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Step 1. As shown by the dose distributions in the axial plane in Figure 7a and 6c, the 100% isodose line did not change in Step 1 even after the hotspot region exceeding 105% of the prescription dose disappeared using the semiautomatic FIF tech-nique. The dose distributions for the manual and FIF plans were almost the same, as indicated in Figure 8　Dose volume histograms (DVHs) of planning target volume (PTV) in the original, manual field-in-field (FIF), and semiautomatic FIF plans. (a) DVHs of PTV in the original (dashed line), manual FIF (dotted line), and two-sub-beam FIF plans in Step 1 (solid line). (b) DVHs of the PTV in the original (dashed line), manual FIF (dotted line), FIF (solid line), and four-sub-beam FIF plans in Step 2 (FIF-4SF, dashed-dotted line).Figure 9　Dose distributions of the original, manual field-in-field (FIF), and semiautomatic FIF plans with the different numbers of sub-beams (FIF and FIF-4SF). (a) Field shape and dose distributions of the original plan. The upper, middle, and lower panels show the field shape, 3D dose distribution in the beam’s eye view, and 2D dose distribution in a slice, respectively. The slice position is indicated in the middle panel with a white dashed line. (b) Same as (a) but it is for the manual FIF plan. (c) Same as (a) but it is for the two-sub-beam FIF plan in the Step 1 (FIF). (d) Same as (a) but it is for the four-sub-beam FIF plan in the Step 2(FIF-4SF). In the 2D dose distributions, the yellow lines are the 100% isodose lines, and the pink lines are the 105% isodose lines. The doses are relative to the prescribed dose.Figure 7b and c. The dose volume histograms (DVHs) of the PTV of the original, manual FIF, and FIF plans are shown in Figure 8a. Dmax remark-ably decreased in the manual FIF and FIF plans, while there was almost no change in D95%.One case was categorized into Step 2 in this study. Figure 9 shows the dose distributions in the 383