Numerous beam directions using 3-D conformal techniques can be employed in treating tumors in the posterior fossa, each with characteristic normal tissue exposure along the entrance and exit trajectory. A representative variety of beam configurations were modeled in a modern computer planning system initially with the entire posterior fossa as the target.These beams were quantitatively scored using criteria based on integral doses for both low dose and high dose effects encompassing a variety of critical normal structures, thus identifying strengths and weaknesses of each beam. By blocking portions of a particular beam accounting for unfavorable scores, a map of "zones" within the posterior fossa ideally treated by a certain beam or beams could be constructed. No universally ideal photon beam arrangement for the entire posterior fossa target could be identified. However, using single beam analysis, the strengths and weaknesses of particular strategies could be quantified.For example, vertex beams treating the cerebellar hemispheres allow the greatest sparing of cochlea and hypothalamus but at the cost of increased low to moderate dose to the supratentorial brain. Using the constructed maps identifying "zones" appropriately treated by a given beam or beams, three-dimensional conformal treatment plans with favorable dose-volume statistics can be designed based on previously defined normal tissue tolerance considerations. It is shown how this approach can be individualized based on specific patient characteristics (e.g., age). We conclude that radiotherapy directed to the posterior fossa can be optimized based on systematic assessment of individual beam contributions to normal tissues. This technique allows fast selection of treatment beams based on known normal tissue anatomical and tolerance information. Further studies will be required regarding long term effects of various radiation doses on specific volumes of normal tissue in order to individualize beam selection. When treating children, knowledgeable consideration of these beam characteristics can help avoid late effects.Key words: Posterior fossa neoplasms, Conformal radiotherapy, Pediatric, Medulloblastoma. IntroductionRadiotherapy (RT) has long played an important role in the treatment of nearly every type of tumor occurring in the posterior fossa in children. Treatment planning techniques have dramatically evolved from one-dimensional to two-dimensional and now to three-dimensional representations of anatomy, beams, and dose distributions. Understanding and localization of targets in the posterior fossa has evolved as well. Historically, tumor targets were localized based on relationships to bony landmarks and external features with correlation to opera- Abbreviations: RT -radiotherapy; CT -computed tomography; MRI -magnetic resonance imaging;3DCRT -three-dimensional conformal radiotherapy; TM -temperomandibular. tive findings. Today, the location and extent of tumor targets can be identified using computed tomography (CT) and magnetic resonance imaging ...