DOSE-EFFECT RELATIONS FOR LOCAL FUNCTIONAL AND STRUCTURAL-CHANGES OF THE LUNG AFTER IRRADIATION FOR MALIGNANT-LYMPHOMA

Purpose: To estimate the dose-effect relations for local functional (v entilation and perfusion) and structural (density) changes of the lung , 3-4 months after irradiation. Methods: Twenty-five patients with mal ignant lymphoma were irradiated with a (modified) mantle field to an a verage dose of 38 Gy, given in 21 fractions. Single photon emission co mputed tomography (SPECT) ventilation (V) and perfusion (Q) scans, and CT scans were performed before and 3-4 months after radiation treatme nt. The three-dimensional dose distribution was calculated using the C T data. After correlation of SPECT and CT data sets, the average post- treatment value of V, Q and lung density per voxel was calculated rela tive to the pre-treatment value, per dose interval of 4 Gy. Subsequent ly, the dose-effect relations in each patient were normalized to the a verage value per voxel in the dose interval of 0-12 Gy. In addition, i n each dose interval of 4 Gy the fraction of patients with changes lar ger than 20% was calculated for all three parameters. The dose-effect relations for perfusion and ventilation normalized to the low-dose reg ions, and the dose-incidence curves for the fraction of patients with changes larger than 20% were fitted for all three parameters, using a logistic model. Results: Marked changes in the distribution of V and Q were found after irradiation. Prior to normalization to the low-dose regions, a change in V and Q was found in most patients in the dose in terval of 0-12 Gy, varying from an increase of 37% to a decrease of 10 %, which was followed by a decreasing trend at higher doses. The incre ase in the low-dose regions indicated a redistribution phenomenon, the magnitude of which was dependent of the irradiated volume. The logist ic fit of the dose-effect relations for Q and V, normalized to the low -dose regions, resulted in values for D-50 of 51 Gy and 54 Gy (given i n 21 fractions on average), respectively, and for the steepness parame ter k of 4.2 and 4.0, respectively. The logistic fit for the dose-inci dence curves for Q, V and lung density resulted in values for D-50 and k of 38 Gy, 37 Gy, 44 Gy and 10.3, 7.8 and 9.4, respectively. Conclus ions: With the combined use of SPECT and CT scans, we have obtained do se-effect relations for local functional and structural damage in the lung, 3-4 months after irradiation.