Dosimetric comparison of different volumetric arc therapy (VMAT) plans created according to different plan parameters in hippocampus-sparing whole brain radiotherapy

"Dosimetric comparison of volumetric arc therapy plans created according to different plan parameters in hippocampus shielded whole brain radiotherapy", Trakya University, Institute of Health Science, Health Physics Department, Master Thesis, Edirne, 2025. Background and Aim: The aim of this study was to compare the target volume and critical organs (lenses, optic nerves, optic chiasm and hippocampus) in virtual treatment plans prepared with Volumetric Arc Therapy technique for hippocampus protected prophylactic cranial irradiation (PCI) in 20 patients receiving Whole Brain Radiotherapy. Material and Methods: Planning tomographs of 20 patients who received Whole Brain Radiotherapy in Trakya University Faculty of Medicine, Department of Radiation Oncology were obtained and evaluated retrospectively. In the plans, dose criteria were determined according to RTOG 0933 protocol and 30Gy dose was determined to be given in 10 fractions. For virtual plans, contours of the whole brain, hippocampus and critical organs were made. A 3 mm margin was given to the hippocampus and the planned risky organ volume hippocampus was created. The planning body risk volume hippocampus was subtracted from the whole brain volume with a 2 mm margin and the remaining whole brain planning target volume volume was created. Six different volumetric arc therapy plans with different gantry, collimator and table angles were optimized for each patient. In our study, a total of 6 different volumetric arc therapy plans, 3 concentric (coplanar) and 3 non-centric (non-coplanar), were created for each patient. In the plans, care was taken to ensure that the target volume and critical organs received doses within the limits determined according to RTOG 933 criteria. Results: When the data of all 6 plans were analyzed, it was observed that the doses to the hippocampus, right and left optic nerve, optic chiasm, right and left eye, right and left lens did not exceed the critical dose limits. The mean values of Dmin for hippocampus were 914.49Gy, 960.95Gy and 976.67 Gy in the 3 coplanar volumetric arc therapy plans with hippocampus protection, while the mean values were 916.14 Gy, 942.04Gy and 924.98Gy in the 3 non-coplanar volumetric arc therapy plans, respectively. The mean values of Dmax were 1443.29Gy, 1457.60Gy and 1525.82Gy in the 3 coplanar volumetric arc therapy plans and 1388.38Gy, 1380.45Gy and 1381.13Gy in the 3 non-coplanar volumetric arc therapy plans, respectively. Conclusion: In our study, lower hippocampus D%100 doses were obtained in VMAT plans with non-coplanar features compared to other plans. However, no significant difference was found between the results. When hippocampus Dmax doses were analyzed, similarly, doses in non-coplanar VMAT plans were found to be significantly lower than other co-planar plans. When all plans are evaluated, it can be seen that non-coplanar treatment plans provide an advantage in both target volume doses and risky organ doses, including the hippocampus. However, high treatment duration and high MU values should be taken into consideration. Keywords: Radiotherapy, hippocampal sparing, volumetric-modulated arc therapy, whole-brain radiotherapy