You can’t definitively tell whether a tumor is cancerous just by looking at it or feeling it. A biopsy, where a small sample of tissue is examined under a microscope, is the only way to confirm a diagnosis. But there are physical characteristics, imaging features, and blood tests that help doctors determine which growths need that closer look and which are almost certainly harmless.
Physical Signs That Raise Suspicion
Most lumps that appear under the skin are benign. Soft, movable bumps that stay the same size over time are generally not concerning. Cancerous lumps behave differently. They typically start small and grow steadily over weeks or months, becoming larger and harder. A mass that feels firm or fixed in place, meaning it doesn’t slide around when you press on it, is more suspicious than one that moves freely.
Other warning signs include pain at the site, redness or warmth over the lump, bleeding or open sores that don’t heal, and rapid changes in appearance. A bump that persists for several weeks without shrinking deserves attention, especially if it appears on the breasts or testicles.
The ABCDE Rule for Skin Growths
For spots on the skin, the National Cancer Institute uses five features to distinguish early melanoma from ordinary moles:
- Asymmetry: One half of the spot doesn’t match the other.
- Border: The edges are ragged, notched, or blurred rather than smooth.
- Color: The color is uneven, with mixtures of black, brown, tan, white, red, or blue.
- Diameter: The spot is larger than about 6 millimeters (roughly the size of a pencil eraser), or it’s growing.
- Evolving: The mole has changed in size, shape, or color over recent weeks or months.
A normal mole is usually a single uniform color, has smooth borders, and stays the same over time. Dysplastic nevi, a category between normal moles and melanoma, tend to be larger with uneven color and harder-to-see borders. Having dysplastic nevi doesn’t mean you have cancer, but it does mean you should keep a closer eye on those spots.
What Imaging Can and Can’t Show
When a doctor orders an ultrasound, CT scan, or MRI, they’re looking for a set of clues. Malignant tumors tend to be larger (a median of 8 cm versus 5 cm for benign masses in one study of soft tissue tumors), sit deeper beneath the skin, and show more blood vessel activity since cancerous growths stimulate new blood vessel formation to fuel their growth. On MRI, malignant tumors are more likely to appear uneven in their internal signal, reflecting areas of dead tissue, bleeding, or mixed cell types.
That said, imaging has real limits. Most soft tissue tumors, whether benign or malignant, have well-defined edges and similar oval shapes. Smooth, clear margins on a scan are reassuring but not a guarantee. MRI can correctly distinguish benign from malignant masses over 90% of the time when several features line up, but no single imaging characteristic is reliable enough on its own. Imaging narrows the possibilities. It doesn’t replace a biopsy.
How a Biopsy Provides the Answer
A biopsy is the definitive test. A pathologist examines the tissue sample under a microscope, looking at features that are invisible from the outside. The key things they evaluate are the cell nuclei: whether they vary wildly in size and shape (a hallmark of cancer called anisokaryosis), whether the internal structure of the nucleus looks disrupted, whether cells are dividing rapidly, and whether cells contain multiple nuclei. Cancerous cells also tend to look less like the tissue they came from. Poorly differentiated tumors, ones whose cells no longer resemble normal tissue, shed large numbers of abnormal-looking cells.
There are several types of biopsy, and the one you get depends on the location and nature of the growth. Fine-needle aspiration uses a thin needle to draw out fluid and cells. It’s quick, and you can typically return to normal activities the next day. Core needle biopsy uses a slightly larger needle to collect a more substantial tissue sample, with most people advised to avoid heavy activity for at least two days afterward. Surgical biopsies, which remove part or all of the suspicious tissue, involve a longer recovery of a few days to a week depending on the incision size.
Biopsy results are highly accurate, with an incorrect diagnosis occurring in roughly 1 to 2% of cases at most institutions. Some specialized cancer centers report error rates as low as 0.4%. It’s worth noting that when cases are reviewed at specialized centers, the diagnosis changes in 11 to 18% of cases, often enough to alter the treatment plan entirely. If you receive an unexpected result, getting a second pathology opinion is reasonable.
What Blood Tests Can Tell You
Blood tests for tumor markers play a supporting role, but they’re not reliable enough to confirm or rule out cancer on their own. These tests measure proteins or other substances that some cancers release into the bloodstream. Common examples include PSA for prostate cancer, CA-125 for ovarian cancer, CEA for colorectal and other gastrointestinal cancers, and HCG for certain testicular and ovarian tumors.
The problem is that non-cancerous conditions can also raise these markers. CEA, for instance, is elevated in 60 to 90% of colorectal cancers but also rises in liver disease, hepatitis, and ulcerative colitis. CEA lacks both the sensitivity and specificity needed to catch early cancers reliably. Tumor markers are most useful for monitoring known cancers during and after treatment, tracking whether the disease is responding or coming back, rather than for making an initial diagnosis.
Blood-Based Cancer Detection Tests
A newer approach analyzes fragments of tumor DNA circulating in the bloodstream, an approach called liquid biopsy. Every tumor sheds small pieces of its DNA into the blood, and advanced sequencing technology can now pick up on mutations and chemical modifications unique to cancer cells. Unlike a traditional biopsy, this requires only a blood draw.
A test developed at Oxford University in early 2025, called TriOx, uses deep genome sequencing combined with machine learning to detect six cancer types (colorectal, esophageal, pancreatic, kidney, ovarian, and breast). In patients with symptoms and confirmed tumors, it showed 94.9% sensitivity and 88.8% specificity. The test can also track tumor activity from early or pre-malignant stages without needing a matched tissue biopsy. These blood-based tests are not yet standard screening tools, but they represent a significant step toward detecting cancer through a routine blood draw.
How Cancer Is Staged Once Confirmed
If a biopsy confirms cancer, the next step is staging, which describes how far the cancer has spread. The most widely used framework is the TNM system: T describes the size and extent of the primary tumor, N indicates whether cancer has reached nearby lymph nodes, and E (or M) indicates whether it has metastasized to distant parts of the body. The combination of these three factors determines an overall stage, typically I through IV, with stage IV reserved for cancer that has spread to distant sites. Staging guides treatment decisions, from whether surgery alone is sufficient to whether additional therapies are needed.