[382] Assessment of EGFR Mutation Status in Needle Biopsies and Cytology Specimens of Lung Adenocarcinoma by Immunohistochemistry Using Antibodies to the Two Major Forms of Mutant EGFR

DC Ang, M Brevet, M Ladanyi, MF Zakowski. Memorial Sloan-Kettering Cancer Center, New York, NY

Background: Mutations of epidermal growth factor receptor (EGFR) gene in lung adenocarcinoma (AD) predict sensitivity to EGFR tyrosine kinase inhibitors (TKI). Small deletions in exon 19 and the L858R point mutation in exon 21 account for 85-90% of EGFR mutations. Molecular testing is best performed on surgically resected tumors but many patients are not operative candidates and are diagnosed by cytology or needle biopsy only. Such tiny specimens are often insufficient for molecular testing in these patients who need systemic chemotherapy or EGFR TKIs. Recently, immunohistochemistry (IHC) with mutation-specific monoclonal antibodies against the L858R mutant and the exon 19 mutant (common 15bp deletion; E746_A750del) has been shown to be sensitive and specific for EGFR mutation status. Here, we evaluated these antibodies in the setting where they would be uniquely useful, cytology or needle biopsy specimens.
Design: We identified 94 lung AD patients with known EGFR status (based on molecular testing of resected tumor) and positive needle biopsy (BX) or cytology (ThinPrep (TP) or cell block(CB)) samples. These 94 samples included 47 with ex19 deletion [deletion sizes: 15bp (n=32), 18bp (n=9), 12bp (n=1), 24bp (n=1)], 35 with L858R, and 12 EGFR wild type cases. IHC with the two EGFR mutation-specific antibodies (Cell Signaling Technology) described above was scored as negative (0-1+) or positive (2+-3+). Both antibodies were performed on EGFR wild type cases. Twelve known negative cases were used as controls.
Results: The sensitivity for EGFR ex19 (15bp only), EGFR ex 19 (including all deletions) and EGFR exon 21 mutation-specific antibodies are 79%, 57% and 66%, respectively. Both antibodies showed a specificity and PPV of 100%. No wild type cases showed immunoreactivity with either antibody.

IHC for EGFR ex19 del and L858R mutant
IHC for ex19 del
Mutation StatusSpecimenPOS(%)NEG(%)
ex19 (15bp del)BX n=2218(92)4(8)
TP n=55(100)0
CB n=74(57)3(43)
ex19 (all bp del)BX n=909(100)
CB n=404(100)
IHC for L858R
L858RBX n=1210(83)2(17)
TP n=43(75)1(25)
CB n=1910(53)9(47)

Conclusions: IHC with EGFR mutation-specific antibodies on small biopsies and cytology specimens shows high specificity and good sensitivity for EGFR mutation status. Application of these antibodies could allow or expedite critical treatment decisions in patients with unresectable lung AD.
Category: Cytopathology

Wednesday, March 24, 2010 1:00 PM

Poster Session VI # 47, Wednesday Afternoon


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