isodoses

Isodoses are lines or surfaces on medical images or treatment plans along which the absorbed radiation dose is identical. In radiotherapy, isodose lines are used to visualize and evaluate how a prescribed dose is distributed within a patient or a phantom. They are generated by treatment planning systems from calculated dose grids that incorporate beam geometry, tissue density, and scatter.

Isodose lines are often expressed as percentages of the prescribed dose, such as 50%, 90%, or 100%. In external-beam planning, common practice aims to have the planning target volume covered by higher-percentage isodose lines, while lower-percentage lines indicate regions of lower dose. In brachytherapy, lines are frequently plotted as percentages of the prescribed dose around the radioactive source, showing how quickly dose falls off with distance.

Applications include assessing target coverage, sparing of organs at risk, and evaluating overall plan conformity and homogeneity. Isodose maps support dose-volume constraints and iterative optimization in the treatment planning system. Clinicians use them to verify that critical structures remain within tolerance and that the target receives an adequate dose.

Limitations exist because isodose representations are mathematical maps based on physical models and CT-derived densities. They do not capture microscopic tissue heterogeneity, biological response, or intra-fraction motion. Dose calculation uncertainties, patient movement, and anatomical changes can alter the actual delivered dose. Therefore, isodose diagrams are planning and quality-assurance tools, not exact boundaries of tissue damage.