Abstract:ObjectiveTo measure and evaluate the setup errors in patients with nasopharyngeal carcinoma (NPC) immobilized with two different techniques, and determine the proper margins extended from clinical target volume (CTV) to planning target volume (PTV).Methods73 NPC patients treated with intensitymodulated radiotherapy (IMRT) were divided into two groups. All the patients received cone beam computed tomography (CBCT) scan (429 times) and CT image matching before every treatment and correction. 35 patients were immobilized using standard plastics pillow with headandneck thermoplastics masks (standard plastics pillow group) and received 201 CBCT scans, 38 patients were immobilized using polyurethane foam sealing agent pillow with headandneck thermoplastics masks (polyurethane foam sealing agent group) received 228 CBCT scans. The shift and rotation setup errors in rightleft (X), superiorinferior (Y) and anteriorposterior (Z) directions were recorded and analyzed.ResultsIn the standard plastics pillow group, the shift errors in X,Y and Z axes were (0.11±0.08)cm, (0.15±0.12)cm and (0.12±0.10) cm respectively, the corresponding rotation errors were 0.59°±0.53°, 0.84°±0.62°and 0.61°±0.51°, and the margins from CTV to PTV were 0.33 cm, 0.46 cm and 0.37 cm. In the polyurethane foam sealing agent pillow group, the shift errors in X,Y and Z axes were (0.09±0.08)cm, (0.13±0.09)cm and (0.11±0.10)cm respectively, the corresponding rotation errors were 0.55°±0.49°, 0.69°±0.51°and 0.58°±0.47°, and the margins from CTV to PTV were 0.28 cm, 0.39 cm and 0.35 cm. The differences of shift and rotation errors in Y axis between the two groups were statistically significant (both P<0.05).ConclusionsThe shift and rotation errors in Y axis can be obviously decreased by immobilization with polyurethane foam sealing agent pillow plus headandneck thermoplastics masks during IMRT of NPC patients. CBCT can be used in the precise measure of setup errors, which is instructive to determine the margins from CTV to PTV.