On voxel based iso-tumor-control probability and iso-complication maps for selective boosting and selective avoidance intensity-modulated radiotherapy

Voxel-based iso-tumor-control probability (TCP) maps and iso-Complication maps are proposed as a plan-review tool especially for functional image-guided intensity-modulated radiotherapy (IMRT) strategies such as selective boosting (dose painting) and conformal avoidance IMRT. The maps employ voxel-based phenomenological biological dose-response models for target volumes and normal organs. Two IMRT strategies for prostate cancer, namely conventional uniform IMRT delivering an equivalent uniform dose (EUD) = 84 Gy to the entire planning target volume (PTV) and selective boosting delivering an EUD = 82 Gy to the entire PTV, are investigated, to illustrate the advantages of this approach over iso-dose maps. Conventional uniform IMRT did yield a more uniform isodose map to the entire PTV while selective boosting did result in a non-uniform isodose map. However, when employing voxel-based iso-TCP maps selective boosting exhibited a more uniform TCP map compared to what could be achieved using conventional uniform IMRT, which showed TCP cold spots in high-risk tumor subvolumes despite delivering a higher EUD to the entire PTV. Voxel-based iso-Complication maps are presented for rectum and bladder, and their utilization for selective avoidance IMRT strategies are discussed. We believe as the need for functional image-guided treatment planning grows, voxel-based iso-TCP and iso-Complication maps will become an important tool to assess the integrity of such treatment plans.