A systematic evaluation of air cavity dose perturbation in megavoltage x-ray beams

The EGS4 Monte Carlo radiation transport code was used to systematically st udy the dose perturbation near planar and cylindrical air cavities in a wat er medium irradiated by megavoltage x-ray beams. The variables of the probl em included x-ray energy, cavity shape and dimension, and depth of the cavi ty in water. The Monte Carlo code was initially validated against published measurements and its results were found to agree within 2% with the publis hed measurements. The study results indicate that the dose perturbation is strongly dependent on x-ray energy, field size, depth, and size of cavity i n water. For example, the Monte Carlo calculations show dose reductions of 42% acid 18% at 0.05 and 2 mm, respectively, beyond the air-water interface distal to the radiation source fur a 3 cm thick air slab irradiated by a s ingle 5 x 5 cm(2) 15 MV beam. The dose reductions are smaller for a paralle l-opposed pair of 5 x 5 cm(2) 15 MV x-ray beams, being 21% and 11% for the same depths. The combined set of Monte Carlo calculations showed that the d ose reduction near an air cavity is greater for: (a) Smaller x-ray field si ze, (b) higher x-ray energy, (c) larger air-cavity size, and (d) smaller de pth in water where: the air cavity is situated. A potential clinical applic ation of those results to the treatment of prostate cancer is discussed. (C ) 2000 American Association of Physicists in Medicine. [S0094-2405(00)02605 -5].