Cancer diagnosis: understanding what Cancer is, and how it is diagnosed

My role as a Medical Oncologist begins before treatment is chosen. It begins with putting the pieces together clearly: what type of Cancer is this, where did it start, has it spread, what is its biology, and which of those findings will actually change the treatment plan?

What is Cancer?

Cancer develops when cells acquire changes that let them grow and survive outside the body’s normal controls. Some Cancers stay confined to their site of origin for a time, while others invade nearby tissues or spread to distant organs. But a Cancer diagnosis is more than the name of an organ. Two people with Cancer in the same organ may have different subtypes, stages, biomarkers, treatment options and priorities. That is why I focus on understanding both the Cancer and the person living with it before recommending treatment.

Why an accurate Cancer diagnosis matters

Accurate diagnosis is the foundation of Cancer care. It helps confirm whether Cancer is present, identify the exact type and subtype, determine the stage and, when relevant, identify biomarkers that can guide treatment. Not every patient needs every test. The aim is not to order the maximum number of investigations. It is to choose the tests that answer a meaningful clinical question, so that a good diagnostic plan reduces uncertainty and helps us make each treatment decision for a reason.

When screening or symptoms lead to further testing

Cancer may first be suspected because of symptoms, an examination finding, an abnormality on a screening test such as a mammogram or colonoscopy, or an unexpected finding on imaging. Blood tests and tumour markers can sometimes add useful information, but an abnormal tumour marker by itself usually does not confirm Cancer. When a solid tumour is suspected, the next step is often to obtain tissue so that a pathologist can establish the diagnosis, although there are important exceptions. See Cancer Prevention, Screening & Vaccination for how screening differs from diagnosis.

Tissue diagnosis: biopsy and histopathology

Biopsy

A biopsy takes cells or tissue from an abnormal area for examination by a pathologist. Depending on the site, it may be done with a needle, through an endoscopic procedure, or during surgery. For many solid tumours a core biopsy is preferred when feasible, because it preserves tissue architecture and usually gives enough material for histology and additional biomarker testing. Fine-needle aspiration (FNAC) remains useful in selected situations, particularly when cytology is enough for the clinical question.

Histopathology

Histopathology is the microscopic examination of tissue. It tells us whether Cancer is present and, when it is, helps identify its type, grade and other features that may influence prognosis or treatment. Imaging shows where an abnormality is and how extensive it looks, but for most solid Cancers it does not replace a definitive tissue diagnosis. When biopsy is unsafe or not clinically necessary, the approach may differ and is individualised.

Immunohistochemistry: understanding the tumour more precisely

Immunohistochemistry (IHC) uses antibodies to identify proteins expressed by tumour cells. These markers can help clarify the tissue of origin, define a subtype and identify biomarkers that may guide therapy. In breast Cancer, for example, oestrogen receptor (ER), progesterone receptor (PR) and HER2 assessment are central to treatment planning. Interpretation is more nuanced than simply positive or negative, because newer therapies may use different levels of biomarker expression, so results are read in the context of the Cancer type, current evidence and the treatment setting.

Molecular and biomarker testing: when it adds value

Modern oncology increasingly uses biomarker testing to understand features of a tumour that may influence treatment. These tests may assess specific genes, gene rearrangements, proteins, immune markers or other molecular characteristics.

Next-generation sequencing and comprehensive genomic profiling

Next-generation sequencing (NGS) can examine many Cancer-related genes at the same time. Comprehensive genomic profiling (CGP) is a broader approach that may use NGS and other methods to evaluate clinically relevant genomic alterations. These tests are particularly useful when a Cancer has several established molecular targets, when the disease is advanced, or when the result could open an evidence-based targeted treatment or clinical-trial option. They are not automatically required for every Cancer or every stage.

The most important question is not “can we test for more genes?” but “will this result change what we do?” I use molecular testing when it is clinically relevant and read the result alongside pathology, imaging, stage, prior treatment, overall health and your priorities. See Targeted Therapy & Molecular Testing.

Imaging: defining the extent of Cancer

Imaging helps identify the location and extent of disease, plan a biopsy or treatment, assess response and look for recurrence when appropriate. The best imaging test depends on the Cancer type and the clinical question.

PET/CT should be ordered for a clear clinical indication and read in context.

Tumour biology: markers such as Ki-67

Ki-67 is a marker of cell proliferation. In some Cancers it adds useful information about how actively tumour cells are dividing, but its value varies by tumour type and it should not be read in isolation. A higher value does not, by itself, define the whole behaviour of a Cancer or decide treatment. Similarly, genomic alterations such as TP53, PTEN or RB pathway changes can say something about tumour biology, but not every alteration is directly targetable or changes routine treatment.

Hereditary Cancer testing: is the Cancer inherited?

Tumour biomarker testing and hereditary (germline) genetic testing answer different questions. Tumour testing looks for changes within the Cancer that may guide treatment. Germline testing looks for inherited genetic changes that may affect a person’s Cancer risk and, in some situations, treatment or screening recommendations for family members.

Hereditary testing may be considered based on the Cancer type, age at diagnosis, pathology, personal and family history, ancestry or treatment implications. Some Cancers now have indications for germline testing even without a striking family history. Genetic counselling helps patients understand what a result may mean for them and for their relatives.

Bringing the diagnosis together: the whole picture

Cancer diagnosis is rarely a single test. Pathology tells us what the Cancer is. Imaging shows where it is and how far it has spread. IHC and biomarker testing may clarify tumour biology and treatment options. Hereditary testing, when indicated, adds information about inherited risk. I bring these results together with the person in front of me: overall health, symptoms, goals, responsibilities, concerns and preferences. This is where personalised oncology becomes more than a molecular report. See Cancer Treatment Planning for what comes next.

Moving forward with diagnostic clarity

Before starting treatment, it matters to understand the diagnosis well enough to know why a particular treatment is being recommended. That may include reviewing the pathology report, clarifying the stage, deciding whether additional biomarker testing is useful, and discussing what each result actually changes. If you have recently been diagnosed with Cancer or are still having tests, ask questions, and keep copies of your pathology, imaging and relevant blood-test reports.

Reviewing the diagnosis, understanding the test results and working out whether any further evaluation is genuinely needed are what come before treatment planning.

This page is general information, not medical advice for your situation. Which tests are needed depends on the suspected Cancer, its location and your overall condition. Medically reviewed by Dr Sneha Kommineni.