Comparison of dose distributions from geometric conformation and intensity modulation for intracranial lesions

Because of the functional importance of structures found in the brain, improved dose conformation to tumor volumes is a continuing goal of radiation therapy. Current beam-shaping devices such as miniature multileaf collimators improve tumor dose conformation in the lateral directions, but require the tumor to be convex in the plane of gantry rotation. If the lesion is concave, normal tissue or critical structures in or near the concavity may receive a higher dose than desired. Intensity modulated radiotherapy offers improved target dose conformation, and therefore improved normal tissue sparing. This work compares dose distributions from geometric conformal stereotactic radiosurgery and step-and-shoot intensity modulated stereotactic radiosurgery using a miniature multileaf collimator. Both sets of treatment plans were generated using software from a commercial vendor (Radionics Software Applications, Inc., Burlington, MA). This reduced the number of variations between plans by using the same set of contours in the planning process as well as the same dose algorithm for dosimetric evaluation and comparison. The resulting dose distributions are compared using dose-volume relations (tissue-volume ratios, dose-volume histograms) and planning/treatment efficiency (planning time, treatment time). Preliminary results indicate that the IMRT plans improved target dose conformation and normal tissue sparing for targets in close proximity to critical structures. For targets far from critical structures, there was slightly improved target dose conformation, but at the cost of significantly increased treatment time. Intensity modulated stereotactic radiosurgery should be used in cases where sparing nearby normal tissue is a high priority. Otherwise, geometric conformal radiosurgery is sufficient