# Photoacoustic Imaging for Management of Breast Cancer: A Literature Review and Future Perspectives
## 1 Introduction  
Globally, the number of breast cancer cases was largest among the different types of cancers, considering male and female populations together, and it was the leading cause of cancer-induced deaths among women in 2015 [1]. Breast cancers are classified as non-invasive breast cancer and invasive breast cancer (IBC). Non-Invasive breast cancers are those which do not spread, and comprise ductal carcinoma in situ (DCIS) and lobular carcinoma in situ (LCIS). Invasive or infiltrating breast cancers can spread to other parts of the body, and they are comprised of invasive ductal carcinoma (IDC) and invasive lobular carcinoma (ILC) [2]. In the year 2015 alone, worldwide, 523,000 women died because of breast cancer, while in the previous 16 years (2005-2015) breast cancer cases had increased by 43% [1]. For American women, it is the second leading cause of deaths due to cancer [3; 4]. The American Cancer Society estimated that the number of new female IBC cases in the United States for 2019 would be 268,600 in addition to 62,930 in situ cases, and also estimated that the number of deaths due to female breast cancer would be 41,760 [5]. Early detection of breast cancer can be an effective strategy for reducing the number of breast cancer-induced deaths [6]. For successful treatment, breast cancer should be detected at an early stage when it is small and yet to spread. For early detection, a reliable screening technique for detecting breast cancer in asymptomatic women was required. Breast cancers that are detected successfullyduring screening tests are more likely to be smaller and are still confined to the breast. The two most important parameters for the successful diagnosis of a breast cancer patient are the size of breast cancer and how far it has spread [7]. Screening is performed on a targeted population with asymptomatic patients who are evaluated for cancer yet to be detected, or precancer. During screening, it is expected that most of the population will not have the disease. Diagnosis is performed on patients who have some symptoms of breast cancer, such as a lump in the breast, breast pain, nipple discharge, etc. Both early diagnosis and screening are important components of a comprehensive cancer control but they differ in parameters, such as resources, as well as infrastructure requirements, impact, and cost. The target population in screening is 50-100 times the number of participants in early diagnosis. Screening involves mostly in situ cases and a very few invasive cases, whereas in early diagnosis most of the cases are invasive [8]. Diagnostic tests are more involved, time-consuming, and expensive than screening tests and are performed to determine the size and location of abnormalities. In the case of breast cancer, sometimes diagnosis also involves evaluation of surrounding tissues and lymph nodes [9]. There are some commonly available medical imaging modalities, such as X-ray mammography, ultrasound (US) imaging, and magnetic resonance imaging (MRI), which are used separately or in combination for the screening of the breast cancer. For an asymptomatic patient, the commonly used diagnosis protocol using imaging is to refer the patient for a biopsy procedure to confirm the breast cancer diagnosis if the patient comes up with a suspected X-ray mammogram and US image [10]. X-ray mammography, in addition to possessing ionization hazards, has limited sensitivity specifically for women under 50 [11; 12; 13]. Although the American Cancer Society recommends regular breast cancer screening with X-ray mammograms for women in certain age groups, it recommends using MRI along with X-ray mammogram for breast cancer detection. For imaging dense breast tissues which are generally found in younger women, X-ray mammogram cannot be used [12; 14; 15]. US imaging is another imaging modality used for imaging breast cancer. When X-ray mammography produces conflicting inference with that of the US image, MRI is done before referring the patient for a biopsy procedure [10].