The comedian cancelled shows last week after saying he was struggling with neck and throat pain.
Comedian Rhod Gilbert is receiving treatment at a cancer hospital, he has said.
The 53-year-old from Carmarthen said he is a patient at Velindre Cancer Centre in Cardiff – a hospital he is a patron of and has fundraised for.
“I know better than anyone that I am in the best possible hands,” he said in a Facebook post, announcing the news.
After previously cancelling shows due to issues with his throat, he paused his Book of John tour last week.
“As a proud patron of Velindre Cancer Centre, I’ve trekked the world, hosted chaotic quizzes and star-studded comedy nights,” he said.
“I’ve met and made friends with so many inspirational staff, patients and their families. It has been a huge part of my life for the last ten years.
“So while I never imagined that I would be a patient here, I know better than anyone that I am in the best possible hands. The NHS care I’m receiving is incredible.”
Gilbert has hosted BBC Two’s Never Mind the Buzzcocks and his own BBC Radio Wales show, while he tried his hand at different jobs in Rhod Gilbert’s Work Experience.
“I wouldn’t wish this on anyone… but who knows, maybe I’ll come out the other end with a new stand-up show and a 40 minute rant about orange squash,” he added.
“Thank you everyone for your support over the last few weeks and months (and years). I’ll be disappearing for a while and won’t be commenting further, at least not for now, whilst I focus on my recovery.”
The comedian had announced last week that dates after 9 July in Swansea, Hereford, Yeovil, and Truro would be postponed.
At the time time, he said: “As many of you will know, I’ve been struggling with pain in my neck and throat over the past few months. I’ve been receiving treatment, and have continued to tour where possible.
“I had sincerely hoped to go ahead with all dates as scheduled, but I’m afraid that is not going to be possible. I am due to have surgery next week and will be hanging up my boots for a while whilst I recuperate.”
In postponing, he said he was working to reschedule shows for early 2023.
Gilbert had done about 200 Book of John shows, and added: “I am genuinely gutted that we didn’t get to do the last seven shows as planned, but, rest assured, we will do everything we can to finish what we started.”