THE UTILITY OF SPECT LUNG PERFUSION SCANS IN MINIMIZING AND ASSESSINGTHE PHYSIOLOGICAL CONSEQUENCES OF THORACIC IRRADIATION

Purpose: Three-dimensional single photon emission computed tomography lung perfusion scans (SPECT) provide a unique quantitative 3-dimension al map of the distribution of functioning pulmonary vascular/alveolar subunits, information not provided by other imaging modalities. This r eport describes our initial experience utilizing these scans to assist in the design of radiation treatment beams and to assess changes in r egional lung function following irradiation. Methods and Materials: Pa tients were immobilized and scanned in the treatment position with app ropriate fuducial markers. Four millicuries of technetium 99M microagg regated albumin were injected and SPECT images of the lung were genera ted. Pre-treatment SPECT images were used to help design radiation bea ms to minimize irradiation of functioning lung. Pre- and post-treatmen t scans were compared to assess changes in regional function. These ch anges in function were then correlated with the regional radiation dos e. Results: Pre-radiotherapy SPECT scans were obtained in 18 patients (11 with lung cancer). Marked variations in regional function were fre quently noted. In patients with primary lung tumors, these variations were not necessarily immediately adjacent to the tumor volume. In gene ral, 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/alveo lar subunits. In these cases, pre-treatment scans were most useful in designing radiation portals to minimize irradiation of functioning lun g. SPECT scans were also used to detect changes in regional lung funct ion secondary to radiotherapy in four patients. With doses in excess o f 40 Gy, reductions in regional function were noted 1-6 months followi ng completion of radiotherapy. These reductions were not necessarily a ccompanied by reductions in conventional pulmonary function tests, whi ch are assessments of whole lung function and may not reflect regional lung injury if the volume affected is small.Conclusions: SPECT lung s cans provide an excellent means of assessing regional lung function, s uperior to that obtainable with planar images. The functional data pro vided by the SPECT images is useful in designing ''optimal'' radiation treatment beams and in assessing the effect of radiotherapy on region al lung functions. Efforts are continuing in our laboratory to develop a dose response curve for regional lung damage using the tools of SPE CT scanning and 3-dimensional dose calculations.