Some patients are having to go to Christchurch for treatment, and others feel they have no choice but to pay for private treatment.
"Patients are telling us that they are having to wait more than double the Ministry of Health guidelines for treatment," Cancer Society Otago-Southland division chief executive Rachael Hart said.
People were waiting up to three months to see a specialist.
"We are concerned that palliative patients that might be marked as low priority would be unable to receive treatment that would curb their pain and extend their life, and we are also concerned that patients with potentially curable tumours, the delay could see their cancers spread and become incurable."
The Southern District Health Board said all patients waiting for their first appointment with radiation oncologists were being told about potential delays and the options available to them.
"We recognise the distress delay causes to our patients, and the challenges delays present for the treatments patients may receive," chief medical officer Nigel Millar said.
"So far as the effect on patients' health and life expectations, this is difficult to quantify. But in some cases delays will have an effect, which is very concerning to us."
The delays are because of the high number of patients and a shortage of staff.
Dr Millar said in the short term the SDHB had organised additional clinics in Dunedin, was looking for additional locum cover and was giving suitable patients the option of having their treatment in Christchurch.
Some patients spoken to by the Otago Daily Times have already paid for private treatment in Christchurch rather than wait.
"When I said I was going to Christchurch for treatment the staff said `great, now there are only 150 people still on the waiting list'," one woman said.
"I just can't get my head around that, that there are so many people waiting ... I worry that the SDHB thinks it was my choice to go to Christchurch, but I feel that I had no other choice."
Another patient said she was still waiting for her first specialist appointment, and had grave fears her disease could spread.
"You get to the point after you process the diagnosis, which wasn't on the agenda at all, that you accept it as much as you can and you just want to throw everything at it, to give you some sort of sense of control in a situation you don't really have any control over.
"When someone tells you you can't because they don't have the resources, that's really hard to sit with."
Dr Millar said the SDHB was working hard to reduce waiting times for radiation oncology patients.
"We are aiming to see and treat the most urgent cases as soon as possible and other patients are seen according to the urgency of their condition.
"Some patients are waiting eight-12 weeks for their first specialist appointment dependent on the type of cancer they have."
A delay could affect the benefit and outcome of treatment, so the SDHB was carefully prioritising all patients to try to minimise that risk, Dr Millar said.
"In the longer term, with the increasing demands on the service, we need a reliable and sustainable approach to meet the needs of our community.
"We are working on ways to achieve this, amd we are aware we have a large challenge to develop any strategies to serve us into the future with the continuing increase in demand."
