Improvement of treatment plans developed with intensity-modulated radiation therapy for concave-shaped head and neck tumors

PURPOSE: To improve dose conformity and normal tissue sparing in patients with concave-shaped head and neck cancers by using tomotherapy and static step-and-shoot intensity-modulated radiation therapy (IMRT) and by comparing results with those of three-dimensional (3D) conformal radiation therapy (CRT) and two-dimensional (2D) radiation therapy. MATERIALS AND METHODS: Treatment planning in 10 patients with concave-shaped head and neck tumors was performed by using tomotherapy and step-and-shoot IMRT, 3D CRT, and 2D techniques. IMRT plans were modified by placing "virtual critical structures" in regions outside the target where hot spots occurred. These modified plans were used for comparison because they provided better dose conformity. Critical structures were the spinal cord, the parotid glands, and the mandible. Comparisons were performed by means of dose-volume histograms, clinical target volume (CTV), target covered by 95% isodose (D-95%) dose received by 5% of the critical structure volume (D,,), maximum dose, mean dose, and normal tissue complication probability for critical structures. RESULTS: Original IMRT plans showed more conformal dose distributions than those in 3D CRT and 2D plans. However, hot spots developed in the posterior and anterior neck. Introduction of virtual critical structures in IMRT plans resulted in removal of these hot spots without affecting target coverage. Modified IMRT plans also demonstrated better CTV coverage than that in 3D CRT and 2D plans. The average D-95% was 97.3% with tomotherapy, 97.1% with step-and-shoot IMRT, 84.7% with 3D CRT, and 69.4% with 2D techniques. D-95% for the spinal cord changed from approximately 45 Gy with 3D plans and 46 Gy with 2D plans to approximately 28 Gy with IMRT. CONCLUSION: IMRT demonstrated better target coverage and sparing of critical structures than that of 3D CRT and 2D techniques. Use of virtual critical structures resulted in removal of hot spots around the spinal cord. (C) RSNA, 2002