A prospective study on volumetric and dosimetric changes during intensity-modulated radiotherapy for nasopharyngeal carcinoma patients

Background and purpose: Significant tumor shrinkage and weight loss may occur during Intensity-modulated radiotherapy (IMRT) for nasopharyngeal carcinoma (NPC). This study aims to evaluate the dosimetric effect of volumetric changes on target volumes and organs at risk (OARs) during IMRT, using reassessment of computed tomography (CT) and magnetic resonance imaging (MRI). Material and methods: Nineteen loco-regionally advanced NPC patients treated with IMRT were recruited prospectively. Repeat planning CT and MRI were acquired at 30 and 50 Gy intervals. Recontouring of target volumes and OARs was based on the fused CT-MRI images. Hybrid plans with recontouring were generated. The assessment of volumetric and dosimetric changes was performed by comparing the hybrid plans with the original plan. Results: There was volume reduction of target volumes and parotid glands over the course of IMRT. Relative to the original plan, the hybrid plans demonstrated significantly higher dose to most of target volumes with greater dose inhomogeneity, higher maximum doses to the spinal cord and brainstem, and higher median doses to the parotid glands. Conclusions: Replanning with repeat CT and MRI scans at 30 Gy is essential to keep a satisfactory dose to the target volumes and avoid overdosing the OARs.