10 Chapter 1 Dose volume histograms A dose volume histogram (DVH) in radiotherapy represents the 3 dimensional dose distribution of target volumes or critical organs in a 2D graph. This DVH graph is a cumulative graph, with the radiation dose (in Gray) on the x-axis and the volume (in percentages) on the Y axis. These DVH graphs can be used to compare different treatment plans or techniques. DVH parameters can be extracted from these graphs and are used in scientific literature and radiotherapy guidelines. Values like D98(Gy) are used to describe the plan quality (target coverage) and represents the minimum dose given to 98% of the target volume. V values are used to describe the dose on OARs. E.g., V20 of the heart represents the volume of the heart in percentage that receives 20 Gy or more. Normal tissue complication probability Normal Tissue Complication Probability (NTCP) models describe the relation between radiation dose distributions in one or more OARs and the risk of complications. These prediction models are developed by using cohorts of patients who were treated in the past. Next to Dose Volume Histogram (DVH) parameters, other clinical risk factors (like age, smoking, or a cardiac history) can be included in these models to correct for confounding or to improve model performance. These multivariable NTCP-models should preferably be based on large patient cohorts, and validated in independent patient cohorts to assess generalizability in other study populations. These externally validated multivariable prediction models are currently considered the highest level of evidence for the prediction of treatment related complications [4]. Radiation induced pulmonary toxicity. As mentioned before, most literature in the 90’s focused on pulmonary toxicity. Radiation-induced pulmonary toxicity may present with different clinical symptoms, varying from mild dyspnea and non-productive cough to respiratory failure requiring mechanical ventilation which could eventually be fatal. Currently, the most widely used NTCP-models for radiation-induced pulmonary toxicity include the mean lung dose as DVH parameter[5,6], next to clinical factors like age, co-morbidities and the location of the tumor [7].
RkJQdWJsaXNoZXIy MTk4NDMw