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Predicting dosimetric indices in IMRT planning for lung cancer patients

Purpose: For lung cancer patients treated with intensity modulated radiation therapy (IMRT) technique, several dosimetric indices of lungs such as the mean lung dose, V30, V20 and V10 have drawn high attention, given their close correlations with the occurrence of radiation pneumonitis. In this study, we developed a method to predict the indices V30, V20 and V10 before treatment planning. Methods and Materials: The work in this study utilises anatomical information to predict dosimetric indices of lungs for IMRT plans. 15 shells surrounding the PTV were created and each shell comprised voxels whose distances to PTV surface were within a certain distance. The overlap regions of the lungs and the 15 shells were determined. A linear predicting function is employed to map volumes of these regions to dosimetric indices, where parameters were fitted using previous IMRT plans from a training database consisting of 43 cases. The effectiveness of the predicting function was validated using another 14 plans. Results: Dosimetric indices of the 14 patients in validation cohort were calculated using the linear predicting function. The predicted indices were compared with the indices derived from the clinically accepted treatment plans. The absolute differences between calculated V30 and the actual clinical values of 14 plans had a mean value of 1.6% and a standard deviation of 1.0%. The mean value and standard deviation for absolute differences of V20 were 1.5% and 1.0%. And for V10, these values were 3.6% and 2.6%. Conclusions: The method proposed here can predict dosimetric indices of lungs before treatment planning using geometric information of delineated PTVs and lungs. The results show that the prediction error of V30 and V20 are less than about 2.0%.