"As the DNA can only be damaged when the cells are undertaking the replication process – not in the G0 phase – the majority of normal cells that take longer to replicate are less affected than the cancerous cells."

It is simply false that DNA cannot be damaged in G0 cells.  The consequences of the damage will be less, but firing high-energy radiation at any molecule under any circumstances can cause damage, I believe.  That is why it is called ionizing radiation - it has enough energy to knock electrons clean out of molecules, and that damage can't always be repaired, even when the cell is not dividing and thus has more time for repair.