Former US President Joe Biden is receiving radiation therapy as part of his treatment for prostate cancer, his spokesman has said.

The spokesman also said that Biden, 82, was undergoing hormone treatment, without giving any further details.

The radiation treatment was expected to span five weeks and marked a new point in his care, a source told NBC News.

In May, Biden's office announced that he was diagnosed with a more aggressive form of the disease, which had spread to his bones. The discovery was made after the Democrat politician reported urinary symptoms which led doctors to find a small nodule on his prostate.

The Biden office said at the time that "he was diagnosed with prostate cancer, characterised by a Gleason score of 9 (Grade Group 5) with metastasis to the bone.

"While this represents a more aggressive form of the disease, the cancer appears to be hormone-sensitive which allows for effective management."

A Gleason score of nine meant his illness was classified as "high-grade" and the cancer cells could spread quickly, according to Cancer Research UK.

Biden left office in January as the oldest serving US president in history and questions about his health dogged his first term, leading him to end his run for re-election late in his campaign.

His former vice-president, Kamala Harris, ran instead as the Democrat's presidential candidate, losing to current US President Donald Trump.

For many years, Biden has advocated for cancer research.

In 2022, he and his wife Jill Biden relaunched the "cancer moonshot" initiative with the goal of mobilising research efforts to prevent more than four million cancer deaths by the year 2047.

Biden himself lost his eldest son, Beau, to brain cancer in 2015.

In recent months, Biden has largely retreated from the public eye. 

In May, he sat down for an interview with the BBC - his first since leaving the White House - where he admitted that the decision to step down from the 2024 race was "difficult".

Prostate cancer is the second most common cancer affecting men, behind skin cancer, according to the American Cancer Society.

The US Centres for Disease Control and Prevention (CDC) says that 13 out of every 100 men will develop prostate cancer at some point in their lives. It says age is the most common risk factor.