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Pathology & Laboratory Medicine

URMC / Pathology & Laboratory Medicine / Education / Case of the Month / Case 14: Rectal Nodule / Diagnosis

 

Diagnosis and Discussion

Diagnosis

Gastrointestinal stromal tumor.

Discussion

The cytologic and immunocytochemical features are consistent with a diagnosis of Gastrointestinal stromal tumor. It is the most common primary mesenchymal neoplasm of the GI tract. Clinically silent lesions (<1cm) are potentially present in up to 35% of the general population. Median age of presentation is 60 years. Female predominance is seen in SDH-deficient GIST.

Stomach is the most common site of involvement (60%), followed by small bowel ( 30-35%) and colorectum (<5%). GISTs usually develop within the muscularis propria presenting as a submucosal mass or mucosal polyp. Patients mostly present with gastrointestinal bleeding (most common), abdominal pain or obstructive symptoms. These tumors may show spindled, epithelioid or mixed type morphology.

Spindle cell GISTs (most common, 70% of cases) show relatively monomorphic nuclei, fine chromatin with inconspicuous nucleoli and pale fibrillary eosinophilic cytoplasm. Paranuclear vacuoles are common in gastric tumors. Epithelioid GISTs (20% of cases) show uniform round cells that may appear epithelioid, plasmacytoid or rhabdoid. The cells have eosinophilic to clear  cytoplasm. Some cases may show prominent nuclear pleomorphism and multinucleation. SDH-deficient GIST have mostly epithelioid morphology.

Immunocytochemistry: These tumors characteristically show cytoplasmic and membranous positivity for CD117. DOG1 is positive in 30-60% of CD117 negative tumors and show cytoplasmic and membranous pattern. 75% of cases are CD34 positive. Loss of cytoplasmic SDHB expression is seen in SDH deficient GIST in addition to CD117 and DOG1 positivity.

Molecular genetics: KIT mutations are seen in the majority (80-85%) of these tumors mostly involving exon 11. PDGFRA mutations are most common in stomach and omentum, involving most commonly exon 8. KIT and PDGFRA mutations are mutually exclusive. SDH-deficient GISTs most commonly involve SDHA gene. Association with NF1 and BRAF (V600E) mutations are also known.

Differential diagnosis: Positivity for CD117 and DOG1 helps to differentiate this lesion from similar appearing entities such as leiomyoma, schwannoma, desmoid fibromatosis, solitary fibrous tumor, melanoma and poorly differentiated carcinoma. Presence of KIT and PDGFRA mutations also help with the differential diagnosis.

References

  • Montgomery, E. Gastrointestinal stromal tumor. Expertpath.https://app.expertpath.com/document/gastrointestinal-stromal-tumor/ccfd07ab-b77d-43e7-8233-86ce3a49c950.
  • Miettinen M, Lasota J. Succinate dehydrogenase deficient gastrointestinal stromal tumors (GISTs) - a review. Int J Biochem Biol 2014 Aug;53:514-9. doi: 10.1016/j.biocel.2014.05.033. Epub 2014 Jun 2.
  • Eisenberg BL, Pipas JM. Gastrointestinal stromal tumor--background, pathology, treatment. Hematol Oncol Clin North Am. 2012 Dec;26(6):1239-59. doi: 10.1016/j.hoc.2012.08.003.

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