Room: Exhibit Hall | Forum 7
Purpose: To determine the possibility of further improving clinical stereotactic body radiotherapy (SBRT) plans using normal tissue complication probability (NTCP) objectives in order to minimize the risk for carotid blowout syndrome (CBOS).
Methods: 10 patients with inoperable locally recurrent head and neck cancer, who underwent SBRT using CyberKnife were analysed. For each patient, three treatment plans were examined: 1) cone-based without delineation of the ipsilateral internal carotid (clinical); 2) cone-based with the carotid retrospectively delineated and spared; and 3) IRIS-based with carotid sparing. The dose volume histograms of the target and primary organs at risk were calculated. The three sets of plans were compared based on dosimetric and TCP/NTCP (tumor control and normal tissue complication probabilities) metrics. For the NTCP values of carotid, the Relative Seriality model was used with the following parameters: D50 = 40Gy, Î³ = 0.75 and s = 1.0.
Results: Here, the results of three patients are presented. The TCP values of the clinical plans (from no overlap to complete overlap of carotid and GTV) were 82.9%, 79.8% and 73.8%, whereas the respective NTCP values of carotid were 25.3%, 32.1% and 61.8%. The respective values for the cone-based plans were 81.6%, 83.2% and 85.8% for the TCP and 1.9%, 7.8% and 59.1% for the carotid NTCP. For the IRIS plans the respective values were 80.0%, 81.9% and 82.0% for the TCP and 0.8%, 10.0% and 58.5% for the carotid NTCP.
Conclusion: A significant improvement in the quality of the clinical plans could be achieved through the delineation of the internal carotids and the use of more modern treatment delivery modalities. In this way, for the same target coverage a significant reduction in the risk of CBOS could be achieved. The range of risk reduction varied depending on the proximity of carotid to the target.