  Breast cancer is one of the leading causes of mortality in women. Early
detection and treatment are imperative for improving survival rates, which have
steadily increased in recent years as a result of more sophisticated
computer-aided-diagnosis (CAD) systems. A critical component of breast cancer
diagnosis relies on histopathology, a laborious and highly subjective process.
Consequently, CAD systems are essential to reduce inter-rater variability and
supplement the analyses conducted by specialists. In this paper, a
transfer-learning based approach is proposed, for the task of breast histology
image classification into four tissue sub-types, namely, normal, benign,
\textit{in situ} carcinoma and invasive carcinoma. The histology images,
provided as part of the BACH 2018 grand challenge, were first normalized to
correct for color variations resulting from inconsistencies during slide
preparation. Subsequently, image patches were extracted and used to fine-tune
Google`s Inception-V3 and ResNet50 convolutional neural networks (CNNs), both
pre-trained on the ImageNet database, enabling them to learn domain-specific
features, necessary to classify the histology images. The ResNet50 network
(based on residual learning) achieved a test classification accuracy of 97.50%
for four classes, outperforming the Inception-V3 network which achieved an
accuracy of 91.25%.
