Kate Middleton didn't wear her engagement ring that once belonged to Princess Diana at the Royal Marsden Hospital in London on Jan. 14. Inside why and more about Princess Kate's new ring.

For her first official solo appearance in over a year, Princess Kate met with cancer patients and the team at London's Royal Marsden Hospital.

Kate Middleton visited the Royal Marsden Hospital in London on Tuesday, Jan. 14, where she received chemotherapy treatment after her cancer diagnosis last year. See all the photos here

Everyone just noticed that Kate Middleton isn't wearing her engagement ring...here's what we know about why she ditched the iconic sapphire.

Kate, the Princess of Wales, made a poignant return Tuesday to the same hospital where she received treatment after being diagnosed last year with cancer.

The Princess of Wales wanted to “show her gratitude to the incredible team, but also highlight the world leading care and treatment the Marsden provides.”

Kate Middleton returned to the hospital where she underwent cancer treatment earlier today, Jan. 14. The Princess of Wales visited The Royal Marsden Hospital in London, where she spoke with other patients undergoing treatment after she and her husband, Prince William, became Joint Patrons of The Royal Marsden NHS Foundation Trust.

The Princess of Wales made a surprise return to the Royal Marsden Hospital to personally thank staff who looked after her during her cancer treatment on Tuesday. At such a critical moment in US history, we need reporters on the ground. Your donation allows us to keep sending journalists to speak to both sides of the story.

Middleton’s famous 12-carat diamond and sapphire engagement ring (you know, the one that used to belong to Prince William‘s late mother, Princess Diana) was MIA. And yes, this