In total, medical records of nearly 1,600 patients treated since 2023 are now being examined following concerns about the service the trust offered.
An independent review into the running of CDDFT, carried out by governance specialist Mary Aubrey, has also been published this week. Among the shortcomings it highlights are:
The Aubrey review says "persistent concerns" had been raised since 2012, when the trust lost its status as a training centre for breast surgery services. In the following years, it "failed to act on repeated warnings and evidence... which allowed unsafe and outdated practices to persist".
CDDFT has apologised to "women and their families who have been harmed because of our substandard care", and it admits that "over many years, warning signs were repeatedly missed or not acted upon".
Kate Driver, who is 31 and from Chester-le-Street, is one of several women we spoke to who had been treated by CDDFT's breast cancer service.
In the run-up to Christmas 2023, Kate found a lump in her right breast. She was referred by her GP to the trust for an appointment with a specialist.
Although this was an NHS appointment, it took place at a private clinic held at Spire Hospital in Washington, where she was seen by Mr Amir Bhatti, the trust's clinical lead for breast surgery.
He carried out a test known as a fine needle aspiration (FNA), during which a sample of cells is extracted to test for cancer.
What she did not know was that FNA tests are not considered best practice for breast biopsies, because they can produce inadequate samples, and lead to the risk of missed, incomplete or delayed diagnoses.
Kate's test results were supposed to be provided within 48 hours, according to the trust's website. In fact, the test was inconclusive, and Kate had to wait several weeks before she was finally told she had breast cancer.
"Waiting so long and having to chase everything up made it a hundred times worse than it needed to be," says Kate. "I couldn't eat, I couldn't sleep. It just made it felt like no-one really cared, no-one was taking it seriously."
More bad news was to follow. Kate was told she would need a mastectomy, but advised that because of her particular type of cancer, she should not have breast reconstruction at the same time.
"They were going to completely remove my right breast and leave me with nothing, just a big scar," she says.
Kate sought a second opinion at another trust in Newcastle, where she was told there was no reason why she should not receive a breast reconstruction immediately after her mastectomy. She chose to be treated there instead.
The County Durham and Darlington NHS trust has apologised to Kate saying her care "fell below acceptable standards".
She made a formal complaint about her treatment by Mr Bhatti, which she says left her traumatised.
Spire Healthcare says that treatment and follow-up care of the clinic's patients was the responsibility of CDDFT.
Mr Bhatti told us he was unable to comment on specific cases because of patient confidentiality and urged patients to contact the trust directly.
Research from 2024 shows that almost half of women diagnosed with breast cancer at the County Durham trust had a mastectomy. According to 2022 parliamentary evidence given by the charity Breast Cancer Now, the UK average was about 27%.
In addition, only 7.5% of mastectomies carried out by CDDFT were followed by immediate breast reconstruction - far below the national recommended minimum of 25%.
Dawn Gillott is another breast-cancer patient whose treatment at the trust is being investigated.
She told us she had been given no option except mastectomy: "I felt like I didn't have a choice. It was 'you're having this and that's all there is to it'."
She says she has been left with a massive scar which has left her feeling "butchered".
"I can't look in the mirror at myself after having a bath because the scar is just a constant reminder that it's horrific."
The operation has also left Dawn in constant pain across her chest and under her armpit.
"Was my mastectomy necessary?" she reflects. "Maybe it wasn't. Maybe I could have had a lumpectomy."
This arrangement was considered "overly generous" by some staff, who were interviewed for a Royal College of Surgeons (RCS) report into treatment and care at the trust, published earlier this year.
The clinics generally worked on an out-of-hours basis, seeing patients in the evenings and at weekends.
The RCS raised concerns about the high turnover of appointments, commenting that it was not unusual for one clinic "to run on until 23:30".
"Obviously there was an incentive to see as many people as possible because of the per capita payment," says Prof Ian Fentiman, emeritus professor of surgical oncology at King's College London.
Additional reporting by Ben Milne