CBCT-guided radiotherapy for locally advanced head and cancer: dosimetric impact of weight loss on VMAT and IMRT plans for selected organs at risk structures (OARs)

Purpose: It is common for head and neck patients to be affected by time trend errors as a result of weight loss during a course of radiation treatment. The objective of this planning study was to investigate the impact of weight loss on volumetric modulated arc therapy (VMAT) as well as intensity modulated radiation therapy (IMRT) for locally advanced head and neck cancer using automatic co-registration of the cone beam computed tomography. Materials and methods: A retrospective analysis of previously treated IMRT plans for ten patients with locally advanced head and neck cancer was done. A VMAT plan was also produced for all patients. We calculated the dose-volume histograms (DVH) indices for spinal cord planning at risk volumes (PRVs), the brainstem PRVs (SC + 0.5 cm and BS + 0.5 cm, respectively) as well as mean dose to the parotid glands. Results: The results show that the mean difference in dose to the SC + 0.5 cm was 1.03% and 1.27% for the IMRT and VMAT plans, respectively. As for dose to the BS + 0.5, the percentage difference was 0.63% for the IMRT plans and 0.61% for the VMAT plans. The analysis of the parotid gland doses shows that the percentage change in mean dose to left parotid was -8.0% whereas that of the right parotid was -6.4% for the IMRT treatment plans. In the VMAT plans, the percentages change for the left and the right parotid glands were -6.6 and -6.7% respectively. Conclusions: This study shows a clinically significant impact of weight loss on DVH indices analysed in head and neck organs at risk. It highlights the importance of adaptive radiotherapy in head and neck patients if organ at risk sparing is to be maintained.