A dosimetric study of helical tomotherapy for nasopharyngeal carcinoma / 中华放射肿瘤学杂志

ABSTRACT

Objective To use helical tomotherapy ( HT ) for determining the difference between actual doses and planning doses to the target volume and organs at risk ( OARs ) in patients with nasopharyngeal carcinoma receiving radiotherapy, and to provide guidance for the clinical treatment. Methods Localization and delineation of the target volume and OARs were performed by computed tomography ( CT) in 21 patients with nasopharyngeal carcinoma receiving radical radiotherapy using HT. All patients underwent megavoltage CT ( MVCT) scan prior to treatment. The obtained MVCT images were used for dose reconstruction in the adaptive module of HT, in which the actual dose was obtained and the non?image?guided dose was simulated. Each single dose distribution and the corresponding CT image were sent to software MIM6. 0 for superimposition, and the overall dose was obtained. The initial plan, image?guided plan, and non?image?guided plan were named Plan?1, 2, and 3, respectively. The dose distribution in the target volume and OARs was compared between the three plans with t ? test or wilcoxon test . Results Compared with those in Plan?1, the D98 values for the planning gross tumor volume ( PGTV) and planning target volume ( PTV) in Plan?2 were significantly reduced by 1. 16% and 2. 3%, respectively ( P=0. 025;P=0. 043);the volumes of the left and right parotids in Plan?2 were significantly reduced by 46. 0% and 46. 5% on average, respectively ( P=0. 000);the distances between the midline and the center?of?mass for left and right parotids were significantly reduced by 6. 9% and 6. 5%, respectively ( P=0. 000);the V26 and Dmean for both parotid glands were significantly elevated by 32. 8% and 25. 2%, respectively ( P=0. 000) . Compared with those in Plan?1, the D98 values for PGTV, PTV?1, and PTV?2 in Plan?3 were significantly reduced by 2. 0%, 1. 9%, and 2. 4%, respectively ( P=0. 001;P=0. 007;P=0. 036);the V26 and Dmean for both parotid glands in Plan?3 were significantly elevated by 33. 6% and 25. 3%, respectively ( P=0. 000);Dmax to the spinal cord was significantly increased by 6. 9%( P=0. 005) . There was no significant difference in D2 to the spinal cord between Plan?2 and Plan?1( P=0. 392) . Conclusions The doses to both parotid glands increase during HT for nasopharyngeal carcinoma, which is closely associated with the shift of the parotid glands toward the midline. Image?guided radiotherapy does not enhance the dose to the target volume, but reduces the dose to the spinal cord.