CD117 (c-kit) Marker: Clinical Significance in Diagnosis and Treatment

Diagnostic Applications

CD117 expression with diffuse cytoplasmic staining pattern confirms GIST diagnosis when combined with compatible morphological features (spindle cell, epithelioid, or mixed cell types) 2, 3
The marker shows intense staining in 75% of cases, with immunohistochemical positivity independent of KIT and PDGFRA mutational status 1, 2
Between 4-5% of GISTs with typical morphological features are CD117-negative, requiring additional diagnostic workup 1, 3, 4
For CD117-negative cases, DOG1 (discovered on GIST-1) immunostaining is highly recommended, as it is expressed in over 35% of CD117-negative GISTs 1, 2
When both CD117 and DOG1 are negative, mandatory mutational analysis for KIT and PDGFRA genes should be performed at reference centers 1, 3, 4

Mutational analysis should be performed on all moderate or high-risk GISTs, specimens prior to neoadjuvant/adjuvant therapy, and all unresectable/metastatic cases to guide treatment selection 4
KIT exon 11 mutations (most common) respond to standard-dose imatinib (400mg) 4
KIT exon 9 mutations may benefit from high-dose imatinib (800mg) 4
PDGFRA p.D842V mutations are imatinib-resistant but may respond to avapritinib 4

Samples should be fixed in 4% buffered formalin; Bouin fixative must be avoided as it prevents molecular analysis 5, 6
Immunohistochemistry should be performed without antigen retrieval, as this may result in false-positive CD117 staining 5
Fresh snap-frozen tissue collection is encouraged for subsequent molecular assessments 6
The pathology report must include CD117 status along with tumor location, size, mitotic count (expressed as number of mitoses per 5 mm² total area), and margin status 1, 2
Mitotic count should replace the conventional 50 high-power field measurement to standardize reporting 6, 4

Perform DOG1 immunostaining immediately 1, 7
If DOG1 is also negative, order KIT and PDGFRA mutational analysis 1, 4
Refer complex cases to a reference sarcoma center with expertise in GIST diagnosis 1, 3
Consider SDHB immunohistochemistry for KIT/PDGFRA wild-type GISTs, particularly in young patients with gastric location 2, 4