A Monte Carlo study of IMRT beamlets in inhomogeneous media.

Intensity Modulated Radiation Therapy (IMRT) has trended toward smaller multiple radiation fields thereby increasing the resolution of the intensity map. Vendors have introduced multileaf collimators with beam apertures of 0.5 cm and less. The beam characteristics of these smaller fields have not been adequately assessed, especially in the presence of inhomogeneities. Most dosimetric devices have significant limitations due to finite size, dose rate, and energy dependence. We studied the effect of inhomogeneities on small beamlets. The 6, 15, and 24 MV beams were modeled using the EGSnrc Monte Carlo code. Point source beams of circular field sizes 0.5, 1.0, 3.0, 5.0, and 10 cm were simulated in a water phantom at 100 SSD. A 3 cm inhomogeneity of lung tissue was incorporated between 3 and 6 cm in the phantom. The depth dose curves and profiles were compared by beam size and density of the inhomogeneity. The Monte Carlo simulations show that for small fields a marked dose decrease in the presence of low-density media due to the lack of lateral electronic equilibrium is observed. As the density and field size increase, the dose reduction is less pronounced and for the 10 cm field there is an increased dose as expected due to lack of attenuation. This data suggests that current TPS may dramatically over- or underestimate the dose in inhomogeneous media for small field sizes that are used for IMRT.