Thousands of hospital appointments are set to be cancelled again next week following the cyber attack on the HSE.
The health service says there will be cancellations across many outpatient services, with many radiology services also currently stalled.
Chemotherapy and dialysis services are continuing as normal and hospitals are working to contact patients around the country.
Cybersecurity experts will continue to assess a decryption code this weekend to see if it will unlock the stolen HSE data.
The CEO of the HSE has warned that the process of getting the health service's IT systems back up and running will be "fraught with risk" in the weeks ahead.
Paul Reid added that it would not be a matter of simply switching back on the systems.
It could take several weeks to have a full recovery of the system, which the CEO of Cancer Care West said doesn't offer much cause for comfort.
Speaking to Newstalk Breakfast with Susan Keogh, Richie Flaherty said the cyber attack is a "cruel development" for many cancer patients.
"We deal with cancer patients on a daily basis and this is causing significant anxiety among cancer patients who already live with significant anxiety due to their diagnosis and treatment," he said.
"We have newly diagnosed patients who are waiting on test results and scans and also patients who are undergoing their treatments who are apprehensive of the delay.
"Coming on top of COVID and the pandemic, just when we were starting to be hopeful of a gradual return to normality with the rollout of the vaccination programme, this is a cruel development for cancer patients."
While the support service had time to prepare and adapt their services around the pandemic, the ransomware attack disrupted appointments and treatment for patients very suddenly, he added.
"That was a different kind of threat and we thought we wouldn't meet anything bigger than that and we came through it and we thought things would return to normal for our support centres in Galway and Letterkenny," Mr Flaherty said.
"We returned to face-to-face counselling, we also had remote counselling, thinking that we were getting back to normality and treatments had continued throughout all of this.
"The delay and the uncertainty around the delay and the effects on treatment, patients are really finding that overwhelming."
He said that all week, the cancer support service has been getting calls from patients asking for information about when treatments might resume or when the systems might be back up and running.
"That's just not possible [to say] because we know the hospitals don't know," Mr Flaherty added.
"We know the staff in the hospitals are working extremely hard to get treatments recommenced but it's important to note the impact the delays are having on patients and their families.
"Family members are really concerned that their sister, brother, mother or father's treatment has been stopped.
"Cancer diagnosis and treatment is really tough and this on top of it is really tough to take.
He is encouraging people to access help if they are feeling overwhelmed by the impact on their cancer treatment.
"I would encourage anyone that's going through or experiencing anxiety or stress from the disruption of their treatment, who may be feeling overwhelmed by the situation, to seek support and to reach out for help," Mr Flaherty said.
"We don't know how long this is going to go on for and we don't want people to go through it alone."
You can contact Cancer Care West here.