Feeling a lump in the breast understandably leads to worry and panic. Is it a harmless cyst? Or could it be a sign of breast cancer? One thing's for sure: You want your uncertainty addressed. So how do you tell if it's a breast cyst or a tumor?

"If you feel a lump of any kind, it's important you have it evaluated," says Dr. Correna Terrell, a breast radiologist at Houston Methodist. "It's not uncommon for a woman to discover a breast cyst at some point in her life, but we never want to miss a breast cancer diagnosis."

From what a breast cyst feels like to how a radiologist tells the difference between a cyst and a tumor, here's everything you need to know if you've discovered a lump.

What causes breast cysts?

"A breast cyst is a round or oval sac in the breast that is filled with fluid," explains Dr. Terrell. "The exact cause is unknown, but breast cysts are thought to be related to natural fluctuations in hormone levels."

Though they can occur at any age, breast cysts are fairly common in women ages 35 to 50. They can vary in size — as tiny as a grain of rice or as large as a golf ball — and a woman might sometimes have more than one. Breast cysts are almost always benign (noncancerous) and don't require treatment unless the cyst is large and needs to be drained.

A breast tumor, on the other hand, is a solid mass — rather than a fluid-filled sac. They can be benign or malignant (cancerous).

"A malignant breast tumor always requires treatment," says Dr. Terrell. "The earlier we detect breast cancer and initiate treatment, the better — which is why any lump or change in the breast should be evaluated."

What does a breast cyst feel like?

You might wonder if there are any breast cyst symptoms, like breast tenderness or pain, that can help you distinguish a harmless lump from a potentially cancerous one. But it's not that simple.

"There are some differences between a breast cyst and a tumor, but they can be subtle — and there are always exceptions," says Dr. Terrell. "It can be difficult to tell a cyst from a tumor by feeling alone."

That said, a breast cyst is more likely to:

• Be painful or tender
• Feel soft and smooth, like a water balloon (though they can sometimes feel firm)
• Be easily moved
• Change with your menstrual cycle (increase and decrease in size or tenderness)

"A breast tumor is typically painless, firm, hard to move and won't change with your menstrual cycle," says Dr. Terrell. "But while these characteristics can provide some indication of whether it's a cyst or tumor, the only way to accurately assess a lump is through imaging."

(Related: 5 Signs of Breast Cancer Beyond Feeling a Lump)

Breast cyst vs. tumor: How to tell if a lump is cancerous

If you feel a breast lump of any kind, it's important to have it evaluated — which includes mammography (depending on your age) and ultrasound.

"A radiologist can tell the difference between a cyst and a tumor using a breast ultrasound," says Dr. Terrell. "A cyst will have smooth walls and be filled with clear fluid, whereas a tumor will look solid."

A small, simple breast cyst typically doesn't need treatment and should go away on its own over time. Alert your doctor if it persists for several more menstrual cycles or becomes painful. A large cyst may need to be drained via fine-needle aspiration, especially if it's painful.

If a breast lump looks solid and has suspicious features on breast ultrasound, your radiologist will recommend a biopsy to further evaluate the mass and determine whether it's cancerous or not. Some solid lumps with benign features on ultrasound can be monitored with follow-up ultrasound exams.

Can breast cancer be misdiagnosed as a cyst?

A clinical breast exam is part of diagnosing a breast lump, but, again, it can be difficult to tell a cyst from a tumor just by feeling it. It's why ultrasound is an essential component of evaluating a lump. Depending on your age, mammography may be, too.

"Without imaging, a breast lump can certainly be misdiagnosed and cancer could potentially be missed," says Dr. Terrell. "Any new lump or change in your breast tissue needs to be evaluated by a breast radiologist, who has the tools and technology needed to provide an accurate diagnosis."

Can a breast cyst turn into a tumor?

A benign breast cyst can't become cancerous and develop into tumor. But while rare, breast cysts aren't always benign.

"Complex cysts — ones that have irregular borders, thick walls and some amount of solid material within the fluid — have a 20-30% chance of being malignant," adds Dr. Terrell. "This is when we'd recommend either biopsying the cyst, so we can rule out breast cancer or doing follow-up imaging in a few months."