RESUMO
BACKGROUND & OBJECTIVE: Intensity-modulated radiotherapy (IMRT) may help to diminish radiation-induced normal tissue damage and improve quality of life of nasopharyngeal carcinoma (NPC) patients. However, highly conformal treatment techniques commonly establish steep dose gradients between tumor and normal tissues, therefore, daily setup variations can significantly compromise the ultimate precision of idealized IMRT delivery. This study was to investigate the setup accuracy of thermoplastic masks used for immobilizing NPC patients treated by simultaneous integrated boost IMRT, and to determine adequate margins to account for those uncertainties. METHODS: Nineteen patients with early stage (T1-2N0M0) NPC received CT scan weekly during their 6-week treatment course of fractionated IMRT. A total of 85 scanning parameters were obtained. The differences in anatomic landmark coordinates in 3-dimensional directions between daily setup and the first day setup were calculated through comparing the CT images with Osiris software. RESULTS: Mean target isocenter translation was (0.89+/-0.69) mm in x-direction, (0.82+/-0.79) mm in y-direction, and (0.95+/-1.24) mm in z-direction. The systematic errors were 0.94 mm, 1.00 mm, and 1.32 mm. The random errors were 0.87 mm, 0.80 mm, and 1.04 mm. The mean total magnitude vector of isocenter motion was 1.87 mm; the 95% confidence interval (CI) was 2.03-7.24 mm. CONCLUSION: During IMRT for early stage nasopharyngeal carcinoma, setting appropriate margin of planning target volume (PTV) by widening 3 mm from clinical target volume (CTV) in x, y, z directions will be sufficient to compensate for the dosimetric uncertainty of target areas due to patient setup error. The measured data in the present study should enable the user of this kind of thermoplastic mask to assign appropriate margins for the generation of planning target volumes (PTVs).
