I have many Botox patients who have many different medical conditions, including cancer. That said, you should ask your oncologist and make sure there is no contraindication for your particular condition.
I have several patients who have cancer, or who have had cancer, who still receive cosmetic Botox. I always try to speak with that patient's oncologist first to coordinate care, understand medications, etc. but there are no studies showing there are contraindications for the two. "This answer has been solicited without seeing this patient and cannot be held as true medical advice, but only opinion. Seek in-person treatment with a trained medical professional for appropriate care."