Dose Deformation and Accumulation for Adaptive Radiotherapy in Nasopharyngeal Carcinoma

To realize dose deformation and accumulation using image registration technology, and evaluate the anatomic changes and corresponding dosimetric variations for patients with nasopharyngeal carcinoma (NPC). 15 patients with NPC treated with IMRT were selected in this study, the original treatment plan(Plan1(CT1)) based on the first CT scan(CT1) and the second plan(Plan2(CT2)) based on the repeated CT scan(CT2) were calculated using the planning system Raystation Version 4.5 (RaySearch Laboratories AB, Stockholm, Sweden). Plan1(CT2) was acquired by deforming the dose of Plan1(CT1) to CT2 using the hybrid deformation image registration method, which express to perform the original plan without re-planning. Assessing the dose distribution and deciding whether to modify the plan, if need it, Plan2(CT2) was planned based on CT2 images, Plan1+2(CT2) that expressing the actual exposure dose after modification was generated by accumulating the dose of Plan1(CT2) and Plan2(CT2). The dose distribution was compared among Plan1(CT1), Plan2(CT2) and Plan1+2(CT2). Compared with CT1, the volume of GTVnx, GTVnd and CTV2 in CT2 decreased by 36.3%, 36.3% and 49.8% respectively, and the left and right parotid gland decreased in volume by 34.6% and 34.6%. Compared with Plan1(CT1), due to the changes of organ size and location, the dose to target organ were significantly decreased in Plan1(CT2) (the dose received by 95% of the target (D95) to GTVnx, GTVnd and CTV2 were decreased by 2.9%, 6.2% and 3.3%, respectively), and the dose to normal tissue were increased (the dose to right parotid gland, brain stem and spinal cord were increased by 5.5%, 1.4% and 8.9%, respectively). With repeated CT and re-planning after 20 times as shown in Plan1+2(CT2), the dose delivered to target were significantly increased (Compared with Plan1(CT2), the dose in Plan1+ 2(CT2) received by 95% of the target (D95) to GTVnx, GTVnd and CTV2 were increased by 2.9%, 3.3% and 3.7%, respectively), and at the same time the dose to normal tissue were decreased as much as possible (the mean dose to the left and right parotid gland were decreased by 3.0% and 7.4%, and the max dose to the brain stem and spinal cord were decreased by 14.1% and 7.7%). During the course of adaptive radiotherapy, the volume of parotid gland and target significantly decreased. Re-planning after 20 times could ensure adequate dose to target and safe dose to the normal tissues.