Accumulated Dose of Intensity-Modulated Radiotherapy for Head and Neck Cancer Using Deformable Registration of Two Sets of Computed Tomography Images

Purpose: The aim of this study was, using deformable image registration (DIR), to evaluate alteration of dose distribution caused by patient’s anatomical structure changes during a two-phase intensity-modulated radiotherapy (IMRT). Methods: IMRT consisted of an initial plan delivering 53 Gy to gross tumor volume (GTV) and 45 Gy to elective volumes and a boost plan delivering 16.96 Gy to GTV. The subjects were 10 patients with head and neck cancer who underwent computed tomography (CT) scans twice (first CT before treatment and second CT before boost). A sum of the initial and the boost plans for the first CT was Original total plan. Using DIR, the original boost and a modified new boost plan were recalculated on the second CT and summed with the initial plan to create total plans: DIR plan and modified DIR plan. Results: Mean dose (Dmean) of the ipsilateral and contralateral parotids were increased by 8.0% (P<0.01) and 6.8% (P<0.05) in DIR plan compared with Original total plan. Compared with DIR plan, modified DIR plan reduced Dmean of the ipsilateral parotid (P<0.01). Dose to 95% of the volume (D95) to clinical target volume for GTV (CTV1) of DIR plan was significantly higher than that of Original total plan (P<0.01) and modified DIR plan (P<0.01). Conclusions: Dose summation using DIR demonstrated that the body shrinking during IMRT significantly increased the doses of both parotids and CTV1. Modified DIR plan compensated the increases in doses of the ipsilateral parotid and CTV1.