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376volume (PTV) should be 95%-107% of the prescribed dose5). In this situation, a method to reduce hotspots should be used, even in WBI, to reduce the risk of cognitive impairment as much as possible.The field-in-field (FIF) technique has been used to reduce unnecessarily high doses6, 7). To apply the FIF technique to WBI, several beams are added to the opposed lateral fields as sub-beams. In the sub-beams, the hotspot regions are blocked to reduce the dose in these regions7-9). In this tech-nique, additional time is required to create the sub- beams manually. If treatment planning using the FIF technique is automated, the burden of treat-ment planners is greatly reduced without compro-mising the quality of the treatment plans10, 11).Several studies have been conducted on automatic planning for conventional techniques, including the FIF technique11-14). Kim et al. retrieved Digital Imaging and Communications in Medicine (DICOM) data from a treatment planning system (TPS) and created FIF plans for breast-conserving therapy12). Yu et al. developed an automatic multileaf collimator shaping technique for WBI using deep learning14). Dth is the dose threshold for the hotspot regions and fixed at 105% of the prescribed dose at the isocenter in this study. D95% is the dose covering 95% of the PTV. Dmax is the maximum dose in the original plan. Dith is the intermediate threshold defined as Dith=(Dmax+Dth)/2.Figure 1　Flowchart of the semiautomatic field-in-field (FIF) techniqueThey adopted a simple two-opposing-lateral-field technique, and therefore, obtained the relatively high average max dose of approximately 110%.In this study, we propose a semiautomatic treat-ment planning method for the FIF technique for WBI using an application programming interface provided by a TPS manufacturer without output-ting DICOM data outside the TPS.Overview of the proposed methodIn the FIF technique for WBI, we developed an automatic technique for creating sub-beams, and adjusting beam weights to make hotspot regions disappear. Because we only automated the sub-beams creation and weight adjusting for the FIF technique, we describe our method as semiau-tomatic. In an automatic planning, whole planning process from the creation of main beams will be automated. The hotspot regions were defined as regions receiving dose above a predefined threshold Dth. Figure 1 shows an outline of the proposed method. The input of the method (an original treat-Materials and Methods