The purpose of this study was to devise and evaluate a method to quantify the dosimetric uncertainty produced by the interplay between the movement of multileaf collimator (MLC) and respiratory motion in lung stereotactic body radiation therapy (SBRT). The method calculates the dose distribution for all control points from a dynamic treatment in all respiratory phases. The methodology includes some characteristics of a patient's irregular breathing patterns. It selects, for each control point, the phases with maximum and minimum mean dose over the tumor and their corresponding adjacent phases, whenever necessary. According to this selection, the dose matrices from each control point are summed up to obtain two dose distributions in each phase, which are accumulated in the reference phase subsequently by Deformable Image Registration (DIR). D95 and Dmin,0.035cc were calculated over those accumulated dose distributions for Gross Tumor Volume (GTV), Planning Target Volume (PTV) - based on Internal Target Volume (ITV) approach - and Evaluation Target Volume (ETV), a novel concept that applies to 4D dose accumulation. With the ETV, DIR and interplay uncertainties are separated. The methodology also evaluated how variations in the breathing rate and field size affects the mean dose received by the GTV. The method was applied retrospectively in five patients treated with intensity modulated radiotherapy (IMRT) - minimum area defined by the leaves configuration at any control point was at least 4cm2-. Uncertainties in tumor coverage were small (in most patients, changes on D95 and Dmin,0.035cc were below 2% for GTV and ETV) but significant over- and under- dosages near ETV, which can be accentuated by highly irregular breathing. Uncertainties in mean dose for GTV tended to decrease exponentially with increasing field size and were reduced by an increase of breathing rate. The implementation of this method would be helpful to assess treatment quality in patients with irregular breathing. Furthermore, it could be used to study interplay uncertainties when small field sizes are used.