Single most likely diagnosis
- Lipoma: A benign tumor composed of fat tissue, which is common in the axillary region and can present as a soft, movable cyst under the arm.
Other Likely diagnoses
- Epidermoid cyst (sebaceous cyst): A benign cyst that can occur anywhere on the body, including under the arm, often filled with a cheesy material.
- Abscess: A collection of pus that has built up within the tissue of the body, which can occur under the arm due to infection.
- Hidradenitis suppurativa: A chronic skin condition that involves recurrent, painful abscesses and cysts in the apocrine-gland-bearing areas of the body, such as the armpits.
Do Not Miss diagnoses
- Lymphoma: Although rare, lymphoma can present with lymph node enlargement that might feel like a cyst under the arm, and it's crucial to rule out this potentially life-threatening condition.
- Metastatic disease: In rare cases, a cystic lesion under the arm could be a metastasis from another primary cancer, emphasizing the importance of a thorough diagnostic workup.
Rare diagnoses
- Dermatofibrosarcoma protuberans: A rare type of skin cancer that can present as a firm, painless lump or cyst under the skin, including under the arm.
- Hygroma (cystic lymphangioma): A rare, benign condition that typically presents at birth or during infancy as a soft, cystic mass, often in the neck but can occur under the arm.