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Evaluation of the patient doses from megavoltage cone-beam CT imaging in the image-guided radiation therapy for nasopharyngeal carcinoma

Objective To evaluate and estimate the patient doses from megavoltage cone-beam CT imaging system in the image-guided radiation therapy for nasopharyngeal carcinoma.Methods 8 MU protocol of the MV CBCT system was selected for the head-and-neck region.The absorbed doses at the different positions in the phantom were measured using a 0.65 cm~3 ion chamber and the cylindrical acrylic phantom.The absorbed doses at the measurement positions of the phantom were calculated and the patient doses to the tumor and critical organs were derived from dose-volume histogram by the TPS mimicking the MV CBCT scanning with 8 MU protocol.Results The error between the measured dose and the calculated dose was less than 3.5%.The average doses to the tumor target,brain stem.spinal Cord and ehiasm were 6.43,6.36,6.83 and 6.90 cGy,respectively,while those to left and right of both optic nerve and parotid were 7.70 and 7.53 cGy,7.70 and 7.53 cGy,respectively.Conclusion The patient doses estimated using the TPS mimicking the M V CBCT image acquiring procedure are accurate and reliable.The patient doses from the MV CBCT imaging must be considered when treatment plan of the patient is designed. 
 
Key words: 
Nasopharyngeal carcinoma;  Megavoltage cone-beam CT; Patient doses