A trial of more than 500 patients found that it did not prolong or improve their quality of life any more than other forms of treatment.
More than 45,000 people are diagnosed with lung cancer every year in the UK.
In a third of cases, the cancer will spread to the brain.
Secondary brain tumours are usually treated with whole brain radiotherapy along with steroids and other treatments to reduce the side-effects of cancer therapies.
But it can have serious side-effects, such as nausea and extreme tiredness, and cause damage to the nervous system.
This study, involving doctors, researchers and patients from hospitals right across the UK, found that there was no improvement in the quality of life of patients after one week of whole brain radiotherapy.
These patients tend to already have a poor prognosis.
Dr Paula Mulvenna, consultant clinical oncologist with Newcastle Hospitals NHS Foundation Trust, said whole brain radiotherapy was used because it was thought to control tumours.
"But in our lung cancer clinics, we were not seeing the improvements we had hoped for in our patients.
"Survival times are poor and have hardly changed since the 1980s.
"What's more, the technique's toxicity can be substantial and it can damage cognitive function."
According to Prof Ruth Langley, from the Medical Research Council clinical trials unit at University College London, radiosurgery - a very precise form of radiotherapy - is a favoured alternative technique, which has minimal side-effects.
But some scientists say there may still be a place for whole brain radiotherapy.
Writing in a linked comment in the Lancet, Dr Cecile le Pechoux from Gustave Roussy Cancer Campus in France said: "We believe that optimised whole brain radiotherapy, given at the right time to appropriate patients, could lead to more individualised strategies."
They said all treatments should be discussed with patients, taking into account the result of this trial.

Regulator Ofgem said there were 3.8 million switches in the first six months of the year.
This was the second six months in a row that the switching total broke three million, and was nearly one million higher than the first half of 2015.
The data follow a major report into competition in the energy market.
According to the Competition and Markets Authority's (CMA's) two-year investigation, two-thirds of UK households were paying "over the odds" for their energy compared with those who have switched to a different tariff.
Ofgem said that customers could save more than £300 by ditching expensive standard variable tariffs for the cheapest deal, often a fixed-term tariff.
The average standard variable tariff of the larger energy suppliers as of 28 July was £1,066. The cheapest tariff available on that date - a one year fixed tariff - was £758, a saving of £308.
"Great deals are there for the taking. There are very competitive offers and many more suppliers to choose from, and that's why more consumers are switching," said Rachel Fletcher, of Ofgem.
To encourage more switching, Ofgem is planning to trial "more effective prompts" on customers' bills to encourage them to compare different tariffs. However, Ofgem said it would not be capping standard variable energy tariffs.
The regulator's response to the CMA report, and the investigation itself, was criticised by some as not being bold enough.
Separately, the Energy Networks Association has launched a new service in England, Scotland and Wales allowing people to contact their network in the case of a power cut by calling 105 or visiting a dedicated website.

Michelle O'Neill hosted a departmental breakfast at the Balmoral Show on Thursday morning.
In attendance was the Chinese consul general Madam Wang Shuying.
Chinese inspectors visited Northern Ireland pork plants in April as part of a process to approve local pork for export.
Ms O'Neill said she was also hoping to agree access to Chinese markets for beef and chicken.
US consul general Greg Burton was also at the event.
Northern Ireland is hoping to sell beef into the United States.
Ms O'Neill said she was also working with Irish agriculture minister Simon Coveney to resolve labelling issues that have hit Northern Ireland lamb sales to meat plants in the Republic of Ireland.
She told guests that the quality of Northern Ireland food was "something that carries our reputation around the world."
She said it was "safe, traceable and sustainably produced" and she would seek to use that reputation to open doors.
It is the second day of the Balmoral Show, the biggest showcase for Northern Ireland's farming and food industries.
Whole brain radiotherapy is of no benefit to people with lung cancer which has spread to the brain, says research in the Lancet.
[NEXT_CONCEPT]
A rising number of energy customers have switched gas and electricity deals amid widespread calls to benefit from savings that are available.
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The agriculture minister is to return to China next month as attempts to access markets there continue.