SPECT V/Q in Lung Cancer Radiotherapy Planning

Curative-intent lung cancer radiation therapy either alone (RT) or combined with immuno-chemotherapy is associated with potential risk of serious radiation-induced lung injury. This review provides a summary of the role of SPECT ventilation perfusion (V/Q) imaging as an emerging adjunct to lung cancer RT planning and treatment dosimetry. Denoted "functional lung avoidance RT" it is hypothesized that preferential dosimetric avoidance of physiologically functional lung may reduce the frequency of radiation-induced lung injury. SPECT V/Q imaging datasets available during the planning process allows the prioritization (or "personalization') of RT dose to minimize the volume of functional lung probabilistically exposed to injurious radiation dose. Selective escalation of target dose and adaptive planning and replanning is also enabled. The emergent importance of the tumor-lung microenvironment and its biologic relationship to local immune effectors in lung cancer provides further incentive to individualize RT planning and delivery. This review examines important normal tissue dosimetric constraints that are part of current standards-of-care and the new dosimetric parameters associated with functional lung avoidance RT. SPECT V/Q has been a valuable tool in investigating the feasibility and efficacy of functional lung avoidance RT but is yet to become main stream due to the lack of large clinical trials. It is encouraging however that functional lung avoidance is feasible in RT dose-target delineation and some of the more promising studies are discussed.