The King made rare comments about his cancer diagnosis as he met patients and toured a new facility at York Hospital on Tuesday.
Charles, 77, emphasised how important it is to be "able to talk", adding that "cancer is everywhere", following his own diagnosis in 2024.
The monarch met staff and patients who have worked on the new Sir Robert Ogden Macmillan Cancer Centre, due to officially open in July, on a surprise visit.
He met Louise Rhodes, from Hopgrove, who has stage four secondary breast cancer, and her mother, Diana Ellison. Lou was involved in the design and development of the centre as a patient representative.
She told the King that her father had sadly passed from cancer in 2024, to which he responded: "It's everywhere," adding: "It's so important to be able to talk."
Speaking about the Macmillan Charity, Charles said: "They are amazing and I'm delighted to have been patron for a very long time."
Wearing a light grey, pin-striped suit, pink shirt and darker pink tie, the royal made his way through the hospital despite 30°C heat and was mobbed by well-wishers taking pictures of him and filming.
He shook hands with staff and patients, asking how they were faring in the heat.
© Alamy Stock PhotoCharles's important visit to cancer hospital
The King, royal patron of Macmillan Cancer Support, met people for whom the centre had been a lifeline during treatment and heard about their roles in shaping the new facility, including the services it will offer and its design, from the layout and the furniture to the colour schemes and the artwork.
© POOL/AFP via Getty ImagesHe greeted Macmillan health professionals and cancer nurse specialists, who explained how the £2.4 million redevelopment would support thousands of people living with cancer in the region each year.
Charles also visited the centre’s purpose-built garden and planted a Macmillan rose with Colm Gough, Macmillan personalised cancer care lead, who is part of the team there.
Earlier in the day, the King was praised for his "innate understanding that we need to be in harmony with nature", as he visited a project aimed at protecting red squirrels at the Animal and Plant Health Agency’s York Biotech Campus.
Is King Charles still undergoing cancer treatment?
King Charles revealed he was undergoing treatment for an undisclosed form of cancer in 2024, just weeks apart from the Princess of Wales announcing her own diagnosis. Prince William later said it was the "hardest year" of his life.
The King is currently still receiving treatment, but revealed in December that it was being reduced in a Stand Up To Cancer campaign video. He called the milestone a "personal blessing and testimony to remarkable advances that have been made in cancer care".
A spokesperson for Buckingham Palace added that he has responded "exceptionally well" to treatment, but the new position will be continuously monitored by doctors.
The King has continued to carry out public engagements across the UK and abroad while receiving treatment, including a recent state visit to the US.
