FISH Versus IHC as the Primary Test for HER-2 Analysis: Cost Analysis, Aneusomy Significance and Gene Heterogeneity in a Consecutive Series.

Abstract

Abstract Objective: To utilize a test algorithm for HER-2 that has the lowest cost per reportable test and establishes the most accurate results for trastuzumab therapy decisions.By restricting hybridization area, we have reduced the probe quantity required without altering sensitivity and specificity. Our FISH reagent cost per test is $32.20. The cost per reportable test is $62.30 for FISH and $56.86 for IHC (Ventana). FISH cost/result is slightly costlier than IHC cost/result. However, reflex to FISH (20-25%) after 2+ IHC analyses is a more expensive test algorithm than primary FISH analysis with reflex to IHC (5.6%). The total cost of the FISH first algorithm/100 cases is $6548 and for IHC first algorithm is $6932. In a recent study of patients being considered for trastuzumab treatment, primary FISH analysis was the most cost-effective diagnostic approach.Studies have shown greater accuracy and precision of FISH vis a vis IHC in predicting response to trastuzumab: Our institution has selected FISH as the primary method for assessing HER-2 status, rather than IHC.HER-2 gene amplification is observed in 15-30% of breast cancers. FISH positivity predicts the response to trastuzumab. Increased gene copies may be due to repeated duplication of HER-2 amplicon or inherent chromosomal instability due to mitotic mis-segregation of cancer cells. Aneusomy is frequently found in cancers and may complicate the interpretation of HER-2 testing results. Tumors with aneusomy of 17 without amplification have a high and uncertain response rate to trastuzumab. There is no clear consensus on how to report tumor heterogeneity in cases due to 17 aneusomy.We report here the MGH experience in 90 consecutive cases using FISH as the primary test for assessing HER-2 status. Using CAP/ASCO guidelines for reporting HER-2 FISH, we determined 15.6% cases amplified, 5.6% equivocal and 78.9% normal. Chromosome 17 and Her2 aneusomy was observed in 27.8% of cases. Our percent aneusomy may be higher than some have reported due to preselection of cases in our reference lab setting. Most of the Her-2 amplified cases had HER-2/CEN-17 ratio >10.In one case the HER 2 amplicon extended widely to chromosome 17 centromeric region. FISH studies using Topoisomerase II alpha probes showed probable deletion of this gene putatively located within the HER-2 amplicon. Even though HER gene copy number was > 20 HER-2/Chromosome17 according to ASCO/CAP was normal (< 1.8). In instances of reported co-amplification of topoisomerase II alpha gene, its status may influence the treatment decision.The incidence of chromosome 17 aneusomy has varied from as low as 4% to as high as >30% in studies of invasive breast cancer influencing the reporting of amplification. There are no existing guidelines on how to report such tumor heterogeneity. We hope this analysis will be useful to other centers evaluating the use of primary FISH algorithm for HER-2 testing. Citation Information: Cancer Res 2009;69(24 Suppl):Abstract nr 6037.