Comparison of the amount of irradiated lung tissue volume and mass in patients with breast cancer treated with an optimized tangential-field technique with and without a deep inspiration breath-hold (DIBH) technique and its impact on the normal-tissue complication probability (NTCP).Computed tomography datasets of 60 patients in normal breathing (NB) and subsequently in DIBH were compared. With a Real-Time Position Management Respiratory Gating System (RPM), anteroposterior movement of the chest wall was monitored and a lower and upper threshold were defined. Ipsilateral lung and a restricted tangential region of the lung were delineated and the mean and maximum doses calculated. Irradiated lung tissue mass was computed based on density values. NTCP for lung was calculated using a modified Lyman-Kutcher-Burman (LKB) model.Mean dose to the ipsilateral lung in DIBH versus NB was significantly reduced by 15%. Mean lung mass calculation in the restricted area receiving ≤ 20 Gy (M(20)) was reduced by 17% in DIBH but associated with an increase in volume. NTCP showed an improvement in DIBH of 20%. The correlation of individual breathing amplitude with NTCP proved to be independent.The delineation of a restricted area provides the lung mass calculation in patients treated with tangential fields. DIBH reduces ipsilateral lung dose by inflation so that less tissue remains in the irradiated region and its efficiency is supported by a decrease of NTCP.