Cancer is usually diagnosed step by step: the doctor asks about your symptoms and examines you, orders blood or urine tests and chooses the imaging that fits the symptom, such as an ultrasound, CT scan, MRI or PET scan [1, 4]. Scans show whether there is a mass, how big it is and whether it has spread; but in most cases only a biopsy can say for certain that it is cancer [1, 2]. After the diagnosis, staging tests and special tests on the tumour decide the treatment [1, 3].
Key points
- No blood test or scan on its own usually confirms cancer; in most cases a biopsy is needed [1].
- The first test depends on the symptom: for example, a chest X-ray for a lasting cough, an ultrasound for testicular or ovarian symptoms, an MRI for brain symptoms and a CT scan when pancreatic cancer is suspected [4].
- A PET scan shows how active cells are and is mainly used for staging and to follow treatment; infection and inflammation also light up on it [1, 9, 10].
- No scan is perfect; if the result was normal but your symptom stays or gets worse, see your doctor again [5, 11, 12].
- Most people who are referred for cancer tests turn out not to have cancer [5].
How is cancer diagnosed? The steps
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Talking and examination
The doctor asks about your symptoms, how long you have had them, your family history and habits such as smoking, and examines you. This step decides which test is needed [4]. See the warning signs in What are the signs of cancer?
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Blood, urine or stool tests
These tests give clues, but on their own they cannot say whether you have cancer; an abnormal result does not always mean cancer either [1]. Read more: Can a blood test detect cancer? and What is a tumour marker?
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Biopsy
A small piece of the suspicious tissue is removed with a needle, through an endoscope or by surgery, and a pathologist examines it under the microscope. In most cases only this step confirms cancer [1]. Read more: What is a biopsy and how long do the results take?
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Special tests on the tumour and the treatment plan
Gene or marker tests on the sample, together with the grade and the stage, decide the treatment [1]. Read more: How to read a pathology report and I have just found out I have cancer: what now?
What does each imaging test show?
| Test | How it works | Mainly used for | Limitation |
|---|---|---|---|
| X-ray (plain radiograph) | Low-dose X-rays [1] | First step for a lasting cough or breathlessness (chest X-ray), and for bone pain or swelling in children and young people [4] | Can miss cancer; a normal result is not always enough [5] |
| Ultrasound | Sound waves and their echoes [1] | Testicular symptoms, soft-tissue lumps, the liver and gallbladder, the ovaries; guiding a biopsy needle [2, 4] | Sometimes the result is unclear and another test is needed [4] |
| CT scan | X-rays and a computer; 3D pictures, sometimes with contrast dye [1, 2] | Chest and abdomen, staging, guiding a biopsy, checking response to treatment and recurrence [2] | In most cases it does not replace a biopsy [1]; more radiation than a plain X-ray [2, 12] |
| MRI | A strong magnet and radio waves; sometimes with contrast dye [1] | Brain, prostate and liver [4, 7, 8] | Like CT, in most cases it does not replace a biopsy [1] |
| PET scan (usually combined with CT) | A labelled sugar is injected, and areas that use a lot of sugar light up [1] | Staging and checking response to treatment in some cancers [2, 10] | Infection, inflammation and injury also light up [9] |
| Bone scan | A radioactive substance that collects in the bones [1] | Seeing whether cancer has spread to the bones [1] | Not every bright spot is cancer; bone damage also shows up [1] |
Your doctor chooses the test according to the symptom and the organ involved; there is no single "best scan" for everyone.
Can a CT scan detect cancer?
A CT scan shows many tumours, their size and position and whether they have spread; that is why it is widely used for diagnosis, staging and follow-up of treatment [2]. But not every spot on a CT scan is cancer [12], and in most cases only a biopsy can say for certain whether a mass is cancer [1]. Lung nodules are a good example; most of them are not cancer: Lung nodule on a CT scan: what size is concerning? For the terms used in reports, read Terms in CT and ultrasound reports.
Can cancer be missed on a CT scan?
Can an MRI detect cancer?
An MRI uses a magnet and radio waves to take pictures of the body in slices, and sometimes a contrast dye is injected to make a tumour show up more brightly [1]. In some parts of the body MRI is better than CT:
- Brain: for brain and nerve symptoms that are slowly getting worse, MRI is the recommended test, with CT only when MRI is not possible [4].
- Prostate: in men suspected of having prostate cancer, an MRI before biopsy found more important cancers, and more than 1 in 4 men did not have a biopsy at all because their MRI showed no sign of cancer [7].
- Liver: in someone with cirrhosis, an MRI or CT scan with contrast dye can often diagnose liver cancer without a biopsy [8].
Breast MRI is also used for women at high risk: Mammogram or breast ultrasound: which test do I need?
Can an ultrasound detect cancer?
Ultrasound uses sound waves and is a good, simple test for many lumps [1, 4]. The UK national guideline recommends ultrasound as the first step for [4]:
- unexplained or persistent testicular symptoms, such as swelling or a lump
- a soft-tissue lump that is getting bigger
- a lump in the upper abdomen that may come from the liver or gallbladder
- persistent ovarian symptoms, such as constant bloating or feeling full quickly (from age 40, depending on the result of a CA125 blood test)
Ultrasound shows a lump and can guide a biopsy needle, but if its result is unclear, another test or a referral is needed [4]. For the breast and thyroid, read: Mammogram or ultrasound? and Thyroid nodules and TIRADS
PET scan vs CT scan for cancer
The two do different jobs. A CT scan shows the shape, size and position of organs and masses [2]. A PET scan shows which parts of the body use more sugar; cancer cells usually use more sugar [1]. Today a PET scan is usually done together with a CT scan, and the combination gives a fuller picture of where a tumour is and how far it has spread [2].
A PET scan is not the first test for everyone; it is mainly used for staging and to check the response to treatment in some cancers [2, 10]. In one study, more than a quarter of PET scans showed a bright spot that was not cancer, most often inflammation [9]. Read more: What is a PET scan and how is it used in cancer?
Why only a biopsy gives a definite diagnosis
A scan shows shadows and shapes, but not the cells themselves. Under the microscope, a pathologist sees whether the cells are cancer, what type of cancer it is and how aggressive it looks; the same sample is also used for gene tests on the tumour [1]. The sample can be taken with a needle (often guided by ultrasound or CT), through an endoscope or by surgery [1, 2].
An important exception is liver cancer in someone with cirrhosis: here the typical appearance of the tumour on an MRI or a CT scan with contrast is often enough for the diagnosis [8]. Worried about a biopsy? Read: Can a biopsy spread cancer? and, to understand the result: What is cancer grade?
How long does a cancer diagnosis take, and why are tests done in a certain order?
Tests usually move from simple and low-risk to more detailed: first the test most closely linked to your symptom, then more detailed imaging and finally a biopsy [1, 4]. Each step decides whether the next one is needed; that is why several tests in a row are sometimes needed.
In England, the aim is that when a GP urgently refers someone with suspected cancer, cancer is confirmed or ruled out within 28 days [6]. For how long biopsy results take, read What is a biopsy and how long do the results take?
Waiting is hard. Remember that most people referred for cancer tests do not have cancer [5]. If you have a new question or worry in the meantime, contact your doctor [5].
When should I call my doctor?
Do not wait; see a doctor sooner if [4, 5]:
- your skin or eyes turn yellow, or you see blood in your phlegm, urine or stool
- you find a new lump that is getting bigger
- you are losing weight without trying
- a test result was normal but your symptom stays, comes back or gets worse
- you have not received a test result or your referral appointment
Severe shortness of breath, heavy bleeding, sudden weakness of one side of the body or a seizure is an emergency; call your local emergency number.
Frequently asked questions
Can cancer be diagnosed with a blood test?
Usually not. A blood test gives clues, but on its own it cannot say whether you have cancer [1]. Read more: Can a blood test detect cancer?
Is a full-body CT scan a good check-up?
There is no evidence that a full-body scan in someone without symptoms does more good than harm [12]. It may find harmless things that lead to more, unnecessary tests, and at the same time miss some conditions [12]. To find cancer early, have the screening tests suited to your age.
Can an ultrasound show cancer?
Is MRI better than CT?
My scan was normal: does that mean I definitely do not have cancer?
Isn't the radiation from all these scans dangerous?
A CT scan gives more radiation than a plain X-ray and may very slightly raise the chance of cancer later in life [12]. To weigh the benefit against the risk, read: Do CT scans cause cancer?
Choosing the test and the referral pathway (for doctors)
For physicians
NICE NG12 definitions. Suspected cancer pathway referral: diagnosis or ruling out of cancer within 28 days of urgent referral; urgent means before 2 weeks and very urgent within 48 hours; direct access means a test for which primary care keeps clinical responsibility, including acting on the result [6]. Examples of first-line direct access tests: urgent chest X-ray at 40 and over with 2 or more unexplained symptoms (or 1 in current or former smokers) among cough, fatigue, shortness of breath, chest pain, weight loss and appetite loss; urgent brain MRI (or CT if MRI is contraindicated) in adults with progressive, sub-acute loss of central neurological function; urgent CT (or ultrasound if CT is not available) for pancreatic cancer at 60 and over with weight loss plus diarrhoea, back pain, abdominal pain, nausea, vomiting, constipation or new-onset diabetes; urgent ultrasound for a soft-tissue lump increasing in size, unexplained or persistent testicular symptoms, an upper abdominal mass consistent with an enlarged liver or gallbladder, and abdominal and pelvic ultrasound for persistent ovarian symptoms, guided by age-specific CA125 thresholds from age 40 and without relying on CA125 alone at 39 or under [4]. Safety netting: review and act on investigation results; be aware of false-negative chest X-rays and faecal occult blood tests; for symptoms that do not meet referral criteria, a planned review or a patient-initiated review if symptoms persist, recur or worsen [5].
Selected evidence. PRECISION (500 biopsy-naive men): in the MRI ± targeted biopsy arm, 28% had a non-suspicious MRI and were not biopsied; clinically significant cancer 38% vs 26% with systematic TRUS biopsy (adjusted difference 12 percentage points; 95% CI 4 to 20) and clinically insignificant cancer 13 percentage points lower [7]. In HCC, multiphase CT or MRI criteria (LI-RADS) allow a non-invasive diagnosis in high-risk patients (cirrhosis) [8]. Benign non-physiological FDG uptake was seen in more than 25% of oncology PET/CT studies, moderate or marked (similar to malignant foci) in half of them; 73% were inflammatory, and correlation with CT may remove the need for further evaluation or biopsy in more than a third of equivocal findings [9]. FDG PET/CT is used for diagnosis, prognosis, staging and response assessment, and the standardised EANM protocol (version 2.0) is needed for SUV harmonisation [10]. Most missed radiological diagnoses are due to interpretation errors (perceptual or cognitive) rather than technical limits, and their prevalence has changed little since the 1960s [11].
References
12 referencesShow all references, 4 with a study cardClose the reference list
Numbers in the text, such as [1], refer to these sources. Sources with a “Study card” button are summarised in the site’s evidence library.
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This page is based on reputable scientific sources and was last updated on 28 October 2026. The content of this website is for educational and informational purposes only and does not replace consultation, diagnosis or treatment by a specialist physician. Consult your treating physician about any treatment decision.
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