Researchers have made a "crucial discovery" which suggests statins may be able to help stop breast cancer coming back.
While most women with ER-positive breast cancer - which accounts for around 70% of all breast cancers - respond to hormonal therapy, around 12,000 women a year see their cancer return.
Now researchers at the Institute of Cancer Research in London have found that cancer cells in these patients can use cholesterol to enable them to keep growing and spreading the disease.
If a therapy - such as statins - could lower the ability of these cells to use cholesterol, it could prove a powerful weapon in preventing cancer coming back.
Experts also believe the discovery could lead to a new kind of personalised medicine for breast cancer - with women undergoing a tumour test to work out if their cancer is likely to return.
If so, women could be swapped to different drugs that destroy a component of their cancer cells entirely.
Many breast cancers rely on the female hormone oestrogen to grow. Traditional hormonal therapies either lower levels of oestrogen in the body, or block oestrogen from reaching the cancer cells. This means the cancer either grows more slowly or stops growing altogether.
But in some cases, these hormonal therapies do not work. The new study found that in some cases, the cancer cells had managed to produce a molecule made from cholesterol, called 25-hydroxycholesterol (25-HC). This molecule can mimic oestrogen and encourage cancer cells to continue growing.
Dr Lesley-Ann Martin, from the Breast Cancer Now Toby Robins Research Centre at the Institute of Cancer Research, said: "During the course of treatment, ER-positive breast cancers, that are 'fed' by oestrogen, often become resistant to standard hormone therapy.
"Our research has demonstrated that these cancer cells can use a cholesterol molecule to mimic oestrogen so that they continue to grow without it.
"This is hugely significant. Testing the patient's tumour for 25-HC or the enzymes that make it may allow us to predict which patients are likely to develop resistance hormone therapy, and tailor their treatment accordingly.
"Our study also demonstrates that statins could be a valuable addition to breast cancer treatment, and that this warrants investigation in clinical trials."
Dr Martin's team investigated breast cancer cells grown in the lab to see why breast cancer can relapse in women taking aromatase inhibitors (AIs), which are generally taken for five years after surgery.
They found some cancer cells were making their own fuel by producing 25-HC, allowing them to continue growing despite a lack of oestrogen.
When the experts blocked the pathway allowing cholesterol production, the proliferation of cancer cells slowed down by 30% to 50%.
These findings, published in the journal Breast Cancer Research, were backed up by two studies involving patients.
In 124 patients, researchers found that the overactivation of certain genes involved in the creation of cholesterol were significantly linked to women having a poorer response to hormonal treatments.
In a separate sample of more than 700 patients, the increased expression of a gene called SQLE was found to be significantly linked to a higher chance of cancer coming back up to 10 years later.
Baroness Delyth Morgan, chief executive of Breast Cancer Now, said: "This is a really crucial discovery. Far too many women have to deal with the potentially devastating consequences of their breast cancer coming back and this research presents an important opportunity to improve the effectiveness of today's most commonly used treatments.
"This study breaks new ground in uncovering how some breast cancers continue to survive without oestrogen and suggests that women could benefit from adding statins to standard anti-hormone treatments.
"But this is early research and greater clinical evidence is now needed to understand the potential risks and benefits of this approach."
Jane Murphy, clinical nurse specialist at Breast Cancer Care, said: "This early study raises an interesting question of whether cholesterol-reducing treatment, such as statins, could help lower the chances of breast cancer returning for some women who have developed a resistance to hormone therapy.
"We know that many women can be consumed with fear of their breast cancer coming back, which can have a huge impact on their ability to move forward after treatment.
"This discovery may, in future, help reduce these concerns for some patients, by allowing doctors to test if their cancer is likely to return, and tailor treatment accordingly.
"However, this research is only in its initial stages. We look forward to seeing further trials to determine whether this approach could work in practice."