## 4 Discussion  
The purpose of this study was to evaluate the facility-specific interplay effect in SSPT for different beam delivery modalities, including FB irradiation with and without rescanning, DIBH, and respiratory gating. A comparative analysis of these beam delivery modalities in terms of the dose distortion and irradiation time is presented in this section. Recommendations for improving beam delivery and limitations of this study are also discussed. The interplay effect was clearly manifested when the moving film was irradiated during FB without motion mitigation. In general, it was observed as hot and cold spots in the high-dose region (inside the PTV) and dose blurring along the motion direction in the low-dose region (outside the PTV). The hot and cold spots caused a decrease in dose homogeneity, and the dose blurring deteriorated the dose gradients (Figure 7, red plot) compared to the stationary dose distribution (Figure 7, magenta plot). The interplay magnitude increased with increasing motion amplitude, resulting in a 12% decrease in homogeneity and a 46% decrease in the GPR (3%/2 mm) relative to the stationary case for 20 mm pk-pk motion. It is generally accepted that rescanning reduces the interplay effect but does not improve the dose gradient [43]. This was confirmed by our results when the moving film was irradiated with rescanning. The implementation of rescanning during FB allowed for the partial mitigation of hot and cold spots in the high-dose region (inside the PTV), resulting in greater homogeneity than FB irradiation without rescanning (95% vs. 88% for 12 mm pk-pk motion), but did not affect the dose blurring that ruined the dose gradient and increased the dose in the low-dose region (outside the PTV, Figure 7, green plot vs. red plot). Rescanning did not improve the dose gradient, but made the dose distribution in the low-dose region more uniform (Figure 5h), in contrast to no rescanning (Figure 5e), where the shape of the distribution appeared uneven and varied between individual repeated measurements. The unevenness and variation in the shape of the low-dose region may be due to the random nature of the interplay effect, and it is expected that rescanning may make it more predictable. Rescanning improved homogeneity to a greater extent than the GPR because the GPR characterized not only the high-dose region but also the low-dose region where rescanning provided no benefit. The mitigation of motion effects with single-fraction rescanning was observed to be similar to that of non-rescanned fractionated irradiation [11]. It is expected that the dose distribution obtained with rescanning will be more uniform under clinical conditions than in our study because multifraction, multifield treatment plans are always used. The number of rescans is the most important adjustable patient-specific parameter [15] that determines the dosimetric and temporal effectiveness of motion mitigation by rescanning. The typical number of rescans to achieve acceptable dose homogeneity with volumetric rescanning varies between studies from 2 to 10 [13,15,17,40,44]. In our study, we applied a hybrid approach with 5 full-volume rescans and 12 layered rescans for the high-energy layer. [PERSON] et al. [40] also used volumetric rescanning with five rescans for 6 mm pk-pk motion and a similar methodology for data analysis (the gamma index calculation with the 3%/2 mm criterion and an LDT of 10%). There were higher values of the GPR (3%/2 mm) in [PERSON]'s study than in our study (95.3% vs. 83.3%), despite the fact that we used the same number of full-volume rescans. The differences in the results between [PERSON]'s study [40] and our study may be related to the differences in the beam delivery systems, particularly the beam size (10.0-14.0 mm vs. 4.7-3.2 mm in the 70-120 MeV energy range) and beam profile (adaptive aperture vs. conventional Gaussian beam).