Actual dose variations of the parotid glands and spinal cord during helical tomotherapy for patients with loco-regional advanced nasopharyngeal carcinoma

Abstract

Objective To quantify dose variations of the parotid glands and spinal cord in nasopharyngeal carcinoma (NPC) patients during helical tomotherapy (HT) by using megavolt (MV) CT and planned adaptive application. Methods Five patients with advanced local-regional NPC were treated by HT with a prescription dose of 70 Gy in 35 fractions in primary tumor and involved lymph nodes. Daily MV CT scans were registered with the planning CT images. The parotid glands were re-contoured every 4 fractions to a total of 45 times, and location of the spinal cord were corrected by MVCT IGRT system every 2 fractions to a total of 85 times. The actual doses delivered to these organs were evaluated by using the planned adaptive application software of HT system. Results At the end of therapy, the mean volume of the parotid gland was decreased by 42% (left) and 33% (right) , respectively. The average V1 of the parotid was increased by 26. 0% (left) and 31.4% (right), and the D50 increased by 15. 8% (left) and 17. 3% (right), respectively. The average Dmax of the spinal cord was increased by 1.3%. Conclusions During HT, the parotid glands receive a higher actual dose due to the volume shrinkage, while the Dmax of the spinal cord changes little. By using adaptive radiation therapy technique, constant monitoring anatomic changes of organs at risk and selective replanning are necessary during radiation therapy for NPC. Key words: Nasopharyngeal neoplasms/radiotherapy; Radiotherapy, helical tomotherapy; Radiotherapy, adaptive; Radiotherapy,image guided