10025 Background: Pediatric malignant solid tumors are increasingly curable; whole lung irradiation (WLI) may be used to treat pulmonary tumor involvement. Few studies have addressed long-term pulmonary function after WLI during childhood. METHODS We conducted a retrospective review of pulmonary function tests (PFTs) in 1 year survivors of pediatric malignant solid tumors treated with WLI. We assessed PFT changes over time and the relationship of abnormal values to clinical parameters (diagnosis, radiation dose, and presence or absence of boost lung irradiation or surgical resection). RESULTS We evaluated 164 PFTs performed on 49 patients with the diagnoses of Wilms tumor (38), rhabdomyosarcoma (5), Ewing sarcoma (3), synovial sarcoma (2), and thymoma (1). Median age at radiation was 6.3 years (range, 0.5-20.2 years); median WLI dose was 12 Gy (range, 10.5-18 Gy). Seventeen (34.7%) had focal pulmonary boost irradiation and 21 (42.9%) had partial lung resection. Across the entire population, forced expiratory volume in 1 second (FEV1) and forced vital capacity (FVC) declined after irradiation with a median follow-up of 9.3 years. The odds of having a normal FEV1 and FVC decreased by a factor of 0.90 (p = 0.0387) and 0.91 (p = 0.0445), respectively, every year, indicating that the proportion of patients with normal values of FEV1 and FVC decreases over time. FEV1/FVC ratio and forced expiratory flow (FEF25%-75%) remained stable. Abnormal FEV1/FVC and FEF25%-75% were more likely in patients who received boost irradiation than in patients who did not (p = 0.0034 and 0.0233, respectively). CONCLUSIONS Pulmonary function worsened over time in this cohort of childhood cancer survivors treated with WLI. Boost irradiation further impaired pulmonary function. Further studies are planned to assess the clinical consequences of these progressive PFT abnormalities and to identify risk factors associated with clinically significant pulmonary dysfunction after WLI. No significant financial relationships to disclose.