Investigating the number of radiation fields in intensity-modulated radiotherapy plans of optic nerve sheath meningioma patients using dose gradient index

Purpose: In optic nerve radiotherapy, vital organs are very close to the target volume, they are highly sensitive to radiation and have low dose tolerance. In this regard, evaluating dose fall-off steepness around the target volume is required to assess various intensity-modulated radiation therapy (IMRT) plans in the treatment of the optic nerve sheath meningioma (ONSM) patients. Materials and Methods: Thirteen ONSM patients were analyzed with three IMRT techniques, including three (IMRT-3F), five (IMRT-5F), and seven fields (IMRT-7F). These plans were studied using D-mean, D-max, D-2%, D-98%, V-100%, uniformity index (UI), homogeneity index (HI), conformity index (CI), and specifically the dose gradient indices (DGIs). Results: The values of D-max and D-mean for IMRT-3F, IMRT-5F and IMRT-7F were (5637.42 +/- 57.08, 5322.84 +/- 83.86), (5670.51 +/- 67.87, 5383.00 +/- 58.45), and (5692.99 +/- 31.65, 5405.72 +/- 51.73), respectively, which were increased with increment in the number of IMRT fields from 3 to 7. The UI and HI indices were significantly different between IMRT-3F and IMRT-7F (p = 0.010 and p = 0.005, respectively), and CI was close to the ideal value (0.99 +/- 0.01) in IMRT-7F. The significant findings of the dose gradient indices represented smaller values in IMRT-7F, which led to a faster dose fall-off, particularly at the 70%-85% isodose levels around the target. Conclusion: Increasing the number of radiation fields in IMRT treatment plans of ONSM patients had a considerable difference in both the dosimetric parameters of the target volume and at-risk organs, as well as the dose gradient indices. Overall, IMRT-7F could be considered as a preferred technique in the treatment of this meningioma.