1.1. Breast cancer and early detection

Nowadays, cancer is a massive public health problem around the world. According to the International Agency for Research on Cancer (IARC) [1], part of the World Health Organization (WHO), there were 8.2 million deaths caused by cancer in 2012 and 27 million of new cases of this disease are expected to occur until 2030 [2].

Cancer, medically defined as a malignant neoplasm, is a board group of disease involving unregulated cell growth [2]. In cancer, cell divides and grows uncontrollably forming malignant tumors, invading nearby parts of the body. The cancer can spread to all parts of the body through the lymphatic systems or blood streams [3].

Cancer can be diagnosed by classifying tumors in two different types such as malignant and benign. Benign tumors represent an unnatural outgrowth but rarely lead to a patient's death; yet, some types of benign tumors, too, can increase the possibility of developing cancer [4]. On the other hand, malignant tumors are more serious and their timely diagnosis contributes to a successful treatment. As a result, predication and diagnosis of cancer can boost the chances of treatment, decreasing the usually high costs of medical procedures for such patients [5].

Breast cancer (BC) is the most commonly diagnosed cancer and the most common cause of death in women all over the world. Among the cancer types, BC is the second most common cancer for women, excluding skin cancer [6]. Besides, the mortality of BC is very high when compared to other types of cancer [7]. BC, similar to other cancers, starts with a rapid and uncontrolled outgrowth and multiplication of a part of the breast tissue, which depending on its potential harm, is divided into benign and malignant types.

Generally, there are two types of BC that are in situ and invasive. In situ starts in the milk duct and does not spread to other organs even if it grows [8]. Invasive breast cancer on the contrary, is very aggressive and spreads to other nearby organs, and destroys them as well [9]. It is very important to detect the cancerous cell before it spreads to other organs; thus, the survival rate for patient will increase to more than 97% [10].

A major class of problems in medical science involves the diagnosis of disease, based upon various tests performed upon the patient. The evaluation of data taken from patients and decisions of experts are the most important factors in diagnosis. The correct diagnosis of BC is one of the major problems in the medical field. As BC can be very aggressive, only early detection can prevent mortality. Clinical diagnosis of BC helps in predicting the malignant cases and timely diagnosis can increase the chances of a patient's life expectancy from 56 to 86% [11].

BC has four early signs: microcalcification, mass, architectural distortion, and breast asymmetries [12]. The various common methods used for breast cancer diagnosis (BCD) are positron emission tomography (PET), magnetic resonance imaging (MRI), CT scan, X-ray, ultrasound, photoacoustic imaging, tomography, diffuse optical tomography, elastography, electrical impedance tomography, opto-acoustic imaging, ophthalmology, mammogram, etc. [13]. The results obtained from these methods are used to recognize the patterns, which are aiming to help the doctors for classifying the malignant and benign cases.

Despite of recent advances in the comprehension of the molecular biology of BC progression and the discovery of new related molecular markers, the histopathological analysis remains the most widely used method for BC diagnosis [14]. Despite significant progress reached by diagnostic imaging technologies, the final BCD, including grading and staging, continues being done by pathologists applying visual inspection of histological samples under the microscope [15].

However, manual classification of images is a challenging and time-consuming task, being highly susceptible to interobserver variability and human errors, resulting in extremely poor critical outcomes, thus markedly increasing the workload of radiologists because of their significant shortage. In addition, medical care costs that are relevant to imaging rapidly increase. Therefore, new methods for diagnosis are required.

Currently, bioimaging quantification is an emerging technique in the field of radiology with a growing implantation in hospital centers. It provides relevant information that is not appreciable by the naked eye in conventional radiological reading. It consists of the generation of quantitative (numerical) data from images, mainly of high resolution, to provide information on which to support a clinical assessment [16]. Biomarkers can be said to be the transition from radiology to personalized medicine.
