For patients undergoing lung cancer radiotherapy, functional lung imaging can be incorporated into treatment planning to achieve differential sparing between the viable and defective non-target lung regions. Hyperpolarized 129Xe gas MRI is a powerful tool for evaluating the pulmonary function due to the extremely high enhancement in spin polarization using spin-exchange optical pumping. As the first implementation of hyperpolarized 129Xe gas MRI pulmonary ventilation imaging-guided functional treatment planning in China, this study aimed to quantify its dosimetric impact on sparing the high-functional lung regions. Ten locally advanced lung cancer patients with a median PTV volume of 233.2 cm3 (range: 119.2-667.9 cm3) were included in this study. Before radiotherapy, each patient underwent an MRI scan with 1 liter (L) xenon-inhalation and a simulation CT scan with 1L air-ventilation. The CT and MRI was then registered in a commercially available deformable registration algorithm where the GTV, CTV, and regular OARs were contoured on the CT and the high-functional lungs contoured on the MRI. Two IMRT plans (64Gy/35fractions) were designed for each patient: an anatomic plan (Plan-A) using the simulation CT alone and a functional plan (Plan-F) using the hyperpolarized 129Xe gas MRI in addition. The two plans were created using a treatment planning system with the same dose optimization objectives except that the lung dose objectives in Plan-A were placed on the high-functional lung instead in Plan-F. These lung dose goals focused on V20Gy, V5Gy and the mean dose. Finally, the dose-volume histograms were compared between the two plans for the PTV and OARs using the Wilcoxon signed-rank test. All plans were judged as clinically acceptable. Plan-A and Plan-F showed similar PTV dose homogeneity index and conformity index. Similarly, the doses to all OARs except the high-functional lungs showed no statistical difference. Compared with Plan-A, the V5Gy, V10Gy, V15Gy, V20Gy, V25Gy, V30Gy, and Dmean of high-functional lungs were significantly reduced in Plan-F (P<0.05 for all indices), from 34.0±13.5%, 24.2±10.8%, 16.7±7.6%, 12.8±6.2%, 9.5±4.8%, 6.4±3.8%, and 7.4±3.0Gy in Plan-A, to 31.8±12.3%, 21.7±8.6%, 14.9±6.2%, 11.1±4.8%, 8.2±4.0%, 5.7±3.4%, and 6.9±2.6Gy in Plan-F, respectively. The sparing is more significant when the high-functional lung is adjacent to the PTV, with the above indices reduced from 46.8%, 39.2%, 32.2%, 26.5%, 19.7%, 15.2%, and 13.6Gy in Plan-A, to 41.8%, 33.4%, 24.8%, 19.4%, 14.2%, 11.8%, and 10.2Gy in Plan-F, for one patient. An emerging technology, hyperpolarized 129Xe gas MRI has been translated into the clinic and used in functional image-guided radiotherapy in China for the first time. Our findings indicate the potential of functional planning in lung functional avoidance for IMRT, particularly for patients who have high-functional lung adjacent to the PTV.