Knowledge-based planning using both the predicted DVH of organ-at risk and planning target volume.

PURPOSE: The purpose of this study was to evaluate the performance of a knowledge-based planning (KBP) method in nasopharyngeal cancer radiotherapy using the predicted dose-volume histogram (DVH) of organ-at risk (OAR) and planning target volume (PTV). METHODS AND MATERIALS: A total of 85 patients previously treated for nasopharyngeal cancer using 9-field 6-MV intensity-modulated radiation therapy (IMRT) were identified for training and 30 similar patients were identified for testing. The dosimetric deposition information, individual dose-volume histograms (IDVHs) induced by a series of fields with uniform-intensity irradiation, was used to predict both OAR and PTV DVH. Two KBP methods (KBP(OAR) and KBP(OAR+PTV)) were established for plan generation based on the DVH prediction. The KBP(OAR) method utilized the dose constraints based on the predicted OAR DVH and the PTV dose constraints obtained according to the planning experience, while the KBP(OAR+PTV) method applied the dose constraints based on the predicted OAR and PTV DVH. For the plan evaluation, the PTV dose coverage was used D(98) and D(2), and the maximum dose, mean dose or dose-volume parameters were used for the OARs. Statistical differences of the two KBP methods were tested with the Wilcoxon signed rank test. RESULTS: For patients with T3 tumors, there was no significant difference between the KBP(OAR) and KBP(OAR+PTV) methods in dosimetric results at most OARs and PTVs. Both KBP methods achieved a similar number of plans meeting the dose requirements. For patients with T4 tumors, KBP(OAR+PTV) reduced the maximum dose by more than 1 Gy in the body, spinal cord, optic nerve, eye and temporal lobes and reduced the V(50) value by more than 3.9% in the larynx and tongue without reducing the PTV dose compared with KBP(OAR). The KBP(OAR+PTV) method increased the plans by more than 14.2% in meeting the maximum dose requirements at the body, optic nerve, mandible and eye and increased the plans by more than 21.4% in meeting the V(50) of the larynx and V(50) of the tongue when compared with the KBP(OAR) method. CONCLUSIONS: For patients with T3 tumors, no significant difference was found between the KBP(OAR) and KBP(OAR+PTV) methods in dosimetric results at most OARs and PTVs. For patients with T4 tumors, the KBP(OAR+PTV) method performs better than the KBP(OAR) method in improving the quality of the plans. Compared with the KBP(OAR) method, dose sparing of some OARs was achieved without reducing PTV dose coverage and helped to increase the number of plans meeting the dose requirements when the KBP(OAR+PTV) method was utilized.