A woman who was concerned about the swirly patches all over her body found out the unusual rash was a rare sign of anal cancer.
The 74-year-old, who is unidentified, had lived with the rash for almost a year before going to see a doctor.
She was given anti-histamines at first - but they didn't make any difference. Her rash then became sorer and she started to suffer stomach pain, vomiting and diarrhoea.
This led to doctors in Kansas investigating other causes of the rash, only to discover that it was a symptom of her anal cancer.
The woman underwent treatment and is now in remission after her health scare, the doctors who treated her have now revealed.
Doctors at the University of Kansas Medican Centre, in Kansas City, told the story in the New England Journal of Medicine.
This type of rash, although is a sign of cancer, is rare, a dermatologist told MailOnline.
Dr Adil Sheraz, a consultant dermatologist in London and British Skin Foundation spokesperson, said: 'Erythema gyratum repens (the rash) is a rare condition and in up 80 per cent of cases is associated with an internal malignancy such as lung cancer or oesophageal cancer.
'Internal malignancies and cancers can present as rashes on the skin. The rash itself is a sign of an internal issue and in some cases can be the first indication of cancers.
'This is not a common sight in dermatology clinics, however when it presents it is often quite striking and diagnostic.'
He added that a rash like this is most often associated with blood cancers.
Anal cancer is rare, with around 1,500 people diagnosed in the UK every year, and 8,300 in the US.
The woman had had the red swirly marks on her thighs, buttocks, stomach and armpits for 11 months.
Dr Megan Prouty, one of the dermatologists involved in the treatment of the woman, said the patient had been initially treated with antihistamines.
But over the next four months, the rash developed into scaly plaques, which could have been a sign of many skin conditions from psoriasis to urticaria.
What concerned doctors the most was that the woman had also been showing symptoms of something more sinister.
Medics performed a colonoscopy on the patient, where a camera was inserted into the intestines.
Many anal cancers are found early because they are in a location that doctors can easily view with anal examinations.
It was then that things became clearer - and the woman was diagnosed with stage two anal cancer.
Dr Prouty said: 'The patient was referred to the dermatology clinic for assessment of the persistent, evolving rash.
'She received a clinical diagnosis of erythema gyrateum repens, a rare rash usually associated with breast cancer, lung or oesophageal cancer.'
The woman was given radiation and chemotherapy, and within three months, the rash had gone and never returned - but she was left with scarring.
It is not clear how long she needed treatment for, but the report said she was in remission after eight months.
Human papillomavirus (HPV), which up to four in five people will be affected with at some point in their life, causes more than 90 per cent of anal cancers.
HPV, the name for a group of viruses that affect your skin and the moist membranes lining your body, may never show symptoms and it can take years for cancer to develop. It is unclear if the woman had HPV.
Source: The Daily Mail