Tumors can occur in the armpit, medically known as the axilla. A tumor describes any abnormal mass or swelling of tissue, which can be either benign (non-cancerous) or malignant (cancerous). While finding an armpit lump causes concern, the majority of masses found here are benign and often temporary. The axilla is rich in lymph nodes and soft tissue, explaining why many different types of growths can develop, ranging from minor skin issues to rare malignancies.
Temporary and Non-Neoplastic Lumps
The most frequent cause of a palpable mass in the armpit is the temporary swelling of lymph nodes, a condition known as lymphadenopathy. Lymph nodes serve as filtering stations for the body’s lymphatic fluid, trapping germs, foreign material, and abnormal cells. When the body is fighting an infection in the arm, chest, or breast, these nodes often enlarge as they become more active, a process called reactive lymphadenopathy. Common infections that cause this type of swelling include viral illnesses like mononucleosis, bacterial infections of the skin, or even a reaction to a recent vaccination. This type of lump is usually tender or painful and will typically resolve on its own once the underlying infection or inflammation subsides.
Skin-related conditions can also manifest as temporary lumps or swellings in the axilla. Folliculitis (inflammation of hair follicles) or hidradenitis suppurativa (a chronic skin condition causing painful nodules) can create noticeable masses under the skin. Cysts or abscesses are also frequent, often resulting from clogged sweat glands, shaving irritation, or the use of antiperspirants. These masses are non-neoplastic, meaning they are not tumors but rather fluid-filled sacs or pockets of infection. They are usually painful and may require drainage if they do not resolve naturally.
Specific Benign Growths
Beyond the temporary inflammatory reactions, some true benign tumors, or neoplasms, can develop in the axilla. These growths represent an abnormal proliferation of normal tissue cells but do not spread to other parts of the body. A common example is a lipoma, which is a soft, non-tender lump composed of fatty tissue. Lipomas are typically slow-growing and feel rubbery or doughy, and they are generally harmless.
Another distinct benign mass is accessory breast tissue, also known as polymastia or ectopic breast tissue, which develops most commonly in the armpit. This tissue forms when remnants of the embryonic “milk line” fail to regress before birth. Accessory breast tissue contains glandular and fatty components and can swell and become painful in response to hormonal fluctuations, much like normal breast tissue. Although benign, this tissue carries the same rare risk of developing breast cancer as the primary breast.
Primary and Metastatic Malignant Tumors
While rare, malignant tumors are a serious possibility for armpit lumps, and they are broadly classified as either metastatic or primary. The axilla is the first and most common site for the spread of cancer from the breast, due to the high concentration of lymph nodes in the area. Breast cancer cells often travel through the lymphatic vessels and become trapped in the axillary lymph nodes, causing them to enlarge. The presence of cancer in these nodes significantly influences the staging and treatment of breast cancer. Other cancers, such as melanoma or other skin cancers from the upper body, can also metastasize to the axillary lymph nodes. Metastatic tumors typically present as hard, fixed, and painless lumps, although a lack of pain does not definitively rule out a benign cause.
Primary cancers that originate directly within the axilla are much less common. Lymphoma, a cancer of the lymphatic system, begins in the lymph nodes themselves and may present as persistent, painless swelling. Sarcoma, a rare cancer that develops from soft tissues like fat, muscle, or nerves, can also occur in the axilla. The function of the axillary lymph nodes as a regional drainage system makes it a frequent site for cancer to appear, even if the original tumor is small or undiscovered elsewhere.
Medical Evaluation and Diagnostic Steps
Any persistent, unexplained lump in the armpit requires medical evaluation to determine the underlying cause. The features that raise suspicion for a serious issue include a lump that is rapidly growing, feels hard and immovable, is painless, or persists for more than two to three weeks. A doctor will begin the evaluation with a thorough physical examination, assessing the lump’s size, mobility, texture, and tenderness, while also checking the breast tissue.
Imaging tests are the next step in the diagnostic process, with an axillary ultrasound typically being the first choice. The ultrasound can determine if the mass is solid, cystic, or a cluster of enlarged lymph nodes. It can also reveal features that suggest malignancy, such as a rounded shape or thickened outer layer of a lymph node. For women over the age of 30, the ultrasound is often complemented by a diagnostic mammogram to evaluate the breast tissue for any unseen primary tumor. If imaging reveals suspicious characteristics, the definitive diagnosis requires a biopsy, where a small tissue sample is removed with a needle and examined under a microscope.