Better narrow beam profiles by exposing a stack of radiochromic films and averaging the readings

Radiochromic film, with its high spatial resolution and tissue-equivalence would be excellent for obtaining crossbeam profiles of narrow radiosurgery beams, if it were not for its nonuniformity. We show that this can be large ly overcome by exposing a stack of three films to a beam, reading each film separately, registering the films, and averaging the readings from all thr ee films at each point. In this way, two-dimensional dose distributions for a 10 mm diameter (full width at half-maximum at isocenter) beam as well as the central square centimeter of a large (10 cm x 10 cm beam were obtained . The effects of averaging three films in a stack are also compared to the results of averaging three readings of the same film. For the films uniform ly irradiated in the center of the large field, the average, standard devia tion, minimum? and maximum values of the dose were obtained. These statisti cs show that the dose distribution obtained by averaging three scans of the same film is better than the distribution from one scan, and the distribut ion from averaging three films in a stack is even better. A distribution wa s judged to be better if the average dose was closer to the actual dose, th e standard deviation was smaller, and the minimum and maximum were closer t ogether. (C) 1999 American Association of Physicists in Medicine. [S0094-24 05(99)00803-2].