Spatial and Dosimetric Variability of Organs at Risk in Head-and-Neck Intensity-Modulated Radiotherapy

Purpose: The accuracy of intensity-modulated radiotherapy (IMRT) delivery may be compromised by random spatial error and systematic anatomic changes during the treatment course. We present quantitative measurements of the spatial variability of head-and-neck organs-at-risk and demonstrate the resultant dosimetric effects. Methods and Materials: Fifteen consecutive patients were imaged weekly using computed tomography during the treatment course. Three-dimensional displacements were calculated for the superior and inferior brainstem; C1, C6, and T2 spinal cord; as well as the lateral and medial aspects of the parotid glands. The data were analyzed to show distributions of spatial error and to track temporal changes. The treatment plan was recalculated on all computed tomography sets, and the dosimetric error was quantified in terms of the maximal dose difference (brainstem and spinal cord) or the mean dose difference and the volume receiving 26 Gy (parotid glands). Results: The mean three-dimensional displacement was 2.9 mm for the superior brainstem, 3.4 mm for the inferior brainstem, 3.5 mm for the C1 spine, 5.6 mm for the C6 spine and 6.0 mm for the T2 spine. The lateral aspects of both parotid glands showed a medial translation of 0.85 mm/wk, and glands shrank by 4.9%/wk. The variability of the maximal dose difference was described by standard deviations ranging from 5.6% (upper cord) to 8.0% (lower cord.) The translation of the left parotid resulted in an increase of the mean dose and the volume receiving 26 Gy. Conclusion: Random spatial and dosimetric variability is predominant for the brainstem and spinal cord and increases at more inferior locations. In contrast, the parotid glands demonstrated a systematic medial translation during the treatment course and thus sparing may be compromised