The vast majority of breast lumps are not cancer. Only about 10% of new breast lumps turn out to be malignant, meaning roughly 9 out of 10 are caused by something benign like cysts, fibroadenomas, infections, or tissue changes related to hormones or injury. That said, every new lump deserves evaluation because the causes range widely, and telling them apart by feel alone isn’t reliable.
Fibroadenomas
Fibroadenomas are the single most common cause of benign breast lumps, especially in younger people. They account for about 95% of breast lumps in adolescents and occur most often between ages 15 and 35, though they can develop at any age. A fibroadenoma is a solid lump that feels firm, smooth, and rubbery, with a round shape and clear borders. The hallmark feature is that it moves easily under the skin when you touch it, almost like a marble. They’re typically painless.
Most fibroadenomas stay small and don’t require treatment. Some shrink on their own over time. Occasionally one grows large enough to change the shape of the breast, and in that case it can be removed surgically. The key distinction with fibroadenomas is their mobility and smooth edges, which set them apart from more concerning lumps.
Breast Cysts
Cysts are fluid-filled sacs that develop in breast tissue, most commonly in people between ages 35 and 50. They’re especially common as you approach menopause, and they rarely develop after menopause once estrogen levels drop. Experts believe natural hormonal fluctuations drive their formation.
The connection to your menstrual cycle is one of the most noticeable features of cysts. Many people find that a cyst becomes larger and more tender in the days before a period, then shrinks afterward. Smaller cysts may not be noticeable at all, while larger ones can feel like a distinct lump that’s painful or tender to the touch. Cysts are very rarely cancerous. If a cyst is large and uncomfortable, a doctor can drain the fluid with a needle, which often resolves it immediately.
Infections and Abscesses
Breast infections can create lumps that feel swollen, warm, and painful. Mastitis, an infection of breast tissue, is most common during breastfeeding but also occurs in people who aren’t lactating, particularly smokers and those with higher body weight. If mastitis isn’t treated early, it can progress to an abscess, a walled-off pocket of pus that forms a firm, sore lump. This happens in roughly 3% to 11% of mastitis cases.
Abscesses typically need to be drained, either with a needle guided by ultrasound or through a small incision. Antibiotics alone usually aren’t enough once an abscess has formed. A less common type called periductal mastitis can cause a lump near the nipple, sometimes accompanied by thick nipple discharge, nipple inversion, or a draining opening in the skin.
Fat Necrosis From Injury or Surgery
When fatty tissue in the breast is damaged, it can die and be replaced by scar tissue, forming a firm, round lump called fat necrosis. The most common trigger is physical trauma to the breast, accounting for 21% to 70% of cases. Breast surgery (reductions, reconstructions, biopsies) is another frequent cause, along with radiation therapy.
Fat necrosis can be tricky because it sometimes mimics the feel of something more serious. The lump may be irregular, fixed to the skin above it, and even cause skin dimpling or nipple retraction as scar bands form between the damaged tissue and skin. These features overlap with signs of cancer, which is why imaging or a biopsy is often needed to confirm the diagnosis. Fat necrosis itself is completely benign and doesn’t increase cancer risk.
Milk Retention Cysts
Galactoceles are fluid-filled lumps caused by a blocked milk duct. They occur during or shortly after breastfeeding and feel smooth and movable, similar to a regular cyst. They’re harmless and often resolve once breastfeeding ends or the duct clears.
Phyllodes Tumors
These are rare growths that can resemble fibroadenomas on imaging and by feel, but they behave differently. Phyllodes tumors tend to be larger (often over 3 cm), may have irregular shapes, and can grow quickly. Most are benign, but a small percentage are malignant. The distinguishing feature is growth speed: malignant phyllodes tumors in one study grew at a rate of about 180% per month, more than ten times faster than any benign type. A lump that’s noticeably increasing in size over weeks rather than months warrants prompt evaluation.
When a Lump May Be Cancer
Cancer accounts for about 10% of breast lumps overall, but the risk rises significantly with age. A woman in her thirties has roughly a 0.5% chance that a new lump is malignant. By her sixties, that rises to about 3.5%. Certain physical characteristics raise concern: a lump that is hard, has irregular or jagged edges, feels fixed in place rather than movable, and feels distinctly different from the surrounding tissue. Skin changes are another red flag, including dimpling that resembles the texture of an orange peel, thickening or color changes of the skin, or crusting around the nipple.
A cancerous lump is often painless, which is worth noting because many people assume that a painless lump is less worrisome. Pain is actually more commonly associated with cysts and infections. The absence of pain doesn’t mean anything is wrong, but it also shouldn’t be reassuring on its own.
How Breast Lumps Are Evaluated
The standard workup for a new breast lump is a diagnostic mammogram combined with an ultrasound. For people under 30, ultrasound is typically the first and sometimes only imaging step, since younger breast tissue is denser and harder to read on a mammogram. These imaging tools help classify the lump on a standardized scale. Lumps rated in the lowest categories carry essentially no expected risk of malignancy. Those in the middle range (about 2% risk or less) are usually monitored with a follow-up scan in six months. Higher categories, where risk can range from 2% to over 95%, typically lead to a biopsy, where a small tissue sample is removed and examined under a microscope.
Ultrasound is particularly useful for distinguishing fluid-filled cysts from solid masses. A simple cyst filled with clear fluid almost never needs a biopsy. A solid mass or a cyst with unusual features inside it gets a closer look. The entire process, from imaging to biopsy results, usually takes one to two weeks.