Dr AlHilali Consultant Clinical Oncologist
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Radiotherapy Understanding treatment

The abscopal effect: can radiotherapy affect cancer outside the treated area?

What the abscopal effect means, why it is not a dependable whole-body treatment and how radiotherapy–immunotherapy combinations are being studied.

Written and clinically reviewed by Dr AlHilali Consultant Clinical Oncologist

Published Clinical review

External-beam radiotherapy is directed at a planned area. Occasionally, a cancer deposit outside that area may also shrink. This is called an abscopal effect.

It is an important research observation, not a dependable way of treating all the cancer in the body by irradiating one site.

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Local treatment, possible distant response

Ordinary external-beam radiotherapy delivers its therapeutic dose to a selected region. A distant response does not mean that the beam has travelled around the body to another tumour.

The proposed explanation involves the immune system. Changes in an irradiated tumour may help immune cells recognise cancer, with effects beyond the directly treated site. The biology is complex and does not reliably produce a visible distant response in an individual patient.

Colourised scanning electron micrograph of a healthy-donor human T lymphocyte, shown in blue and yellow.
A T lymphocyte from a healthy donor, photographed by scanning electron microscopy and colourised by the source. This is immune-cell context, not a cancer cell, a treatment result or a demonstration of an abscopal effect. Full frame retained; resized and compressed. Micrograph: NIAID / NIH. Source · Public domain

Why are researchers interested in immunotherapy?

Some immunotherapies help the immune system act against cancer. Researchers are studying whether particular combinations with radiotherapy can improve outcomes.

That does not mean any radiotherapy schedule and any immunotherapy can be combined successfully. Timing, cancer type, the sites treated and the risks to healthy tissues all matter. The fact that two treatments have plausible complementary mechanisms is not proof that a particular combination helps patients.

The guide to chemotherapy, immunotherapy and targeted treatment explains how these approaches differ.

A distant change on a scan needs careful interpretation

Someone may receive systemic treatment as well as radiotherapy. If an untreated deposit then shrinks, it can be difficult to determine how much each treatment contributed. One report or one scan does not establish a general treatment strategy.

This is why researchers need well-designed studies and outcomes that matter to patients, not only an interesting biological explanation. Clinical trials can assess both benefit and harm.

What should I expect from my own treatment?

Ask which sites are being treated, what radiotherapy is intended to achieve there, and what treatment is planned for cancer elsewhere. Do not assume that an abscopal effect will occur or that systemic treatment is unnecessary.

General information only. Your oncology team can explain the purpose of your radiotherapy and whether research on combined treatment is relevant to you.

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