Monte Carlo evaluation of 6 MV intensity modulated radiotherapy plans for head and neck and lung treatments.

Intensity modulated radiotherapy (IMRT) beams may have strong fluence variations and are advantageous at disease sites such as lung and head and neck (H&N), where neighboring tissues have very different electron densities. We use Monte Carlo (MC) dose calculations to evaluate the dosimetric effects of these inhomogeneities for 10 clinical IMRT treatment plans for five lung patients and four H&N patients. All beams are 6 MV photons. "Standard plans" were first produced on a clinical treatment planning system which optimizes beam intensity distributions to meet dose and dose-volume constraints and calculates dose using a measurement-based pencil-beam algorithm with an equivalent pathlength inhomogeneity correction. Patient anatomy and electron densities were obtained from patient-specific CT images. The dose distribution of each beam was recalculated with the MC method, using the same CT images, beam geometry, beam weighting and optimized fluence intensity distributions as the corresponding standard plan. For the lung cases, the MC calculated dose distributions are characterized by reduced penetrations and increased penumbra due to larger secondary electron range in the low-density media, which is not accurately accounted for in the pencil beam algorithm. For the lung cases, the PTV was underdosed; except for one dose-volume index, underdose was less than 10%. Individual H&N fields are affected to different degrees by tissue inhomogeneities, depending on specific anatomy, especially the size and location of air cavities in relation to the beam orientation and field size. For four H&N plans, PTV coverage changed by less than 2%; for the fifth, there was less than 10% difference between the standard and the MC plans. Critical normal tissue DVHs (cord, lung, brainstem) are changed by <10% at the high dose end and mean lung doses are changed by <6%.