Stereotactic body radiotherapy in patients with multiple lung tumors: a focus on lung dosimetric constraints

Introduction: Lung dosimetric constraints with stereotactic body/ablative radiotherapy (SBRT/SABR) for multiple lung lesions are not well-characterized in published literature. Classically, the lung is considered a 'parallel' organ, for which injury to functional subunits could result in partially compromised function of that organ/tissue. Therefore, with SBRT/SABR for >1 thoracic target (especially involving both lungs), lung dosimetry requires special consideration.Areas covered: Current cooperative group and multi-institutional studies of SBRT/SABR for oligometastases rely on lung constraints from expert opinion, including constraints of exposure (i.e., volume of lung receiving more than a threshold dose or mean lung dose) and/or critical volume (i.e. volume of lung receiving less than a threshold dose; also termed complementary volume). For radiation pneumonitis, which reflects inflammatory lung injury, it remains unclear which type of constraint is more predictive of toxicity risks.Expert opinion: With SBRT/SABR for multiple lung lesions, it is prudent to use both exposure and critical volume constraints. Treatment on alternate days (for radiation plans with separate treatment fields) or staging treatment may also lower lung toxicity risks. Further study on lung normal tissue complication probability in the setting of multiple lung targets is urgently needed, particularly analyses of critical volume metrics, which are relatively poorly studied.