We compared simulator images of medial tangential fields taken in two positions: (1) with the ipsilateral arm abducted, holding a 'L-bar' armrest and (2) with both arms extended above the head in a forearm support. The average maximum heart distance as well as the central lung distance decreased significantly by 3.4 (SE 0.9) and 4.7 (SE 1.1) mm, respectively, when the new forearm support was used. The estimated normal tissue complication probability for excess cardiac mortality decreased by on average 3.1% (SE 1.3%). For some patients, a greater amount of the axilla was included in the field. We recommend the use of the forearm support during breast cancer treatment with tangential fields to decrease the amount of heart and lung inside the fields.