Dosimetric advantages of O‐ring design radiotherapy system for skull‐base tumors

The purpose of this study was to investigate whether a new O‐ring design radiotherapy delivery system has advantages in radiotherapy planning for skull‐base tumors. Twenty‐five patients with skull‐base tumors were included in this study. Two plans were made using conventional (Plan A) or new (Plan B) techniques. Plan A consisted of four dynamic conformal arcs (DCAs): two were horizontal, and the other two were from cranial directions. Plan B was created by converting horizontal arcs to those from caudal directions making use of the O‐ring design radiotherapy system. The micromultileaf collimators were fitted to cover at least 99% of the planning target volume with prescribed doses, 90% of the dose at the isocenter. The two plans were compared in terms of target homogeneity, conformity, and irradiated volume of normal tissues, using a two‐sided paired t‐test. For evaluation regarding target coverage, the homogeneity indices defined by the International Commission on Radiation Units and Measurements 83 were [Formula: see text] (mean ± standard deviation) and [Formula: see text] , the conformity indices defined by the Radiation Therapy Oncology Group were [Formula: see text] and [Formula: see text] , and the Paddick's conformity indices were [Formula: see text] and [Formula: see text] , in Plans A and B, respectively. For evaluation of irradiated normal tissue, the Paddick's gradient indices were [Formula: see text] and [Formula: see text] in Plans A and B, respectively. All of these differences were statistically significant (p‐values [Formula: see text]). The mean doses of optic nerves, eyes, brainstem, and hippocampi were also significantly lower in Plan B. The DCA technique from caudal directions using the new O‐ring design radiotherapy system can improve target homogeneity and conformity compared with conventional DCA techniques, and can also decrease the volume of surrounding normal tissues that receives moderate doses. PACS numbers: 87.55.‐x, 87.55.D‐, 87.55.dk