More than 130 breast cancer patients were given lower doses of radiotherapy after their operations than they needed because of a hospital's miscalculations, an NHS Trust admitted yesterday.

Some of the women face a "slightly" increased risk of the disease returning because of the mistakes made at Derriford Hospital, Plymouth. The hospital offered its "sincere apologies" to the 132 patients who had "slightly smaller" doses of radiotherapy than intended between April 1999 and last July.

Some patients given cancer treatment in that period have died. But Plymouth Hospitals NHS Trust refused to disclose how many and insisted there were no more than would be expected in normal circumstances. "A number of patients have died from a variety of causes as would be expected over this period," a hospital spokesman said. The surviving women have been invited to see a consultant oncologist to discuss whether they are at increased risk of cancer returning.

In 107 cases, the dose given was at least 95 per cent of the correct quantity. But 20 women received between 5 and 20 per cent less than the right amount. All patients treated since the beginning of August had been given the correct dose of radiotherapy, as had 581 other patients treated between April 1999 and July last year.

Terence Lewis, medical director of the trust, said there was always a risk of breast cancer recurring. But if a patient received slightly less than the intended dose of radiotherapy then the risk of cancer returning might be slightly higher.

"On the basis of expert advice we have calculated the increase in risk for each patient. For [most] the increase is so slight that the under-dose will have no appreciable effect whatsoever. For a small group the increase in risk of local recurrence is slightly greater but still very small in relation to the underlying risk that the disease will recur in any case."

The error was detected by medical physicists who calculate radiotherapy dosages. The Royal College of Radiologists and the Institute of Physics and Engineering in Medicine had run a six-month investigation.

Paul Roberts, the trust's chief executive, said: "We have done our best to ensure patients receive personal notification of this review before they hear of it in the media. I regret this may not be the case for some of those concerned."

The Department of Health has been kept fully informed.