ERBB2 and cancer: Curve estimation regression models showed that a minimum of 469 or 953 invasive cancer cells per case is needed to reach an average margin of error below 0.1 for HER2/CEP17 ratio (power model (β0 = 3.497; β1 =  − 0.578; R2 = 0.707); p < 0.001) or for the average of HER2 copy number per cell (power model (β0 = 7.479; β1 =  − 0.629; R2 = 0.634); p < 0.001), respectively.