The utility of SPECT lung perfusion scans in minimizing and assessing the physiologic consequences of thoracic irradiation

Three-dimensional single photon emission computed tomography (SPECT) lung perfusion scans provide a unique quantitative 3-dimensional map of the distribution of functioning pulmonary vascular/alveolar subunits, information not provided by other imaging modalities. This report describes the initial experience utilizing these scans to assist in the design of radiation treatment beams and to assess changes in regional lung function following irradiation. Patients were immobilized and scanned in the treatment position with appropriate fuducial markers. Four millicuries of technetium 99M microaggregated albumin were injected and SPECT images of the lung were generated. Pre-treatment SPECT images were used to help design radiation beams to minimize irradiation of functioning lung. Pre- and post-treatment scans were compared to assess changes in regional function. These changes in function were then correlated with the regional radiation dose. Pre-radiotherapy SPECT scans were obtained in 18 patients (11 with lung cancer). Marked variations in regional function were frequently noted. In patients with primary lung tumors, these variations were not necessarily immediately adjacent to the tumor volume. In general, patients with poor pulmonary function pre-treatment, in whom one would like to spare as much normal lung as possible, had the most non-uniform distribution throughout the lung of functioning vascular/alveolar subunits. In these cases, pre-treatment scans were most useful in designing radiation portals to minimize irradiation of functioning lung. SPECT scans were also used to detect changes in regional lung function secondary to radiotherapy in four patients. With doses in excess of 40 Gy, reductions in regional function were noted 1-6 months following completion of radiotherapy. These reductions were not necessarily accompanied by reductions in conventional pulmonary function tests. 50 refs., 7 figs., 2 tabs