@Randomizer We don't know it was a grenade, it might have been explosives tied to the beggar's back, which was facing Itoh's upper torso at the moment of the explosion.
I don't know much about explosives, but I seem to recall having read somewhere that they don't necessarily explode radially in all directions.
And his upper trunk wasn't annihilated. It was ripped off and thrown away by the explosion. We can see a remnant of his spine, crooked sideways like a fence stake after a hurricane. The rest of the body is probably a bloody mess somewhere in the room.