A peanut-sized lump at the lateral border of the pectoralis major muscle is one of the clinical manifestations of paramammary breast cancer. Mammaryofaccessorymamma is a cancer that occurs in the paramammary gland, mostly in the axillary area, but rarely in the subclavian area, abdomen and vulva. Four cases of adenocarcinoma of the parammary gland have been reported, including those with complete nipple, areola and mammary gland, and those with nipple only, and some of them have lactation. The prerequisite for the development of paramammary cancer is that the parammary gland must have mammary glandular tissue, and parammary cancer cannot occur in a parammary gland that has only a nipple or areola, or both without a gland. Due to the existence of parammary glands with their own glands, during physiological changes such as menstruation, pregnancy and lactation, the corresponding area may become distended and painful simultaneously with the breast, and even lactate with breast lactation. Therefore, paramammary glands with glands are as likely to develop breast cancer as normal breasts. Differential diagnosis of a peanut-sized lump at the lateral border of the pectoralis major muscle: 1. Axillary lymph enlargement: Lymph nodes increase in size due to internal cell proliferation or tumor cell infiltration, which is a common clinical sign. It can be detected by touching the armpit area. 2.Breast lumps: Breast lumps usually refer to a disease in which lumps grow inside due to the different composition of breast tissue. Breast lumps are the most common breast disorder, and numerous benign diseases are also manifested in the form of breast lumps, so it is most important to distinguish between benign and malignant breast lumps. Except for breast cancer which is malignant, lumps produced by breast fibroadenoma, breast hyperplasia, breast cumulus cyst and breast fat necrosis are all benign and can be treated by acupuncture points and reflex zone massage. New lumps now appear for the following reasons: cysts; bacterial infection of the nipple wound; fibrocystic mastopathy; and the presence of benign tumors that do not spread. Hormonal abnormalities, suffering from malignant tumors. 3.Subcutaneous emphysema: When there is gas accumulation in the subcutaneous tissue of the chest, it is called subcutaneous emphysema.