Purpose: Patient respiratory motion density function (pdf) has been successfully included in 4D inverse planning in managing lung cancer radiotherapy. The resulting dose distribution is comparable to the one planned using the real‐time tracking technique. However, the motion pdf varies during and between treatment deliveries resulting dose deviation from the one originally planned. In this study, we applied clinically observed pdf variation to evaluate the effect of the variation on the dose distribution obtained from the 4D inverse planning. Method and Materials: 4D inverse planning performed for each of 4 lung cancer patients was used. The reference pdf was obtained using 4D CT with target excursion of 1.5∼3.0cm. Treatment pdf was constructed by deviating the second and higher orders of moments of the reference pdf, meanwhile the baseline (the first order of moment) was assumed to remain 0. This assumption is supported by online CBCT guided correction. The ranges of the higher order moments' deviation were determined from clinical measurements of 8 lung cancer patients. The treatment dose in organs of interest was then simulated using the treatment pdf. The minimal dose (D99%) for target (GTV), and the DVH and EUD in lung, heart and cord were used for the evaluation. Results: Dosimetric effect of respiratory pattern (pdf) variation is dominated by its 2nd order moment (or the standard deviation) variation. The minimal dose in GTV decreases −0.9% ± 1.1%, −2.8% ± 0.6% and −5.5% ± 2.1% respectively with the reference standard deviation increase of 0.5, 1 and 2 mm. The pdf variation has minimal effect on lung, heart and cord EUDs. Conclusion: Variation in respiratory motion pattern causes dose reduction in target. However, the deviation is relatively insensitive to the variation, ass long as the baseline position of the target remains in the treatment course.