grammatical number - Is plural-possessive needed before "organization"

I'm not sure which one is correct:
"The Physical Therapist Organization" or "The Physical Therapists' Organization".

I'm leaning more towards without the plural-possessive but will I be grammatically wrong? Please don't respond with saying that I can use "Therapy" instead. I need to use the word "Therapist" or "Therapists'". Any help will be well appreciated.