Screening women for ovarian cancer would not save lives, research suggests.
Certain cancers are routinely tested for via cervical screening, mammograms and bowel assessments.
More than 4,000 women die from ovarian cancer every year in the UK, prompting some people to wonder whether a screening programme could detect the tumours at an earlier stage, boosting a woman's chances of beating the disease.
To learn more, scientists from the UK Collaborative Trial of Ovarian Cancer Screening (UKCTOCS) analysed over 200,000 women aged 50 to 74. The risk of developing ovarian cancer is highest between 75 and 79 years old.
The women were allocated to three groups: no screening, a vaginal ultrasound once a year, or annual "multimodal screening" that involved a blood test and the scan.
Over the next 16 years, the multimodal approach picked up on ovarian cancer cases earlier in their onset, but this did not translate to lives being saved.
The scientists have warned that rolling out an ineffective programme could do more harm than good, adding that "we have to reluctantly accept screening for ovarian cancer is more than a decade away".
"UKCTOCS is the first trial to show screening can definitely detect ovarian cancer earlier; however, this very large, rigorous trial shows clearly [that] screening using either of the approaches we tested did not save lives," said co-lead author Professor Usha Menon, from University College London (UCL).
"We therefore cannot recommend ovarian cancer screening for the general population using these methods.
"We are disappointed, as this is not the outcome we and everyone involved in the trial had hoped and worked for over so many years.
"To save lives, we will require a better screening test that detects ovarian cancer earlier and in more women than the multimodal screening strategy we used."
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The women were enrolled onto the trial between 2001 and 2005, with the last screening taking place in 2011.
Of the more than 200,000 women, around half were assigned to the no-screening group. The remaining 100,000 participants were split between only having a scan, and having a scan and a blood test.
The blood test looked for changes in levels of the biomarker CA125. This naturally occurring protein is produced by some of the cells that line the ovaries and tends to rise in cancer patients.
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Results, published in The Lancet, reveal the blood test alone picked up on 39% more cancers at stage one or two than no screening.
Stage one tumours are defined as the disease only being in the ovaries. In stage two, the malignant cells have grown outside of the ovaries into the pelvis.
Perhaps surprisingly, the blood test detected 10% fewer stage three or four tumours than those that were picked up in the no-screening group, likely via symptoms.
In stage three ovarian cancer, the disease has spread outside of the pelvis into the abdomen or lymph nodes. Stage four is defined as the tumours reaching organs some distance away, like the liver or lungs.
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The UKCTOCS scientists found no difference in the grade of cancers detected between the ultrasound-only and no-screening groups.
In 2015, the scientists attempted to assess if screening wards off ovarian cancer deaths, but an insufficient number of fatalities had occurred to gauge the effectiveness of the blood test or scan.
After looking at five more years of data, the scientists have concluded screening does not save lives.
Of the women who developed ovarian cancer, 0.6% in the no-screening, blood test and multimodal groups died of the disease.
"Our trial showed screening was not effective in women who do not have any symptoms of ovarian cancer," said study author Professor Mahesh Parmar, from UCL.
"In women who do have symptoms, early diagnosis – combined with this better treatment – can still make a difference to quality of life and, potentially, improve outcomes.
"On top of this, getting a diagnosis quickly, whatever the stage of the cancer, is profoundly important to women and their families."
Although it is unclear why spotting low-stage tumours was not linked to a reduced risk of death, the scientists have speculated the cancer may still not have been picked up early enough.
It is possible ovarian cancer "is going to be aggressive whatever you do, even if you find it at an early stage", said Professor Parmar.
"If that's the case, that would be very depressing indeed," he added.
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While the study demonstrated screening helps spot the disease earlier in its onset, the scientists have stressed a screening programme must be known to save lives before it can be rolled out.
"Population screening for ovarian cancer can only be supported if a test is shown to reduce deaths in a future randomised controlled trial," said co-lead author Professor Ian Jacobs, from the University of New South Wales.
"I remain hopeful a new effective screening test will be found eventually, but it will take many years to conduct a large trial of the test.
"Realistically, this means we have to reluctantly accept population screening for ovarian cancer is more than a decade away."
Many of the participants have donated their samples and data for future research.
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The scientists have stressed that women should be aware of ovarian cancer's symptoms.
"Screening is for people without symptoms, so it's still important if you notice unusual or persistent changes to talk to your doctor," said Michelle Mitchell, chief executive of Cancer Research UK, which partially funded the study.
"Symptoms of ovarian cancer can be quite vague and similar to symptoms caused by less serious conditions, which can make spotting the disease tricky.
"Whether it's needing to go to the toilet more often, pain, bloating or something else, raise it with your GP.
"In most cases it won't be cancer, but it's best to get it checked out."
Other common symptoms include feeling full quickly after eating and a loss of appetite.
Persistent indigestion or nausea, pain during sex, back discomfort, unexplained weight loss and persistent tiredness may also occur.
The NHS recommends women see a GP if they have been bloated more than 12 times a month or have other symptoms that do not ease, particularly if they are over 50 and have a family history of cancer.
After being assessed by a GP, a suspected patient may be referred for a CA125 test, ultrasound scan or X-ray.