How childhood cancer is diagnosed — the process, step by step

The first step is almost always a visit to a paediatrician or general doctor who takes a thorough history and examines the child. The doctor is looking for signs that do not fit a common illness: persistent fever without an obvious infection, unusual pallor, lumps, unexplained weight loss, or neurological changes. If any of these are present, the doctor orders a full blood count (CBC) with differential and, depending on findings, further tests. Childhood cancer is rarely diagnosed at the very first appointment — it usually requires a sequence of investigations before a diagnosis is confirmed. You are not expected to know which test to ask for; your role is to describe what you have noticed and ask the doctor whether a blood count and specialist referral are appropriate.

A full blood count (CBC) with differential is the most important initial blood test. It measures red blood cells, white blood cells, and platelets. Leukaemia — the most common childhood cancer — often shows up as an abnormally high or very low white cell count, low red cells (causing anaemia), and low platelets (causing bruising or easy bleeding). Other blood tests that doctors commonly add include LDH (lactate dehydrogenase), uric acid, and a peripheral blood smear, which allows a pathologist to look at individual blood cells under a microscope. Elevated LDH and abnormal blast cells on a smear raise strong suspicion for leukaemia. Blood tests alone cannot confirm most other solid tumours — imaging and biopsy are needed.

The imaging test used depends on what part of the body is involved. Ultrasound is usually the first scan ordered for an abdominal lump because it uses no radiation and gives immediate information about whether a mass is solid or fluid-filled. Chest X-ray is often done early to check for masses in the chest or enlarged lymph nodes. MRI is the preferred scan for brain tumours and for soft-tissue tumours of the limbs or spine because it shows detail without radiation. CT scan is used to measure tumour size and check whether cancer has spread to other organs. PET-CT helps map whether cancer cells are present anywhere in the body and is used for staging certain lymphomas and solid tumours. Your doctor will tell you which scan is needed and why.

In most cases, yes — a biopsy is necessary to confirm that a mass is cancerous and to identify the exact type of cancer, which is essential for choosing the right treatment. A biopsy involves removing a small sample of tissue from the tumour, either with a needle (core needle biopsy) or through a small surgical procedure. The sample is then examined by a pathologist under a microscope and tested for specific genetic markers. In leukaemia, a bone marrow test (bone marrow aspiration and biopsy) is performed instead of a tumour biopsy, because the cancer cells are inside the bone marrow. The biopsy result, not the scan, is the definitive diagnosis.

From the time a parent first notices a worrying symptom to a confirmed cancer diagnosis, the timeline varies widely. In straightforward leukaemia cases where the blood count is clearly abnormal, a diagnosis can be made within days. For solid tumours — such as a brain tumour or kidney tumour — the process takes longer: getting imaging, performing a biopsy, and waiting for pathology results typically takes one to three weeks. Delays sometimes happen if symptoms mimic a common illness early on, or if initial tests are normal and it takes further visits for the picture to become clearer. If you feel the process is moving too slowly, asking for an urgent paediatric oncology referral is entirely appropriate.

A bone marrow test involves inserting a needle into the hip bone (iliac crest) to collect a small amount of bone marrow fluid and a tiny core of marrow tissue. It is the key test for diagnosing leukaemia. In children, this procedure is almost always done under sedation or general anaesthesia, which means the child is fully asleep and does not feel the procedure. There may be some soreness at the hip for one to two days afterwards, which is managed with simple pain relief. Parents are often more anxious about this procedure than children are — the use of anaesthesia specifically aims to make the experience as gentle as possible. The results are usually available within 24 to 72 hours.