King Charles and Queen Camilla held a private dinner for staff from the hospital where both the monarch and his daughter-in-law the Princess of Wales were recently treated.
The intimate event for staff at The London Clinic took place at Clarence House in London on Tuesday evening, and followed a reception for the hospital's employees.
Earlier in the day, Charles met cancer patients in his return to royal duties following his diagnosis and treatment. He also had his own treatment before the dinner took place, saying during the engagement: "I've got to have my treatment this afternoon as well."
Nonetheless, the King appeared to be in great spirits as he visited University College Hospital Macmillan Cancer Centre longside his loyal wife, speaking to medical specialists and patients during the outing.
In a touching moment, Charles warmly held hands with cancer patients as he took the time to hear their stories. During one conversation, His Majesty confessed it was a "bit of a shock" when he was given his own cancer diagnosis.
Asked how he is doing by one patient undergoing chemotherapy, the King replied: "Not too bad. It's always a bit of a shock, isn't it, when they tell you?" The King was asked how he was feeling, saying: "I'm all right thank you very much, not too bad."
He is understood to be planning to have further treatment at a later date. Tuesday's outing marks the first in a number of external engagements His Majesty will undertake in the weeks ahead after confirming the positive effect of his cancer treatment.
In the same week back in February, it was announced that the King and Princess Kate had both been admitted to the London Clinic. In an eerie parallel, both royals had surgery for conditions that were initially not believed to be cancerous but later were discovered to have been malignant.
How did King Charles announce his cancer diagnosis?
The King revealed on 5 February that he had been diagnosed with a form of cancer.
A statement from Buckingham Palace said, in part: "During the King's recent hospital procedure for benign prostate enlargement, a separate issue of concern was noted. Subsequent diagnostic tests have identified a form of cancer.
"His Majesty has today commenced a schedule of regular treatments, during which time he has been advised by doctors to postpone public-facing duties. Throughout this period, His Majesty will continue to undertake State business and official paperwork as usual."
The statement continued: "The King is grateful to his medical team for their swift intervention, which was made possible thanks to his recent hospital procedure."
Princess Kate's moving cancer announcement
On 22 March, just before the Easter holiday, the Princess of Wales released an incredibly moving and powerful personal video message.
The Princess addressed her diagnosis, saying: "This of course came as a huge shock, and William and I have been doing everything we can to process and manage this privately for the sake of our young family.
As you can imagine, this has taken time. It has taken me time to recover from major surgery in order to start my treatment.
But, most importantly, it has taken us time to explain everything to George, Charlotte and Louis in a way that is appropriate for them, and to reassure them that I am going to be OK."