Gradients and intensity modulated radiotherapy

A consequence of the advent of intensity modulated radiation therapy (IMRT) is the presence of large dose gradients surrounding the tumor, and the regions at risk (RAR's) near the tumor. Unlike standard radiotherapy, these gradients are not the chance result of a few beam choices, but rather they are strategically placed by the optimization software. A first issue is the magnitude of these gradients. Optimal dose plans were simulated for four clinical cases using experimental helical tomotherapy optimization software. It was found that high gradients could be divided into two categories: avoidance gradients and incidental gradients. Avoidance gradients are associated with tumor/RAR boundaries and tend to be larger, on the order of 30% per cm or higher. Incidental gradients are a secondary consequence of the optimizer doing its job: placing a large amount of dose confined to the target volume. Incidental gradients tend to be on the order of 15%-20% per cm.