How can I have a separate pass for the fog I created with Octane Volume object?

Dear Octane Users,

I want to use falloffmap with opacity channel but at the same time, I want the polygons facing backwards totally invisible.
I have multiplied the side with falloffmap, it didn't work.
I have used mix material node where "side" was assigned to amount, falloff map and rgbspectrum ...

I have the same question. Thank you.