Monte Carlo evaluation of tissue inhomogeneity effects in the treatment ofthe head and neck

Purpose: To use Monte Carlo dose calculation to assess the degree to which tissue inhomogeneities in the head and neck affect static field conformal, computed tomography (CT)-based 6-MV photon treatment plans. Methods and Materials: We retrospectively studied the three-dimensional tre atment plans that had been used for the treatment of 5 patients with tumors in the nasopharyngeal or paranasal sinus regions. Two patients had large s urgical cavities. The plans were designed with a clinical treatment plannin g system that uses a measurement-based pencil-beam dose-calculation algorit hm with an equivalent path-length inhomogeneity correction. Each plan emplo ys conformally-shaped 6-MV photon beams. Patient anatomy and electron densi ties were obtained from the treatment planning CT images. For each plan, th e dose distribution was recalculated with the Monte Carlo method, utilizing the same beam geometry and CT images. The Monte Carlo method accurately ac counts for the perturbation effects of local tissue heterogeneities. The Mo nte Carlo calculated dose distributions were compared with those from the c linical treatment planning system. Results: The degree to which tissue inhomogeneity affects the dose distribu tions of individual fields varies with the specific anatomic geometry, espe cially the size and location of air cavities in relation to the beam orient ation and field size. Most off the beam apertures completely enclose the ai r cavities within or adjacent to the gross tumor volume (GTV). Equivalent s quares {including blocking) ranged from approximately 5 to 9.5 cm. A common feature observed for individual fields is that the Monte Carlo calculated doses to tissue directly behind and within an air cavity are lower. However , after combining the fields employed in each treatment plan, the overall d ose distribution slows only small differences between the two methods. For all 5 patients, the Monte Carlo calculated treatment plans showed a slightl y lower dose received by the 95% of target volume (D-95) than the plans cal culated with the pencil-beam algorithm. The average difference in the targe t volume encompassed by the prescription isodose line was less than 2.2%. T he difference between the dose-volume histograms (DVHs) of the GTV was gene rally small. For the brainstem and chiasm, the DVHs of the two plans were s imilar. For the spinal cord, differences in the details of the DHV and the dose to 1 cc (D-1cc) of the structure were observed, with Monte Carlo calcu lation generally predicting increased dose indices to the spinal cord. Howe ver, these changes are not expected to be clinically significant. Conclusion: For 6-MV photons, the effects of both normal tissue inhomogenei ties and surgical air cavities on the target coverage were adequately accou nted for by conventional pencil beam methods for all of the cases studied. Although differences in details of the DVHs of the normal structures were o bserved, depending on whether Monte Carlo or pencil-beam algorithm was used for calculation, these differences are not expected to be clinically signi ficant. In general, the pencil-beam calculation corrected for primary atten uation by the equivalent path-length is a sufficiently accurate method for head-and-neck treatment planning using 6-MV photons. 2001 Elsevier Science Inc.