ESTIMATION OF PNEUMONITIS RISK IN 3-DIMENSIONAL TREATMENT PLANNING USING DOSE-VOLUME HISTOGRAM ANALYSIS

Purpose: Investigations to study correlations between the estimations of biophysical models in three dimensional (3D) treatment planning and clinical observations are scarce. The development of clinically sympt omatic pneumonitis in the radiotherapy of thoracic malignomas was chos en to test the predictive power of Lyman's normal tissue complication probability (NTCP) model for the assessment of side effects for nonuni form irradiation. Methods and Materials: In a retrospective analysis i ndividual computed-tomography-based 3D dose distributions of a random sample of 46/20 patients with lung/esophageal cancer were reconstructe d. All patients received tumor doses between 50 and 60 Gy in a convent ional treatment schedule. Biological isoeffective dose-volume histogra ms (DVHs) were used for the calculation of complication probabilities after applying Lyman's and Kutcher's DVH-reduction algorithm. Lung dos e statistics were performed for single lung (involved ipsilateral and contralateral) and for the lung as a paired organ. Results: In the lun g cancer group, about 20% of the patients (9 out of 46) developed pneu monitis 3-12 (median 7.5) weeks after completion of radiotherapy. For the majority of these lung cancer patients, the involved ipsilateral l ung received a much higher dose than the contralateral lung, and the p neumonitis patients had on average a higher lung exposure with a doubl ing of the predicted complication risk (38% vs. 20%). The lower lung e xposure for the esophagus patients resulted in a mean lung dose of 13. 2 Gy (lung cancer: 20.5 Gy) averaged over all patients in correlation with an almost zero complication risk and only one observed case of pn eumonitis (1 out of 20). To compare the pneumonitis risk estimations w ith observed complication rates, the patients were ranked into bins of mean ipsilateral lung dose. Particularly, in the bins with the highes t patient numbers, a good correlation was achieved. Agreement was not reached for the lung functioning as a paired organ. Conclusions: Reali stic assessments for the prediction of radiation-induced pneumonitis s eem to be possible. In this respect, the implementation of DVH-analysi s in 3D planning could be a helpful tool for the evaluation of treatme nt plans.