By Hanna Söderqvist|Updated Oct 06, 2026|6 min read
Both can show up as a lump on a scan report, but one is a pocket holding something and the other is extra tissue — and the test that settles it differs.
If a report or a doctor's quick comment left you with the word "cyst" or "tumor" and not much else, the core difference is simple: a cyst is a closed sac or pouch that can be filled with air, fluid, pus or other material, while a tumor is an abnormal growth of body tissue (National Library of Medicine (MedlinePlus), undated). One is a container; the other is extra tissue.
That difference matters because it shapes what happens next — imaging that checks whether a lump is solid or fluid-filled, drainage, watchful follow-up, or a biopsy. Which path applies to you is a decision for your doctor, based on where the lump is, how it looks on imaging, and your own history.
Cyst vs. Tumor
Question
Cyst
Tumor
What it is
A closed sac, pouch or capsule in the body (National Library of Medicine (MedlinePlus), undated)
An abnormal growth of body tissue (American Cancer Society, undated)
What's inside
Air, fluid, pus or other material inside the sac wall (National Library of Medicine (MedlinePlus), undated)
Cells that have divided and grown excessively, usually forming a solid mass (National Library of Medicine (MedlinePlus), undated)
What causes it
Often a blockage or an infection (American Cancer Society, undated); for some types, such as epidermoid cysts under the skin, the cause is unknown (National Library of Medicine (MedlinePlus), undated)
Something goes wrong in the cell cycle, disturbing the normal balance of cell growth and cell death (American Cancer Society, undated; National Library of Medicine (MedlinePlus), undated)
Relationship to cancer
Almost always benign, though some cysts contain cells with abnormal changes and it is possible for them to turn into cancer (American Cancer Society, undated)
Can be benign (not cancer) or malignant (cancer), and malignant cells can invade nearby tissue or break away and spread, called metastasis (National Library of Medicine (MedlinePlus), undated)
How it's typically diagnosed
Imaging such as ultrasound, which can show whether a lump is solid or fluid-filled; a thin needle may be placed in the lump, and if fluid drains out it is a cyst, with the fluid sometimes sent to a lab (Harvard Health Publishing, undated)
Sometimes seen directly on exam, as with some skin or oral cancers, but most are deep inside the body and need imaging plus a biopsy (National Library of Medicine (MedlinePlus), undated)
How it's typically treated
Often left alone and watched; some are drained or removed surgically depending on type and location, and some are removed for testing because they resemble skin cancer (National Library of Medicine (MedlinePlus), undated)
Depends on type: benign tumors are often treated with surgery and usually don't grow back, while cancerous bone tumors, for example, often need chemotherapy plus surgery after biopsy (National Library of Medicine (MedlinePlus), undated)
Either one can need urgent attention. Get same-day care, or go to an emergency department, for sudden severe pelvic or abdominal pain with vomiting, fever or faintness (which can signal a ruptured or twisted ovarian cyst), for a lump with spreading redness, heat and pus, or for a sudden severe headache, new weakness, vision loss or a first seizure — benign tumors can be dangerous when they press on vital organs such as the brain (National Library of Medicine (MedlinePlus), undated).
Why the Confusion Happens
The overlapping word is growth — and, close behind it, mass. The American Cancer Society glossary defines a tumor as an abnormal growth of tissue, and in the same glossary describes a cyst as an abnormal sac-like growth or mass. A radiology or pathology report may use "mass," "lesion" or "growth" for either one, so the sentence that worried you may not be saying what you think it is.
Benign is the second shared word. It means not cancer, and it belongs to both sides: almost all cysts are benign (American Cancer Society, undated), and tumors are sorted into benign and malignant (National Library of Medicine (MedlinePlus), undated). Seeing "benign" does not tell you which of the two you have.
The third source of mix-ups is what you can feel. A lump under the skin, in a breast, or on an imaging scan feels and looks much the same from the outside, which is why a breast ultrasound is used to determine whether a lump is solid or fluid-filled (Harvard Health Publishing, undated). Scale adds to it: up to 10% to 20% of people have pituitary tumors, and most of those are noncancerous (National Library of Medicine (MedlinePlus), undated) — so "tumor" on a report is a description of tissue, not a verdict. Ask your provider which word applies to your finding and what test, if any, comes next.
Frequently asked questions
My ultrasound report says "complex cyst." Does that mean it's a tumor?
This question is about a cyst, not a tumor: "complex" describes a sac that has solid-looking areas, thick walls or internal divisions rather than clear fluid alone. Imaging alone often can't classify those, so a provider may recommend repeat imaging, drainage with the fluid sent to a lab, or a biopsy (Harvard Health Publishing, undated). Your doctor decides which, based on location and your history.
Is a benign tumor the same thing as a cyst?
No — a benign tumor is solid overgrown tissue that grows in one place and cannot spread or invade elsewhere, while a cyst is a sac filled with fluid, air, pus or other material (National Library of Medicine (MedlinePlus), undated). Benign tumors can still cause problems by pressing on vital organs such as the brain, and treatment often involves surgery, after which they usually don't grow back.
Can a cyst turn into cancer?
This one is about cysts. Almost all are benign, though some contain cells showing abnormal changes and it is possible for those to become cancer (American Cancer Society, undated). Because a cyst can also look like skin cancer, one may be removed so it can be tested (National Library of Medicine (MedlinePlus), undated) — a biopsy is the test used to tell benign from malignant.
Hanna writes about the quieter corners of mental health: boredom, loneliness, the ordinary weight of being tired. She approaches the subject with patience, resisting easy resolutions in favor of honest complication.
