Dosimetry difference between field-in-field intensity modulated radiation therapy and fixed field inversely optimized intensity modulated radiation therapy in whole brain radiotherapy

  Objective  To compare dosimetry difference between field-in-field intensity modulated radiation therapy (FIF-IMRT) and fixed field inversely optimized intensity modulated radiation therapy (FFIO-IMRT) in whole brain radiotherapy.

  Methods  Totally 20 patients with whole brain radiotherapy were selected. The Pinnacle³ 9.10 treatment planning system was used to design plan. 6 MV X-ray was used for both two plans, and the prescription dosage was 40 Gy, with 2 Gy per time for 20 times totally. FIF-IMRT plans to select 270 and 90 degrees as the main fields, adjust the collimator angle appropriately to make the eyes overlap, and manually add 1 or 2 sub-fields in each main field direction to reduce the high dose. FFIO-IMRT plans to equalize the intensity of 7 fields and achieve the clinical dose distribution by repeatedly changing the optimized parameters. The dosimetric parameters of two radiotherapy schemes were compared.

  Results  Compared with FFIO-IMRT, the V_105% and D_50% of the planned target area (PTV) in FIF-IMRT increased significantly, while D_min, D_max, V_95%, V_110%, D_2%, D_98% decreased significantly (P < 0.05). CI of FIF-IMRT was significantly lower than that of FFIO-IMRT, but HIV was significantly higher than that of FFIO-IMRT (P < 0.05). The number of planned subfields, machine hops MU, treatment time and planning design time of FIF-IMRT were significantly lower than those of FFIO-IMRT (P < 0.05). The FIF-IMRT plan was significantly lower than the FFIO-IMRT plan in terms of organ-threatening left and right crystals, right and left eyeballs, spinal cord and right optic nerves, especially in crystals. There were significant differences except for left and right optic nerves between two plans (P < 0.05).

  Conclusions  ① Both treatment plans could meet the requirements of clinical dosimetry. ② Although the CI of FIF-IMRT is worse than FFIO-IMRT, but FIF-IMRT is better in protection of organs. ③ Since the single treatment time of FIF-IMRT significantly shorten, the errors caused by organ movement are reduced, the loss of MLC is reduced, and the execution efficiency of the machine is improved. ④ The FIF-IMRT plan is simple designed and easy to implement in primary hospitals. Therefore, the FIF-IMRT is recommended for whole brain radiotherapy.