Hi Robert,
Thanks for your question.
Histiocytic sarcoma in cats is generally uncommon. There are a few different forms, and some are more worrisome/life-threatening than others. Determining this would require biopsy (collecting a small piece of the affected tissue) and sending it for review by a pathologist at the lab. This will give you the information necessary to know how to proceed, as well as an idea of prognosis.
I hope this is helpful for you, and best of luck!
Dr. Kim Hester