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ment plan) was a conventional treatment plan using two-opposing-lateral fields for WBI, which was created in a TPS (Eclipse version 11.0; Varian Medical Systems, USA). The method consists of two steps: in Step 1, an algorithm for creating sub-beams and adjusting beam weights is applied to the original treatment plan and an FIF plan with two sub-beams is created with a predefined threshold Dth for hotspot regions; and in Step 2, the dose index of the PTV in the FIF plan is evaluated to determine whether further adjustment is required. When the reduction in the D95% which represents the dose covering 95% of the PTV in Step 1 exceeds 1%., FIF treatment planning with four sub-beams is performed to alleviate the reduc-tion of the D95%.Steps 1 and 2 were implemented in a research version of the TPS (Eclipse, version 13.7; Varian Medical Systems, USA) using the Eclipse Scripting Application Programming Interface (ESAPI) and automatically performed. The programming language used in the ESAPI was C# (Microsoft Corporation, USA). The research version of Eclipse (13.7) was used because Eclipse version 11.0 did not allow a change of a treatment plan using ESAPI. We retrospectively applied the automatic FIF script to the treatment plans of patients treated using the two-opposing-later-al-field technique.Figure 2　Beam’s eye views with the multileaf collimator shapes of (a) the main beam and (b) sub-beam in the field-in-field plan. The projection of the hotspot region in the original plan is indicated by Hotspot in (b).Automatic FIF techniqueFIF plan in Step 1In the original treatment plan, hotspot regions that received a dose above Dth were identified in the three-dimensional (3D) dose distribution calcu-lated in the TPS. The two beams in the original plan (main beams) were duplicated as sub-beams, which initially had the same multileaf collimator (MLC) positions as the main beams and no beam weights. In the FIF technique, the fields of sub-beams are shaped to block the hotspot region using the MLC. Figure 2 shows the beam’s eye views (BEVs) of (a) one of the main beams and (b) the corresponding sub-beam. The positions of the MLC in the sub-beams were determined to block the hotspot region without a margin in the BEV. Because the hotspot region tends to be located in the frontal and occipital lobes at the edge of the irradiation field, the isocenter was not blocked in the sub-beams in almost all cases.To identify the hotspot region, the 3D dose distri-bution of the original plan was extracted using the ESAPI. In the ESAPI, the 3D dose distribution was defined in the DICOM coordinate system (DCS) fixed in a patient. The hotspot region defined in the DCS was projected onto the BEV using the following three steps: (1) the coordinate transfor-mation of translation from the DCS (x, y, z) to the coordinate system (X, Y, Z), where the isocenter was at the origin (isocenter coordinate system, 377