Field cancerization effects as well as isolated
tumor cell foci extending well beyond the invasive
tumor margin have been described previously to account for local recurrence rates following
breast conserving surgery despite adequate
surgical margins and breast
radiotherapy. To look for evidence of possible
tumor cell contamination or field cancerization by genetic effects, a pilot study (Study 1: 12 sample pairs) followed by a verification study (Study 2: 20 sample pairs) were performed on
DNA extracted from HER2-positive
breast tumors and matching normal adjacent mammary tissue samples excised 1-3 cm beyond the invasive
tumor margin. High-resolution molecular inversion probe (MIP) arrays were used to compare genomic copy number variations, including increased HER2 gene copies, between the paired samples; as well, a detailed histologic and immunohistochemical (IHC) re-evaluation of all Study 2 samples was performed blinded to the genomic results to characterize the adjacent normal tissue composition bracketing the
DNA-extracted samples. Overall, 14/32 (44 %) sample pairs from both studies produced genome-wide evidence of genetic aberrations including HER2 copy number gains within the adjacent normal tissue samples. The observed single-parental origin of monoallelic HER2 amplicon haplotypes shared by informative
tumor-normal pairs, as well as commonly gained loci elsewhere on 17q, suggested the presence of contaminating
tumor cells in the genomically aberrant normal samples. Histologic and IHC analyses identified occult 25-200 μm
tumor cell clusters overexpressing HER2 scattered in more than half, but not all, of the genomically aberrant normal samples re-evaluated, but in none of the genomically normal samples. These genomic and microscopic findings support the conclusion that
tumor cell contamination rather than genetic field cancerization represents the likeliest cause of local clinical recurrence rates following
breast conserving surgery, and mandate caution in assuming the genomic normalcy of histologically benign appearing peritumor breast tissue.