Sam Lister (left), Health Correspondent for The Times, says the mistakes made in breast cancer screenings in Manchester look like isolated cases but raise questions across the NHS

What has happened?

A consultant radiologist working in two hospitals in Manchester made a series of mistaken breast cancer diagnoses during two years of screenings. Out of 2,500 screenings, 21 cases of invasive breast cancer were missed. Although four women were swiftly diagnosed, 17 women had their survival chances "significantly altered" by the delay.

What went wrong?

The two units where the mistakes occurred - Trafford General Hospital and North Manchester General - had only one radiologist checking screenings. In another hospital where the radiologist worked and where scans were double-checked, there were no reported errors in diagnoses. According to the hospitals involved there were no faults in the equipment used to carry out the screenings. An independent team will now examine exactly how the mistakes happened.

Was this a one off?

There are two important things to remember. Firstly, these mistakes occurred in cases where - unusually - patients were referred directly to hospital by their GPs after showing possible symptoms of breast cancer.

In all cases, NHS best practice guidelines recommend an appointment with a multi-disciplinary team at a breast cancer clinic, where numerous specialists will see the patient, rather than direct referrals from a GP to a single radiologist. So the circumstances are rare. These mistakes did not occur during the national screening programme for women between the ages of 50 and 70.

The other reassuring element to this case is that it does look like a one-off. And even though the radiologist was working alone and there was no one to check the scans, mistakes were spotted by junior colleagues and patients were recalled.

Are there wider implications?

The obvious question this raises is whether there should be any NHS breast screening units that only employ one radiologist. Both the hospitals where the radiologist worked were fairly small with relatively few patients, so two radiologists may have seemed like an indulgence. But cancer charities will be asking whether breast screening should be reorganised so that all units are larger, with more patients and more pairs of eyes to double check scans.

The errors will also highlight the national shortage of radiologists. The Royal College of Radiologists says the UK needs twice as many imaging specialists as are currently employed on NHS wards.

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