Doctor told me that symptoms of Pleural effusion won't go away in X RAY even after treatment is it possible?

X-Ray does not show. symptoms, only density/ability to absorb X-Rays so they do not reach the film. Basically standard X-Ray images are just shadows, "shadograms" if you will, using higher frequency (above visible light) electromagnetic radiation. The shadows, on X-Rays, of pleural effusions/lung densities, etc. may linger longer than the effusion itself due proteins which take longer for body to remove than water.
Yes. The xray signs of a pleural effusion may persist long after the symptoms are gone.