Effect --> Generate --> Circle, mode: Stencil Alpha
If I may add, Myl. is suggesting there are other ways of generating a mask besides using the mask property of the layer.
Apply your radial blur to an adjustment layer with a circular mask drawn at the exact center of your layer. Make the Adjustment layer 4X the size of your composition by selecting the layer and pressing Ctrl/Cmnd + Shift + Y , locking the aspect ratio and then multiplying either x or y value by 2. Now draw your mask at the center of the comp. You can do this by clicking at the center point then holding down the Ctrl/Cmnd + Shift and dragging with the elliptical mask tool selected. Set the mask to subtract. Now apply radial blur.
Here comes the trick. Radial Blur's position is based on comp size rather than layer size. To make the effect track with the movement of the adjustment layer you need to apply an expression to the Center value. It's really complicated so hold on to your hat.
That's it. It should work for you just fine.