IMPACT OF SETUP VARIABILITY ON INCIDENTAL LUNG IRRADIATION DURING TANGENTIAL BREAST TREATMENT

Purpose: This study aimed to determine the variability in treatment se tup during a 5-week course of tangential breast treatment for patients immobilized in a customized hemibody cradle, to assess the relationsh ip between the height of the lung shadow on the tangential port film a nd the percentage of lung volume irradiated, and to estimate the impac t of setup variabilities on irradiated lung volume. Methods: One hundr ed seventy-two port films were reviewed from 20 patients who received tangential beam treatment for breast cancer, The height of the lung sh adow at the central axis (CLD) on each port film was compared to the c orresponding simulator film as an assessment of setup variability, A t hree-dimensional dose calculation was performed, and the percentage of total lung volume within the field was correlated with the CLD, The t hree-dimensional dose calculation was repeated for selected patients w ith the location of the treatment beams modified to reflect typical se tup variations. Results: The CLD measured on the port films was within 3 mm of that prescribed on the simulator film in 43% (74 of 172) of t he port films. The variation was 3-5 mm in 26%, 5-10 mm in 25%, and >1 0 mm in 6%, The height of the lung shadow correlated with the percenta ge of lung volume included in the radiation field (r(2) = 0.6), Typica l variations in treatment setup resulted in less than or equal to 5% f luctuation in the absolute volume of ipsilateral lung irradiated. Conc lusion: The current immobilization system used in our clinic provides a clinically acceptable reproducibility of patient setup, The height o f the lung shadow is reasonably well correlated with the percentage of irradiated lung volume, During a typical 5-week course of radiotherap y, the ipsilateral irradiated lung volume fluctuates <5%. (C) 1997 Els evier Science Inc.