RESUMEN
An accurate determination of the penumbra of radiosurgery profiles is critical to avoid complications in organs at risk adjacent to the tumor. Conventional detectors may not be accurate enough for small field sizes. The Monte Carlo (MC) method was used to study the behavior of radiosurgical beam profiles at the penumbral region; the BEAM code was also used in this work. Two collimators (2.2- and 0.3-cm diameter) were calculated and compared with empirical measurements obtained with the detectors normally used. The differences found between film dosimetry and MC revealed a systematic error in the reading procedure. In the process, a water phantom was simulated with a layer of the same composition as that of the film. MC calculations with film differed by a small amount from those obtained with the water phantom alone. In conclusion, MC may be used as a verification tool to support dosimetrical procedures with conventional detectors, especially in very small beams such as those used in radiosurgery. Furthermore, it has been proved that the film energy dependence is negligible for fields used in radiosurgery.