Breast cancer occurs when abnormal cells in the breast grow out of control. Breast cancer is the most common female cancer in the U.S., and the second-leading cause of cancer death in women (lung cancer is the leading cause of cancer deaths). Breast cancer is more common in women, but men can get it too.
Stage 1 breast cancer is cancer still small and localized to the breast and either has not spread to the lymph nodes or a tiny area of cancer has spread to the sentinel lymph node (the first lymph node to which cancer is likely to spread).
Symptoms of stage 1 breast cancer include:
- Skin irritation or dimpling
- Swelling of all or part of the breast
- Redness, scaling, flaking, or thickening of the nipple or breast skin
- A change in the size or the shape of the breast
- Nipple turning inward or change in the appearance of a nipple
- Nipple discharge that is not breast milk
- Breast pain
- Nipple pain
- New lump in the breast
- A lump in the armpit
What Causes Breast Cancer?
Breast cancer results from damage to a cell’s DNA, but the cause of this damage is unknown.
Risk factors for developing breast cancer include:
- Inherited genetic mutations, such as mutation in the BRCA1 and BRCA2 genes
- Family history of breast cancer, especially a first-degree relative (mother, sister, or daughter)
- Personal history of breast cancer or noncancerous breast disease
- Having dense breasts
- Age over 50 years
- Physical inactivity/sedentary lifestyle
- Previous radiation therapy treatment to the chest or breasts
- Early onset of menstrual periods (before age 12) or late menopause (after age 55)
- History of use of the drug diethylstilbestrol (DES)
How Is Breast Cancer Diagnosed?
Breast cancer is diagnosed with a patient history and a physical examination. During a physical exam, a healthcare provider may find a lump in the breast. Tests used to confirm a diagnosis may include:
- Mammogram
- Breast ultrasound
- Magnetic resonance imaging (MRI) of the breast
- Biopsy
If the breast cancer is suspected of metastasizing (spreading), imaging studies may be indicated, such as:
- Bone scan
- Computerized tomography (CT) scan of the chest
- CT scan of the abdomen and pelvis
- Positron emission tomography (PET) scan
What Is the Treatment for Breast Cancer?
Stage 1 breast cancer is highly treatable and treatment usually involves surgery, radiation, a combination of the two, or hormone therapy.
Treatment for breast cancer depends on the extent of the tumor and may involve one or more of the following:
- Surgery
- Lumpectomy – removal of the tumor and some healthy tissue that surrounds it
- Mastectomy – removal of the entire breast which may be followed by breast reconstruction (not usually used for stage 1 breast cancer, but may be a treatment in later stages)
- Radiation therapy
- May be combined with surgery in stage 1
- Hormone therapy
- Chemotherapy
- Not often used for stage 1 breast cancer
- Targeted therapy
- Immunotherapy
From 
https://www.uptodate.com/contents/breast-cancer-guide-to-diagnosis-and-treatment-beyond-the-basics?search=breast%20cancer&source=search_result&selectedTitle=8~150&usage_type=default&display_rank=8
https://www.cdc.gov/cancer/breast/index.htm
https://www.nationalbreastcancer.org/breast-cancer-stage-0-and-stage-1