Following complaints from various GPs and consultants, over 46,000 x-rays, CAT scans and ultrasounds scans are being reviewed at University Hospital Kerry.
The period of time in question ranges from March 2016 to July 2017, according to RTÉ, and involves more than 26,000 patients.
Three patient cases involving delays in cancer diagnosis at the hospital were identified during the summer, according to the broadcaster.
This lead to an initial investigation of a sample of scans.
The National Director of the HSE's acute hospital division then approved to review almost 50,000 scans in August.
So far, the hospital has reviewed around 18,000 scans.
21 patients have been called into the hospital, but no major missed cases were identified.
The South/SouthWest Group, which covers University Hospital Kerry, released a statement to RTÉ, saying that they are investigating patient safety concerns at the hospital.
It said that a "look back" process with independent oversight is also being carried out.
The full review is expected to be completed within the next ten weeks.
Those with any concerns can call the helpline number 1800-742-900 from 9am-5pm Monday to Friday.