Penumbra - Limit the shadow effect to a specific area


I’m trying to use penumbra to draw shadows in a rectangular sub-section of my game/screen. However, after some experimentation it seems that penumbra always uses the full screen size, as i have to call it before any other draw calls. Is there really no way to limit the affected area to a smaller part of my game window or am I just not thinking about this issue in the right way?