The union late yesterday afternoon said it would not strike from 7am today as planned.
It would return to pay talks provided health boards were prepared to enter into "genuine negotiations" on its demands for increased pay and better conditions.
If no genuine negotiation occurred, strike notices would be reissued.
But in a strong statement last night, Southern District Health Board chief executive Brian Rousseau said the union was "out of touch with reality" and "holding patients to ransom".
"Nationally, there have been 730 notices of industrial action issued by the medical radiation therapists' union.
"How much more are all district health boards expected to take? Enough is enough."
Mr Rousseau was unrepentant when asked if his comments might inflame the situation or jeopardise negotiations.
He said he was "not worried", and health boards were "not going going to sit back and take it [strikes] any more".
This week's strike would have affected at least 520 radiology appointments, and some elective surgery, potentially increasing waiting lists stretched from months of rolling strike action, he said.
"Southern DHB is thoroughly fed up with ongoing strikes ... and the unacceptable impact they are having on patients, DHB staff and the whole community.
"This is not low-level industrial action, as Apex says.
It's certainly not low-level for people having their operations cancelled or having to wait for X-rays to be done."
The last-minute cancellation of the strike was "behaviour typical of this union", he said.
It had left departments "once again frantically having to reschedule appointments".
"[The union] needs to get real and accept that what the DHBs have offered is similar to what 90% of other unionised employees have accepted and is the best that we can afford in these difficult financial times.
"Apex is out of touch with reality and continues to hold patient services to ransom.
"We have now had enough."
No-one from the union could be contacted for comment last night.
