Material : effects on mask's edge


I have a 2d texture mask coming from a render target and I would like to know if this is possible to eather soften the mask’s edges or to draw an inner shadow from them, using only the mask inside the material ?

I thought maybe lowering the resolution of the image to blur the edges but would look a bit strange.

This is an example of the mask generated :

Any help or idea appreciated.

It works great thanks !!!

How about spiral blur?