A new method to determine dose-effect relations for local lung-function changes using correlated SPECT and CT data.

PURPOSE To determine dose-effect relations for regional lung-function changes after radiotherapy. METHODS Single Photon Emission Computed Tomography (SPECT) was performed to quantify regional ventilation and perfusion. CT scans were used to calculate the three-dimensional (3-D) dose distribution. Both SPECT and CT scans were performed prior to radiotherapy and 5 months after the start of the treatment. To obtain combined 3-D information on ventilation, perfusion and dose, the SPECT data were correlated with the corresponding CT data. The relative changes in ventilation and perfusion were calculated in each SPECT voxel (voxel size about 6 x 6 x 6 mm) and related to the dose in that voxel. The average relative changes were determined per dose interval of 4 Gy. This procedure was evaluated using the data from five patients treated for Hodgkin's disease with mantle field irradiation with a prescribed total dose of 40-42 Gy. RESULTS Dose-effect relations for perfusion were observed in all patients, while in four of the five patients, a dose-effect relation was found for ventilation. The maximal uncertainty of the calculated radiation dose was 11%: a difference between the position of the patient during treatment and during CT scanning caused a maximal dose uncertainty of 6%, while the accuracy of the dose calculation algorithm itself was estimated to be within 5%. CONCLUSION The results indicate that the combined use of SPECT and CT information is an effective method for determining dose-effect relations for regional lung function parameters in each individual patient.