A rare message from Kate Middleton has been made public as she continues her cancer treatment. 

In a letter to the British Army’s Irish Guards regiment, the Princess of Wales expressed regret for missing the Colonel’s Review, a rehearsal of the annual Trooping the Colour parade in honor of King Charles III‘s birthday. The preliminary event would have seen Kate, 42, take the salute as the unit’s honorary Colonel for the first time.

“I wanted to write and let you know how proud I am of the entire Regiment ahead of the Colonel’s Review and Trooping the Colour,” read the letter, posted on the Irish Guards’ X page. “I appreciate everyone trooping this year has been practicing for months, and dedicating many hours to ensuring their uniforms and drill are immaculate.”

The message continued, “Being your Colonel remains a great honour, and I am very sorry that I’m unable to take the salute at this year’s Colonel’s Review. Please pass my apologies to the whole Regiment, however I do hope that I am able to represent you all once again very soon. Please send my very best wishes and good luck to all involved.”