As the Princess of Wales stepped inside the Royal Marsden Hospital, it could at first glance have been any other royal visit she has made over the past 14 years.

“Good morning,” she said formally, with a firm handshake. “Thank you for having me.”

And that was the last bit of normality about it.

For this was not only the first solo engagement the Princess had made in more than a year, but a return to the hospital where, she revealed, she had her own chemotherapy.

It was the first time she had stepped through its front door, after months of quiet side-door visits for treatment under the strictest secrecy.

It is clear that the experience has changed her, and her royal life, forever.

As she wrapped a protective arm around a mother who burst into tears telling her about her teenage daughter’s treatment, rubbing her shoulder when she could do little else of practical help, this was a Princess who had walked that path and stands ready to help others along it too.

She swapped tales of treatment (she got “attached” to the port she had to deliver her medicine during her course of chemo, she said, and did not wear a cold cap to protect her hair); passing on advice about wearing warm clothes to fend off the side effects, and keeping on with the things that “bring you joy”.

At one point, she appeared to compare herself to a plant, laughing as she said she had needed “loads of water and loads of sunlight”.

The illness could be harder on loved ones than the patient themselves, she ventured, nodding gratefully when someone praised how she had managed her three children – Prince George, Princess Charlotte and Prince Louis – “amazingly well”.

It was clear, in short, that this visit meant something.

There were more smiling conversations than one might expect: some relief from both patient and royal visitor in finding a fellow traveller on the cancer journey.

When the Princess exclaimed “oh my goodness”, grimacing and touching a knee at a particularly difficult discussion, it was with the confidence of someone who knew exactly what was really going on.

This, remarkably, is the second time in a year that a senior member of the Royal family has returned to work after their own cancer diagnosis to visit a chemotherapy ward.

In April, the King – joined by the Queen – made a similar visit to thank the staff and researchers at the Macmillan Cancer Centre in London, expressing the same sentiment of gratitude for the care and compassion they give to patients in their darkest times.

The Princess’s visit was a surprise to most; a more personal, emotional engagement than she has ever done before.

She was naturally reticent, and wary of saying anything that would overshadow the people she was there to see. However, the patients of the Royal Marsden made her seem more willing to open up.

“It’s the uncertainty of that initial diagnosis,” she said, in a conversation with doctors, nurses and medical staff. “Understanding the diagnosis, it’s a massive amount of information to take on as a patient.”

Here, in her own words, was the explanation behind her gradual return to work.

“Sometimes from the outside we all think you’ve finished treatment and you go back to things,” she said. “But it’s hard to get back to normal.”

It was a “real challenge”, she said, nodding animatedly as a nurse discussed a longer-term side effect of losing track of conversations to add that yes, “the words totally disappear”.

In words echoing the sentiments of many cancer patients before her, she said it had made her “appreciate all the small things in life that you take for granted”.

The conversations stretched well beyond the usual royal small talk.

While the Princess’s rare personal revelations will make the headlines, the vast majority of them were questions: how were people feeling, what was their treatment plan, how were their families coping?

This was a Princess with a new purpose. A year of serious illness changes everyone, but fewer have a greater chance to use it for good.

During the visit, the Princess was announced as the new joint patron of the Royal Marsden, alongside her husband Prince William, who has visited many times before officially and could not have expected what a part the hospital would come to play in his personal life.

Diana, a Princess of Wales remembered for her hospital bedside manner, was its former president.

As the Princess paused on her way out of the hospital, 20 minutes later than scheduled, she heard the very live, very immediate stories of the people who were on its wards that day.

“I’m sorry,” she said to one, offering a hug. “I wish there was more I could do to help.”

There is more she can do to help, and she has concrete plans to do so.

Hours after the visit, the Princess issued a message – signed “C” for Catherine – confirming that she was in remission and spelling out her hope of doing more to boost “patient and family well-being” – an area in which she is now indisputably an expert.

“There is light at the end of that tunnel,” she told one mother, holding her firmly by the shoulder.

Her visit, not as a Princess but as a fellow patient, illustrated that.