Should siblings be tested or screened — after a child's cancer diagnosis?

Whether a sibling needs testing depends entirely on the type of cancer the affected child has and whether a hereditary cause has been identified or is suspected. For the majority of childhood cancers — which arise from sporadic (non-inherited) mutations — a sibling's cancer risk is not meaningfully above the general population risk, and routine testing is not recommended. However, if the child's oncologist or genetics team has identified a hereditary cancer syndrome (such as Li-Fraumeni syndrome, hereditary retinoblastoma, or certain Wilms tumour predisposition syndromes), siblings may be offered specific genetic testing or placed on a surveillance programme. Always ask the oncology team directly whether hereditary testing is relevant to your family's situation.

There is no single blood test that screens a healthy sibling for cancer in general. However, when a hereditary cancer syndrome is confirmed in the family through genetic testing, a targeted blood or saliva test can check whether a sibling carries the same inherited gene change. This test looks for a specific, already-identified change in the family — it cannot check for all possible cancer genes at once. If the sibling does carry the same change, the genetics team will recommend appropriate surveillance (for example, regular abdominal ultrasound for Wilms tumour predisposition). A genetics referral is the right starting point to understand what is and is not possible for your family.

The great majority of childhood leukaemias arise sporadically — from genetic changes in a blood cell during the child's development — and are not inherited or passed from sibling to sibling. In most families, a brother or sister does not face a significantly elevated risk. The exception is a small number of cases linked to inherited conditions that affect blood cell development. Your child's haematology-oncology team will be able to tell you whether any genetic testing in the family is indicated based on the specific subtype and clinical features of your child's leukaemia. Do not assume that testing is needed without that guidance — unnecessary testing can cause additional anxiety without benefit.

A hereditary cancer syndrome is a condition caused by an inherited change in a specific gene that significantly raises the lifetime risk of developing one or more types of cancer. In children, examples include Li-Fraumeni syndrome (which raises risk of several cancers, including soft-tissue sarcomas and brain tumours), hereditary retinoblastoma (affecting the eye), DICER1 syndrome (associated with several paediatric tumour types), and Beckwith-Wiedemann syndrome (which raises Wilms tumour risk). These syndromes are identified by the oncology and genetics team through clinical assessment and genetic testing. When one child in a family has a hereditary syndrome, siblings and parents may also be tested to see whether they carry the same inherited change.

Genetic counselling starts with a detailed family and medical history review by a trained cancer genetics specialist. The specialist will explain whether the affected child's cancer type has a known hereditary form, whether testing is recommended, and what the results would mean for siblings and other family members. If a specific gene change has already been identified in the affected child, targeted testing for that change can be offered to siblings. The genetics team will also explain what surveillance or preventive options exist for a sibling who carries the same change. At CION, we can arrange a genetics referral as part of coordinated paediatric oncology care — with time given to answer every question without jargon or rushed decisions.

The timing of testing for a sibling depends on the specific syndrome and the ages at which surveillance or intervention would be useful. For conditions where surveillance begins in early childhood — such as Beckwith-Wiedemann syndrome, where abdominal ultrasound every three months is recommended in the first few years of life — testing is done as early as possible after the hereditary diagnosis is confirmed. For syndromes where the relevant cancers tend to occur in adulthood, testing may be deferred until the sibling is old enough to participate meaningfully in the decision. The genetics team tailors the timing to the specific clinical context. There is no universal answer, which is why a specialist referral is the essential first step.