Adaptive Intensity-Modulated Radiation Therapy For Nasopharyngeal Carcinoma

Purpose:To investigate the impact of adaptive radiation therapy (ART) on dosimetric consequence for the targets and organs at risk (OARs) in nasopharyngeal carcinoma (NPC) during intensity-modulated radiation therapy(IMRT), and to evaluate the potential benefit and feasibility of this technology.Materials and Methods:Eleven undifferentiated non-keratinizing NPC patients treated with IMRT were recruited prospectively. After 5 and 15 fractions, each patient had repeat planning CT (CT1、CT2) and two new ART plans (replanl, replan2) with recontouring the targets and OARs were generated, and used for subsequent treatment. To simulate the situation of no replanning (NART),2 hybrid plans denoted as Planl and Plan2 were generated using the original contours pasted on the CT1 and CT2 datasets by CT-CT fusion. Dosimetric comparisons were made between the NART plans and the corresponding ART plans.Results:Compared with NART plans, conformity index (CI) of PTVnd, PTV1 and PTV2 were improved during the course of radiation, and homogeneity index (HI) have a little changed in all target volumes. Significant improvements in target dose coverage were observed in ART plans,. Median V95 for PTVnx was increased (P=0.013). D95, D99, V100, V95, and V93 for PTVnd were increased by 2.16±2.52 Gy,4.52±6.24 Gy,5.47±7.27%,2.6±3.5%, and 2.01±2.77%, respectively. D99 and V100 for PTV1 were increased by 0.71±0.97 Gy and 0.56±0.65%, respectively. D95, D99, V100, V95, and V93 for PTV2 were increased by 0.47±0.61 Gy,1.06±1.45 Gy,0.99±1.13%,0.50±0.61%, and 0.38±0.50%, respectively.Compared with NART plans, the 3-phase radiotherapy protocol showed improved dosimetric results to the critical structures. The maximum dose to the brainstem and temporal lobe were decreased by 2.34±2.63 Gy and 0.73±1.06 Gy, respectively (P=0.015 and 0.046, respectively). The mean doses to the Supragolttis and glottis were significantly decreased by 3.95±3.99 Gy and 2.98±3.85 Gy, respectively. Dmean and V30 for the left parotids were significantly decreased by 1.36±1.90 Gy,3.12±4.42%, respectively. Median dose to the right optic nerve and V55 for the skin also had a statistically significant decline.The rest of the OARs, such as eyes, lens, optic chiasm, mandible and TMJ (Temporal-mandibular Joint), esophagus, oral cavity, and cochlea, had no significant dosimetric changes.Conclusions:In NPC treated with IMRT, adaptive replanning after 5 and 15 fractions results in an improvement in target coverage and conformity index, meanwhile decreased dose to some OARs, which make up the dosimetry change during the actual delivery.