Lymphatic atlas-based target volume definition for intensity-modulated radiation therapy planning

Despite the improvements in current imaging modalities such as CT and MRI, the detection of normal or malignant lymph nodes remains a challenge due to the large variability in lymph node characteristics and the variation in imaging quality and the limited imaging resolution. A computerized lymph node atlas could be the ideal tool for target volume definition based on the distribution of normal lymph nodes surrounding the verified malignant nodes to improve the accuracy of intensity-modulated radiation therapy planning. The standard lymph node topography in the newly constructed 3D lymph node atlas offers a detailed topographical distribution of discrete nodal locations in relation to surrounding organs at risk. In the present paper, the recently developed lymph node atlas is used for selection and delineation of target volumes in the head and neck, thorax and pelvic region. Image registration techniques were implemented to integrate the topography of the lymph node atlas into the patient's data set. By combining the knowledge-based lymph node distribution with the patient's data set, more detailed definitions of the target volumes were obtained to facilitate biologically based treatment plan optimization. The response values of the biologically optimized treatment plans were used to derive the probability of tumor control and the probability of complications in organs at risk. The treatment outcome of the lung reference plan showed a lower probability of recurrence in comparison to planning without the lymph node atlas. The lymph node atlas can improve and standardize the target volume definition by including more accurate anatomical knowledge for target volume definition and biologically optimized radiation therapy planning.