Background: Stereotactic ablative body radiation (SABR) is a novel and sophisticated radiation modality that involves the irradiation of extracranial tumors through precise and very high doses in patients with oligometastatic lung disease and primary lung tumors. Case presentation: A 52-year-old female with subclinical idiopathic interstitial lung disease (ILD) and oligometastatic lung disease from squamous urethral cancer who was treated with SABR for a metastatic lesion located in the right lower pulmonary lobe. The patient received a hypo-fractionated course of SABR. A 3D-conformal multifield technique was used with six coplanar and one non-coplanar statics beams. A 48Gy total dose in three fractions over six days was prescribed to the 95% of the PTV. The presence of idiopathic ILD and other identifiable underlying lung conditions were not taken into account as a constraint to prescribe a different than standard total dose or fractionation schedule. Six months after the SABR treatment, a CT-scan showed the presence of a pneumomediastinum with air outside the bronchial tree and within the subcutaneous tissue without co-existing pneumothorax. To our knowledge, this is the first case of pneumomediastinum appearing 6months after SABR treatment for a lung metastasis located in the perihiliar/central tumors region as defined by the RTOG protocols as the proximal bronchial tree. Conclusion: Radiation oncologist should be aware of the potential risk of severe lung toxicity caused by SABR in patients with ILD, especially when chemotherapy-induced pulmonary toxicity is administered in a short time interval.