An event was held on the site last evening.
The contractor is currently preparing the site to enable the start of construction later this month.
The new £50+million Radiotherapy Unit will offer an increased capacity in Northern Ireland for radiotherapy services and will cater for over half a million people living on both sides of the border.
Experts say it will make a real difference to people being treated for cancer and their families.
Speaking from the site last evening Chairman of the Western Trust Gerard Guckian said the initial site preparations and building works have now commenced and when finished the building will become one of the most important and modern health care facilities for people living with cancer in the North West.
Una Cardin, Radiotherapy Unit Manager said they are working behind the scenes to ensure this unique cross-border project is delivered on time and to the highest standard.
The unit is expected to be completed by 2016.