Multi-beam set-up in the prone and supine position. A 6-beam set-up used in the multi-beam intensity-modulated radiotherapy (MB-IMRT) plans for right-sided breast tumors in supine (a) and prone position (b). Gantry angles expressed in the Elekta coordinate system. The most inclined medial beam has the gantry angle of a tangential beam set by virtual simulation . The gantry angles are 0°, |α|, |2α|, 180° - 0.5|α|, 180° + 0.5|α|, and 180° + 1.5|α| for supine MB-IMRT. The lateral gantry angles in prone MB-IMRT are |α|, |α|+/−24°, the medial gantry angles are |β|, |β|+/− 12°.