may be noted extending from individual cells. Dong etal. [4] evaluated the cytologic features of nine cases of epithelioid GIST. Nucleoli and intranuclear cytoplasmic inclusion were the two features evaluated which were present in all cases. Other cytoplasmic features present in a majority these cases included irregular nuclear contours, binucleation and collagenous stroma. Although nuclear pleomorphism is typically not a prominent cytologic feature of GIST's, nuclear atypia may be more common in epithelioid tumors

The cytologic differential diagnosis of spindled GIST's includes other stromal neoplasms of the gastrointestinal tract. Although true smooth muscle and peripheral nerve sheath tumors do occur in the gastrointestinal tract, the tumors are much less common than GIST's in all sites except the esophagus where leiomyomas are more common. In addition, at sites such as the liver, lymph nodes or mesentery other mesenchymal tumors (primary or metastatic), malignant melanoma (spindled) and spindled carcinomas must also be considered.

As noted earlier, CD117 expression by immunohistochemistry essentially defines GIST. As such, the cytologic diagnosis of GIST (as with surgical pathology) rests on obtaining cytologic material for immunostaining and demonstration of CD117 expression. The appropriate material for this testing is a matter of preference based on personal experience, laboratory capabilities, and limitations related to the aspirator or patient. In our laboratory cell blocks have been most reliable as demonstrated in this case. Destained papanicolaou smears and cytospin preparations are also acceptable. In tumors which produce particularly hypocellular aspirations, needle core biopsies may be preferable or necessary if possible. The immunohistochemical profile and differential diagnosis is summarized in the table below.

  KIT (CD117) CD34 SMA Desmin S-100
GIST pos pos (60-70%) Pos(30-40%) neg(rare +) neg(5% pos)
Smooth muscle tumor neg pos(10-15%) Pos pos neg
Schwannoma neg pos Neg neg pos

Adapted from Fletcher et al. Human Path 33:459 (2002)

The cytologic differential diagnosis of epithelioid GIST as in this case is much more diverse and includes hepatocellular carcinoma, metastatic carcinoma from a variety of primary sites, malignant melanoma, neuroendocrine carcinoma (carcinoid) and epithelioid sarcomas (especially leiomyosarcoma). Since, again, the diagnosis rests at least partially on immunostaining for CD117, the first key in cases such as this is including GIST in the differential diagnosis and insuring that material for immunostaining is obtained. Obviously this should not be a problem for tumors of the bowel wall. The larger problem is in cases without an obvious intestinal mass or with disseminated intraabdominal or metastatic disease which obscures the site of the primary tumor.

Hepatocellular carcinoma is prominent in the differential diagnosis on patients with single or multiple liver lesions. HCC may have tissue fragments or cluster and typically has a distinct vascular pattern with transgressing endothelium or trabeculae with peripheral endothelium (the trabecular pattern when present essentially excludes GIST). However, vessels in tissue fragments may appear similar to the transgressing endothelium of HCC. Other features shared with GIST include stripped atypical nuclei, prominent nucleoli and intranuclear cytoplasmic inclusions. HCC typically has more distinct cytoplasmic borders and bile if present, is diagnostic. Immunohistochemical analysis should clearly distinguish HCC from GIST. Similarly other carcinomas will also be distinguished by epithelial markers and CD117.

Metastatic malignant melanoma can share the pattern of discohesive epithelioid cells with prominent nucleoli and intranuclear cytoplasmic inclusions. Melanoma may also show a mixture of epithelioid and spindled cells as well. Cytoplasmic melanin is not a feature of GIST, but is only identifiable in a minority of cases of metastatic melanoma. CD117 positvity has been described in melanoma, although it is quite uncommon and a small percentage of GIST's mark with S100. Other melanoma markers,HMB-45 and Mart-1 are usually negative in GIST.

Neuroendocrine tumors, especially low to intermediate grade neuroendocrine carcinoma, (carcinoid, atypical carcinoid) also share the cytologic pattern of dishesive epithelioid cells with round nuclei and moderate amounts of granular cytoplasm. The cytoplasmic granularity of neuroendocrine tumors may be somewhat coarser, especially on Diff Quik stained smears. An organoid arrangement may be seen in both GIST and neuroendocrine carcinoma, however rosettes are not typically seen in GIST. The typical bland granular chromatin of GIST may appear similar to bland "salt and pepper" chromatin of a neuroendocrine tumor. Both tumors may have some spindled to oval nuclei and cytoplasmic processes. Adding to the similarity is the fact that both tumors may occur in the small bowel or stomach and have diffuse intraabdominal spread or liver metastases. Immunostaining with chromogranin and synaptophysin as well as epithelial antigens and absence of CD117 all distinguish neuroendocrine tumors from GIST.

Prognostic factors / Prediction of behavior:
The identification of individual prognostic factors has long been recognized as a challenge in stromal tumors. No individual cytologic feature is predictive of behavior. In general, GIST's show a spectrum of clinical behavior ranging from apparently benign tumors to aggressive malignant neoplasms. Size and proliferative activity (mitotic rate) have been the most reliable predictors of outcome. However, although the majority of tumors may behave in a benign fashion, occasional small bland tumors, with a low proliferative activity may recur or metastasize. As such, it is probably best to consider that all GIST's have at least some malignant potential. Diagnoses can then be qualified with an assessment of risk based on the factors present in the individual case. As with other tumor types, tumors arising in different portions of the intestine may have differing behaviors. In general, intestinal (small or large) GIST's may have greater malignant potential than comparable gastric tumors. The uncommon esophageal GIST is usually malignant.

References

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