Antonyms are words that are opposite in meaning to another word. Therefore, it is not possible to come up with antonyms for "Juvenile Pilocytic Astrocytomas" which is a type of brain tumor that occurs in children and young adults. However, some possible antonyms to the word "juvenile" could be mature, adult or elderly. As for the word "pilocytic", some antonyms could be necrotic, degenerative, or unhealthy. Similarly, antonyms for "astrocytomas" could be non-cancerous, benign or healthy. It is important to note that the concept of antonyms is not applicable to all types of words, especially to medical terminologies.