def add_rt_image(rt_image)
      raise ArgumentError, "Invalid argument 'rt_image'. Expected RTImage, got #{rt_image.class}." unless rt_image.is_a?(RTImage)
      @rt_images << rt_image unless @rt_images.include?(rt_image)
    end
def add_rt_image(rt_image)
      raise ArgumentError, "Invalid argument 'rt_image'. Expected RTImage, got #{rt_image.class}." unless rt_image.is_a?(RTImage)
      @rt_images << rt_image unless @rt_images.include?(rt_image)
    end
Adds a RTImage Series to this Plan.

 @param [RTImage] rt_image an RTImage instance to be associated with this plan