Cancer doctors say difficult decisions are having to be made to postpone some patients' care during the coronavirus crisis.
The NHS says all essential treatment should continue during the outbreak.
And a cancer hub has been set up by the NHS to carry out urgent surgery.
But treatments such as chemotherapy can weaken the immune system, and potentially put patients at greater risk from Covid-19. Some of those affected have been expressing concern.
Postponement is 'unbearable'
Roisin Pelan is 38 and lives in Lancashire. She has incurable breast cancer and had been taking chemotherapy tablets every day. Every three months she also visits the hospital to receive the drug intravenously.
Last month she was told her chemotherapy treatment would be stopped for 12 weeks.
"It's terrifying they've stopped treatment that I know is keeping me alive," she says.
"To have that taken away is just unbearable. How do we know it's only going to be 12 weeks? This pandemic could go on a lot longer."
NHS England has told trusts that all essential and urgent cancer treatments must continue but specialists should discuss with patients whether it is riskier for them to undergo it or delay.
Dr Jeanette Dickson, the President of the Royal College of Radiologists, says that, for the majority of patients, treatment is continuing as usual, but admits it is hard for those whose care has been interrupted.
She says staffing levels are also stretched.
"It is especially difficult for patients who have been well on treatment up to now. And it's difficult for the staff. No one knows whether we can continue to deliver treatments over this week or next week," Dr Dickson said.
But doctors are finding new ways of working during the outbreak.
Twenty-five-year-old Dan Ruston's chemotherapy pills to treat a tumour in his stomach are being posted to his home in Cheshire.
"I take one in the morning and one at night, very simple. So I don't have to brave it in the outside world," he says.
The hubs match patients requiring urgent operations to surgeons across different "Covid-light" hospitals, meaning there are fewer patients with coronavirus being cared for there and less chance more vulnerable people could be exposed to the virus. More hubs are expected to open across the country in the coming weeks.
Thirty-two-year-old Louise Andrews is a patient under Westminster and Chelsea Hospital but had a lump removed from her breast by a surgical team at the Royal Marsden Hospital.
"I was relieved. We were literally just waiting by the phone everyday hoping that someone would call to say they could fit me in anywhere. Moving forward was so important to me."
'Straining every sinew'
Prof Peter Johnson, clinical director for cancer, admits the coronavirus epidemic is putting a huge strain on NHS resources.
"But we are straining every sinew to make sure diagnosis and treatments can continue," he says.
"In some circumstances it may be safer to delay treatment or treat patients in different ways to normal and clinicians and patients have to make those decisions together."
Prof Johnson also revealed there has been a sharp drop in the number of referrals for investigations for suspected cancer and has urged anyone who is worried about themselves to speak to their GP.
Lynda Thomas, chief executive officer at Macmillan Cancer Support, said: "We know this is a very anxious time. One in three calls to our support line last week were from patients concerned about the coronavirus.
"We will be working closely with the NHS to monitor and support this vital care being delivered."