Integration of computerized tomography imaging with single photon emission in a commercial system for developing radiotherapy fields: application to conformational irradiation for lung carcinoma

PURPOSE: Single photon emission computed tomography (SPECT) of lung perfusion permits to map functioning lung parenchyma with higher sensitivity than CT. Delivering higher radiation doses is used to increase local control in lung carcinoma; this strategy is based on radiobiological and clinical studies. Lung parenchyma is a dose-limiting tissue in patients irradiated for lung cancer. Functional mapping based on SPECT and CT findings permits to design radiation beams such as to minimize irradiation of functioning lung. MATERIAL AND METHODS: CT and SPECT were used to examine a patient with non small cell lung carcinoma (stage IIIB, T4N0, left lung) candidate to conformal irradiation. Images were spatially correlated based on lung contours and using CT findings as reference. SPECT images were normalized to mean right lung value and expressed as perfusion (functional) contours. CT images and perfusion contours were transferred to the treatment planning system (Cadplan V 2.79, Varian-Dosetek Oy): in this way both functional (SPECT) and anatomical (CT) data were available for planning. A comparison was made between two irradiation techniques defined at TPS with (technique B) or without (technique A) SPECT contour information. The prescribed dose was 70.2 Gy. Rival plans were compared using dose volume histograms of target and risk organs. Both functional and anatomical regions were considered in the lung, together with single lung(s) and lung parenchyma. A second perfusion SPECT was obtained 5 months after irradiation and correlated with pretreatment CT images. RESULTS: SPECT lung scans showed marked heterogeneity in the left lung, which was found neither at CT nor at classic lung function tests. The lung volume with perfusion exceeding 80% of average corresponds to about 70% of the anatomical volume. Mean doses to anatomical and to functional lung parenchyma were 24 Gy and 19 Gy, respectively, with technique A and 23 Gy and 18 Gy, respectively, with technique B. Thirty-five percent and 20%, respectively of anatomical and functional lung parenchyma received > or = 25 Gy (V25) with technique B. The figure for functional lung parenchyma was reduced by 5% with technique B. Optimal design of irradiation field geometry decreased the area of functional parenchyma given high doses, which sparing was greater with smaller irradiation volumes. CONCLUSIONS: We have integrated the functional data provided by SPECT lung perfusion into a commercial irradiation planning system. Lung function mapping permits to design irradiation portals sparing larger areas of functional lung parenchyma.