BBC News
By Louise Cullen
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The General Medical Council (GMC) is monitoring the training of doctors at the Regional Cardiothoracic Surgery Department in Belfast.
It follows concerns about training being raised with the Northern Ireland Medical and Dental Training Agency.
The GMC has implemented enhanced monitoring in which it helps bodies to address concerns and find solutions.
The department is part of the Belfast Health Trust, which said all training was regularly reviewed.
The unit provides surgical treatment of the heart, lungs and other chest cavity organs for the whole of Northern Ireland.
It is based at the Royal Victoria Hospital in Belfast, with some surgeries taking place in the Belfast City Hospital.