School of Medicine

ANSWER AND DISCUSSION:

Diff-Quik slides revealed a cellular specimen comprised of neoplastic cells ranging from epithelioid with oval nuclei to clearly spindled forms. Some cells showed cytoplasmic vacuoles, ranging from small to medium in size. On Papanicolaou stain, the nuclei showed a symmetric granular chromatin pattern without conspicuous nucleoli. Focal nuclear pleomorphism was noted. Cytoplasmic immunoreactivity for CD117 (KIT), DOG1 and caldesmon was present. Tumor cells were also focal positive for SMA, and negative for all other markers tested including desmin, S100, CD34, STAT6, calretinin, inhibin, and HMB45.  Given the cytomorphologic features and immunohistochemical staining profile, this is a gastrointestinal stromal tumor (GIST), probably of mesenteric origin based on the imaging findings.

GISTs are mesenchymal tumors arising from intramural gastrointestinal interstitial cells of Cajal or interstitial cells of Cajal-like cells in the extra-gastrointestinal sites. They are mainly encountered in adults with a mean age of 60 years. The involved sites include stomach (~50%) > small intestine > colon > esophagus > extra-gastrointestinal (peritoneal > retroperitoneal) with isolated reports at other sites as well. The vast majority of GISTs are sporadic, and their pathogenesis involves activating mutations in either the platelet derived growth factor receptor-α (PDGFR α) gene (~10%) or, more often, in the proto-oncogene KIT gene (~75%) leading to tyrosine kinase constitutive phosphorylation and activation of downstream pathways. This causes increased cell proliferation and survival.  More recently, succinate dehydrogenase (SDH) deficient GISTs have been identified and a minority have syndromic associations, such as the non-hereditary Carney triad and the autosomal dominant Carney–Stratakis syndrome. SDH deficient GISTs are uncommon, occur exclusively in the stomach, and have a different driving mechanism. Either genomic or epigenetic defects of the SDH complex lead to succinate accumulation, prolyl-hydroxylase inhibition, increased hypoxia-inducible factor 1-α (HIF1-α), and tumorigenic transcription. Succinate accumulation also inhibits TET DNA-hydroxylases causing impairment of DNA demethylation.

GISTs typically appear on imaging studies such as CT or MRI as well-defined, non-encapsulated masses that demonstrate homogeneous enhancement after the administration of contrast. Larger lesions may contain areas of necrosis, hemorrhage, and cystic degeneration. Small GISTs are often found incidentally during endoscopic or radiographic examinations performed for other reasons, whereas large tumors tend to present with symptoms attributed to intramural mass effect including abdominal discomfort or a sensation of fullness or bloating. Patients may also present with signs and/or symptoms of gastrointestinal mucosal ulceration or tumor perforation. For the gastrointestinal based tumors that arise in the often inaccessible intramural locations, endoscopic ultrasound-guided fine needle aspiration biopsy is a common method of sampling for diagnostic pathologic evaluations.

While incidentally detected tumors less than 1.0 cm in size usually behave indolently, the overall rate of metastasis is approximately 50%, most commonly to the liver. For both initial staging and subsequent monitoring of disease status and treatment response, contrast-enhanced CT scanning of the abdomen and pelvis is considered the imaging modality of choice. The treatment of choice is usually complete surgical resection regardless of the tumor’s site of origin. Risk assessment is determined by the tumor size, mitotic rate, and site of origin with tumors arising from the small bowel, colon, rectum, or mesentery generally conveying a less favorable prognosis as compared with gastric GISTs. Tumors arising from extra-gastrointestinal sites have the least favorable outcomes overall, although, not surprisingly, some of these may represent metastatic lesions from an undetected primary gastrointestinal tumor. Outcomes vary by mutation status as well, with KIT exon 9 and 11 and PDGFRA exon 18 (non-D842V) mutated GISTs having the worst prognosis. Mutation status is also predictive for response to the tyrosine kinase inhibitor Imatinib and the highest rate of response is seen in tumors with the KIT exon 11 mutation. Non-resectable tumors or those with a high risk of aggressive behavior are initially managed with Imatinib.

Microscopically, the appearance of GISTs is variable and three morphologic subtypes are described: spindle (70%), epithelioid (20%), and mixed (10%). The spindle subtype shows short and poorly defined fascicles or whorls with high cellular density and nuclear palisading. The epithelioid subtype shows cells arranged in sheets, nests, or trabeculae with variably collagenous stroma. Cytologically, the spindled cells are relatively uniform with indistinct cell borders and pale, eosinophilic cytoplasm. They have oval to spindled nuclei, often with tapered ends, and small inconspicuous nucleoli. The epithelioid cells are typically uniform, round to polygonal with indistinct cell borders, have uniform, round nuclei with occasionally prominent nucleoli and scant to abundant pale, eosinophilic or clear cytoplasm. At times, clear, perinuclear cytoplasmic vacuoles and focal nuclear pleomorphism are apparent.

Approximately 98% of GISTs are strong and uniformly immunoreactive for DOG1 (cytoplasmic or membranous), and up to 95% express KIT, with cytoplasmic, membranous, or paranuclear (Golgi) staining. CD34, caldesmon, and SMA may be detectable in some GISTs, but all are nonspecific. S100 and desmin are rarely expressed. The patient’s tumor was compatible with this immunoprofile. Immunochemistry for SDHB and SDHA can be considered when applicable to identify SDH-deficient tumors, which still usually express KIT and DOG1 but do not respond to imatinib. However, such testing would usually be reserved for a gastric GIST that showed the multinodular growth pattern characteristic of SDH mutants.

The principal differential diagnostic considerations for GIST are smooth muscle tumors (particularly leiomyomas), schwannomas, and solitary fibrous tumors. Leiomyomas of the gastrointestinal tract usually involve the esophagus and colorectum. They are less common in the stomach. These grow in intersecting fascicles, have dense eosinophilic cytoplasm and distinct cell borders on H&E stains. FNAs yield spindle-shaped cells that can resemble those of a GIST. The distinction between a smooth muscle tumor and a GIST depends on differential immunohistochemical expression of CD117 / DOG1 and the muscle marker desmin.  Smooth muscle markers caldesmon and SMA are less helpful as they are positive in some GISTs. Interstitial cells of Cajal are sometimes present, usually in small numbers, in leiomyomas and scattered immunoreactivity for KIT and DOG1 should not be taken as diagnostic of GIST, especially if tumor has strong diffuse staining for desmin. CD34 and S100 are not expressed.

Schwannomas are encapsulated, benign peripheral nerve sheath tumors composed predominantly or entirely of Schwann cells. Microscopically, they consist of short fascicles of spindled cells that are variable present in admixed cellular (Antoni A) and hypocellular (Antoni B) areas. Cytology shows well-organized spindle cells arranged in fascicles with inconspicuous fibrillary cytoplasm, hyperchromatic nuclei, which may have pointed or "fish hook" ends, and no or inconspicuous nucleoli. Immunochemistry shows diffuse and strong nuclear positivity for S100 and no expression of DOG1, CD117, and muscle markers. CD34 may be focally immunoreactive.

Solitary fibrous tumors (SFT) are mesenchymal tumors with fibroblastic differentiation displaying patternless growth of spindle cells associated with prominent dilated branching vessels in a hyalinized to collagenous stroma. Cytology classically demonstrates bland spindle cells with fusiform nuclei, scant elongated cytoplasmic processes, and naked nuclei. Immunochemistry is almost invariably positive for CD34 and STAT6. CD34 will stain many GISTs as well, however, unlike GISTs, SFTs are negative for DOG1, CD117, S-100, and muscle markers.

Perivascular epithelioid cell tumors occasionally enter into the differential diagnosis with GIST. They are distinctive mesenchymal neoplasms arising from perivascular epithelioid cells that classically demonstrate combined myomelanocytic immunophenotype. Microscopically, they comprise of nests, trabeculae, or sheets of epithelioid to spindled cells with clear to eosinophilic cytoplasm and are associated with the walls of larger vessels in some cases. They can have variable stromal hyalinization or sclerosis. Malignant tumors often feature prominent nuclear atypia, pleomorphism, and mitotic activity. Cytology usually shows the distinctive perivascular pattern of growth, but no such component was seen in this case. Immunochemistry is classic for expression of myomelanocytic markers including SMA, desmin, caldesmon, HMB45 and melan-A, and HMB45 was negative in this case.

An ovarian sex cord stromal tumor, particularly one on the fibrothecoma spectrum, was of concern in this case, given the radiologic impression and cytologic appearance. Fibromas are benign, nonfunctional, stromal tumors that comprise 4% of ovarian neoplasms. Histologically, fibromas are composed of bundles of spindle-shaped cells arranged in a whorled or storiform patterns. The lesional cells have a fibroblastic appearance, and collagen is usually abundant. Intracellular edema and intracytoplasmic lipid are sometimes observed. Cytological aspirates are hypocellular and contain bundles of fibroblastic spindle-shaped cells. Immunochemistry is positive for FOXL2, WT1, CD56, and SF1. Additional markers that may be positive include inhibin, calretinin, and SMA. Desmin and caldesmon are not expressed. Thecomas are benign tumors of theca cells, are more common in postmenopausal women, and are usually solid and unilateral tumors. They may be functional, secreting estrogens or androgens, or may be nonfunctional. Microscopically, the tumors are composed of monomorphic, oval- or spindle-shaped cells with cytoplasm that show variable degrees of luteinization. Cytological aspirates from thecomas are usually hypocellular. These are composed of isolated elongated cells and loose clusters. The nuclei are spindle-shaped, with finely granular chromatin. The cytoplasm is clear and contains numerous lipid vacuoles. Immunochemistry is similarly positive for calretinin, inhibin, FOXL2, WT1, CD56, and SF1. CD117 and DOG1 are not expressed. The immunoprofile in this case excludes these sex cord stromal tumors.

This case highlights the significant role of cytologic diagnostic evaluations in patients with a poor performance status and difficult to access lesions when biopsy specimen procurement is not a clinically practical option.

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BOARD STYLE REVIEW QUESTIONS

Q1. Histology of an abdominal mass in a middle-aged woman shows a non-descript, spindle cell proliferation with associated dilated, thin-walled, branching vasculature and collagenous stroma. Tumor cells are strongly and diffusely immunopositive for CD34, and negative for Desmin, S100, CD117, and DOG1. Which of the following statements is most accurate?

A) SMA is a critical immunostain which has not yet been performed.

B) The overall findings strongly suggest a CD117 negative GIST.

C) GIST is unlikely here, and STAT6 is more specific than CD34 for the diagnosis of SFT.

D) SDH testing must be the next step.

 

Q2. A middle-aged man with small bowel obstruction is found to have an intramural, small intestinal mass and many liver masses. Biopsy of a liver mass shows a spindle cell neoplasm that is diffusely immunopositive for CD117 and DOG1, and negative for STAT6, desmin, and SOX10. Molecular testing identifies a KIT exon 11 mutation. Which of the following statements is most accurate?

A) GIST is an unlikely diagnosis.

B) SDH deficient GIST is likely in this patient.

C) Additional immunochemical testing is critical.

D) Imatinib treatment would likely be beneficial.