What is the role of CD117 (Cluster of Differentiation 117) in guiding treatment decisions for a patient diagnosed with Gastrointestinal Stromal Tumor (GIST)?

CD117 in Gastrointestinal Stromal Tumor (GIST)

CD117 (KIT) immunohistochemistry is the primary diagnostic marker for GIST, with 95% of cases showing positivity, and its expression—combined with mutational analysis of KIT and PDGFRA genes—directly determines treatment selection with tyrosine kinase inhibitors like imatinib. 1

Diagnostic Role of CD117

Primary Diagnostic Marker

• CD117 positivity with diffuse cytoplasmic staining confirms GIST diagnosis when combined with compatible morphological features (spindle cell, epithelioid, or mixed cell types) 1, 2
• Approximately 95% of GISTs are CD117-positive by immunohistochemistry 1
• The staining pattern may be cytoplasmic, membranous, or paranuclear (Golgi pattern) 3
• Immunohistochemistry should be performed without antigen retrieval, as this causes false-positive CD117 staining 1, 2

Critical Technical Requirements

• Fix tumor samples in 4% buffered formalin; never use Bouin fixative as it prevents subsequent molecular analysis required for treatment decisions 1
• The pathology report must include CD117 status, tumor location, size, and mitotic count expressed as number of mitoses per 5 mm² total area (not the outdated 50 high-power field measurement) 1, 2

Management of CD117-Negative Cases (5% of GISTs)

Sequential Diagnostic Algorithm

When a suspected GIST is CD117-negative 1, 4:

1. Immediately perform DOG1 immunostaining, which is positive in over 35% of CD117-negative GISTs 1, 2
2. If DOG1 is also negative, mandatory mutational analysis for KIT and PDGFRA genes must be performed at a reference laboratory enrolled in external quality assurance programs 1
3. Refer complex cases to a sarcoma reference center with GIST expertise 1, 2

Important Caveat

CD117-negative GISTs are more likely to harbor PDGFRA mutations (particularly exon 18 mutations including D842V) and should not be denied imatinib therapy without mutational analysis, as some contain imatinib-sensitive mutations 4

Treatment Decision-Making Based on CD117 and Mutational Status

Mutational Analysis is Mandatory For:

• All moderate or high-risk GISTs at any site 1, 2
• All resected GISTs showing tumor rupture 1
• All biopsies diagnostic of GIST prior to neoadjuvant or adjuvant therapy 1, 2
• All unresectable and/or metastatic GIST specimens 1
• Tumors >2 cm (except possibly very small non-rectal GISTs unlikely to require medical treatment) 1

Treatment Selection Based on Mutation Type

KIT exon 11 mutations (most common, 65-66% of GISTs):

• Respond to standard-dose imatinib 400 mg daily 1, 2, 5
• These are imatinib-sensitive mutations 1

KIT exon 9 mutations (8-13% of GISTs):

• May benefit from high-dose imatinib 800 mg daily (though not universally approved) 1, 2
• Associated with non-gastric tumor location 1

PDGFRA p.D842V mutation:

• Imatinib-resistant 1, 2
• Consider avapritinib as targeted therapy 1, 2
• These mutations occur in approximately 5% of cases and primarily involve exon 18 1

KIT/PDGFRA wild-type GISTs:

• Perform SDHB immunohistochemistry to identify SDH-deficient GIST, particularly in young patients with gastric location 1, 2
• Consider germline testing for SDH mutations and NF1 syndrome 1

FDA-Approved Indications

Imatinib is FDA-approved for:

• Kit (CD117) positive unresectable and/or metastatic malignant GIST 5
• Adjuvant treatment following resection of Kit (CD117) positive GIST 5

Standard dosing:

• Adults with metastatic/unresectable GIST: 400 mg daily 5
• Adjuvant treatment: 400 mg daily 5
• All doses should be taken with a meal and large glass of water 5

Common Pitfalls to Avoid

• Never rely solely on CD117 expression to predict imatinib response—mutational analysis is essential as CD117 protein expression does not guarantee the presence of imatinib-sensitive mutations 6
• Do not exclude GIST diagnosis based on CD117 negativity alone—proceed with DOG1 staining and mutational analysis 1, 4
• Avoid antigen retrieval during CD117 immunostaining to prevent false-positive results 1, 2
• Do not use Bouin fixative as it impairs molecular analysis critical for treatment planning 1
• Approximately 15% of GISTs have no detectable KIT or PDGFRA mutations (wild-type), requiring alternative diagnostic and therapeutic approaches 1