The health watchdog says a string of serious failures by the Whanganui District Health board meant a patient's lung cancer was not picked up until it was too late.
The Health and Disability Commissioner has ordered the DHB and two doctors to apologise to the patient's family.
The commissioner Anthony Hill said a heavy smoker in her 60s was sent to hospital in 2008 to have all her decayed teeth removed.
Because of a heart murmur, a chest x-ray and ultrasound were ordered first, however none of those involved in her care saw the abnormal result and the operation was completed.
A year later, the woman complained of chest pain and cancer was discovered, but it was inoperable and she later died.
Mr Hill said her diagnosis was delayed and the quality of care was poor.
He said the woman's care was marked by very concerning communication and system failures.
Mr Hill said improvements had been made and more would follow shortly.
He said a significant number of changes have already been made.