If a Cohen is in the middle of Shmone Esre or towards the end, must he go up to Birkat Cohanim as he ends, or go out? Should he put off his davening the Amida and wait for Birkat Hacohanim or go out?

The Biur Halacha at the beginning of Siman 128 (Orach Chaim) brings several opinions in this matter. The general agreement of Poskim is that the Cohen does not interupt his Amidah (Shmoneh Esrei) under any circumstances. If there are other Cohanim in shul he may begin dovening in the shul. If there are no Cohanim in the shul, some prefer that the Cohen dovens outside (so he will not be inadvertantly called up) while others say this is not necessary.