Finding a mass in the armpit, formally known as an axillary mass, can cause immediate concern due to its proximity to breast tissue and the lymphatic system. When this palpable finding involves the lymph nodes, it is called axillary lymphadenopathy. While any new lump warrants medical attention, the vast majority of armpit lumps are benign and not cancerous.
Why Most Armpit Lumps Are Not Cancer
The most frequent reason for an armpit lump is an immune response causing the swelling of the axillary lymph nodes. These small, bean-shaped organs filter lymph fluid from the arm, chest wall, and breast. Functioning as command centers, these nodes collect and neutralize pathogens using white blood cells. When the body fights an infection or responds to an irritant, the nodes swell, resulting in a noticeable lump.
Localized infections can trigger this response, such as folliculitis (inflammation of a hair follicle) or a boil (a deeper infection). These common skin issues often occur in the armpit due to shaving, friction, or poor hygiene. A sebaceous cyst, which forms under the skin due to an obstructed gland, is another frequent, non-malignant cause.
Systemic immune responses, such as those caused by viral illnesses like the flu or infectious mononucleosis, can lead to temporary lymph node enlargement. A common cause of axillary swelling is a recent vaccination administered in the arm, such as the flu or COVID-19 vaccine. The vaccine stimulates the immune system, and this localized activation causes the nearby lymph nodes to swell temporarily.
Other benign conditions include lipomas (soft, harmless growths made of fatty tissue) and fibroadenomas (non-cancerous growths of fibrous tissue). Hidradenitis suppurativa, an inflammatory skin condition, involves recurrent abscesses and nodules in areas with sweat glands, which can be mistaken for a serious mass. These non-malignant lumps typically resolve on their own or with targeted treatment.
Recognizing Malignant Axillary Masses
When an armpit lump is malignant, it usually represents one of two primary scenarios: a cancer that originated in the lymph nodes or one that has spread from a nearby site. The first scenario involves a primary cancer of the lymphatic tissue, known as lymphoma. Lymphoma causes white blood cells to grow and multiply abnormally, resulting in the swelling of the lymph nodes where these cells reside.
The second, more common scenario involves metastatic disease, where cancer cells travel from a primary tumor to the axillary lymph nodes. Because approximately 75% of the lymph fluid from the breast drains into the armpit, breast cancer is the most frequent source of metastatic cells found here. The presence of cancer cells in the axillary nodes is important for staging and diagnosing breast cancer.
A lump can also be a sign of melanoma, a skin cancer that has spread from the trunk or upper limb to the regional lymph nodes. Less common cancers, such as certain leukemias, can also cause lymph node swelling. In all cases of malignancy, the lymph node is enlarged either because it has trapped cancer cells or because the cancer originated within the node’s structure.
Characteristics That Differentiate Lumps
The physical characteristics of an armpit lump offer initial clues to distinguish between a benign finding and a malignancy. Lumps caused by infection or inflammation, which are typically benign, are often accompanied by pain or tenderness upon touch. This pain results from the inflammatory process as the immune system fights a localized issue.
Conversely, a lump caused by cancer is frequently painless, which prompts further investigation. The mobility of the mass is another important differentiator during an exam. Benign lumps, such as an infected lymph node or a cyst, usually feel movable or rubbery and can be gently shifted beneath the skin. A cancerous mass, however, is often described as being fixed or matted, feeling firmly adhered to the surrounding tissue.
Texture provides further detail: most non-cancerous lumps feel soft, compressible, or spongy. Malignant masses tend to feel hard, firm, or rock-like, and may have an irregular shape or rough edges. Benign lumps often fluctuate in size, appearing quickly and then gradually shrinking as the underlying infection or irritation resolves.
A lump that is malignant usually persists for an extended period, often two to four weeks or more, and may continue to grow steadily. Cancerous lymphadenopathy may also be accompanied by systemic symptoms, including unexplained weight loss, persistent low-grade fever, and drenching night sweats. These “B-symptoms” indicate a generalized disease process rather than a localized immune reaction.
The Diagnostic Process and Next Steps
Any persistent or suspicious armpit lump necessitates professional medical evaluation to determine its exact cause. The diagnostic process begins with a detailed physical examination, where the healthcare provider assesses the lump’s size, texture, mobility, and tenderness. The provider will also review the patient’s medical history for recent infections, injuries, or vaccinations, which might explain the swelling.
If the lump is not clearly benign, imaging tests are the next step. Ultrasound is often the first-line modality used to visualize the mass, determining if it is solid or fluid-filled and examining the lymph node’s internal structure. For women, a mammogram may be ordered to evaluate breast tissue if a metastatic cause is suspected.
On ultrasound, a normal or benign lymph node typically appears oval-shaped with a preserved fatty center. Suspicious nodes may appear rounded with a thickened outer cortex and loss of the central fatty area.
If imaging reveals features concerning for malignancy, a definitive diagnosis requires a tissue sample. This is performed through a minimally invasive procedure such as a fine-needle aspiration (FNA) or a core needle biopsy, guided by ultrasound, which allows a pathologist to examine the cells for cancer.