Dosimetric Evaluation of Body Contour Changes to Target Volumes and Organs at Risk for Cervix and Head and Neck Radiotherapy Plans

Purpose: To investigate how much dose discrepancy would be caused by the anatomy changes during the radiotherapy (RT) course. Methods: Ten cervical cancer and ten nasopharyngeal carcinoma (NPC) CT datasets from RT patients were enrolled. The body contour from different directions changed to simulate the weight loss or gain for cervical cancer patients, who had been treated with external-beam RT using intensity-modulated radiation therapy (IMRT) or volumetric modulated arc therapy (VMAT). Moreover, the body contour from facial and shoulder superior-inferior positional change had been also assessed for NPC patients using IMRT or VMAT. The new CT (n-CT) was generated by the body contour changes with different directions based on original CT datasets. The dosimetric parameters to target volumes and organs at risk (OARs) were evaluated in Eclipse based on n-CT. Results: The target volumes and OARs were influenced by the body contour changes. Body contour expansion resulted in coverage loss, whereas body contour shrinkage increased the dose to the OARs. These findings were generally consistent for both IMRT and VMAT plans. Over a course of research, the dose to 95% of the target volumes for cervical cancer decreased by up to 2.83% per cm for IMRT and 2.87% per cm for VMAT (P < 0.05). And the influence on H&N plans was that the dose to 95% of the target volumes (low risk regions) decreased by up to 4.45% per cm. Conclusions: The RT staff could determine whether resimulation and replaning or not according to which body contour directions were changed.