For early breast cancer patients, radiation therapy following breast-conserving surgery is essential. The conventional fractionated radiotherapy (CFRT) in postoperative breast cancer patients involves delivering 50 Gy in 25 fractions over five weeks [1]. With the advancement of radiotherapy techniques, hypofractionated radiotherapy (HFRT) is being increasingly applied in the treatment of breast cancer, which delivers a total dose of 40–42.72 Gy in 15–16 fractions [2, 3]. HFRT aims to increase the dose per fraction while reducing the total dose, thus shortening overall treatment time. The study shows that HFRT can offer a curative effect comparable to, or in some cases, even superior to conventional radiotherapy [4, 5]. Simultaneous integrated boost (SIB) in radiotherapy involves delivering a higher dose to high-risk lesions while administering a specific baseline dose to the overall treatment area. Compared to sequential radiotherapy, SIB significantly shortens treatment time [6], while also being effective in treating tumors. HFRT with simultaneous integrated boost (HFRT-SIB) has a higher prescription dose, which may result in a higher dose to the heart. In recent years, deep inspiration breath hold (DIBH) has been shown to reduce the mean heart dose and mean left anterior descending artery dose [7]. However, a portion of patients cannot tolerate DIBH [8].

Intensity-modulated radiotherapy (IMRT), volumetric modulated arc therapy (VMAT), helical tomotherapy (HT), and Tomodirect (TD) represent the most advanced radiotherapy technologies currently in use. While VMAT and HT are known to improve target volume conformity and uniformity, they also increase low-dose radiation exposure to typical structures [9, 10], making some clinicians hesitant about using these modalities in patient treatment. In contrast, both IMRT and TD, as advanced fixed-field irradiation methods, can reduce the exposure volume to normal organs.

IMRT, as denoted in reference [11], utilizes a traditional C-arm accelerator and computer-controlled blades. These blades selectively block portions of the treatment volume and enable the delivery of radiation fields with varying intensities tailored to individual therapeutic needs. TD delivers radiation doses from fixed gantry positions, using a modulated irradiation approach through a moving bed and multileaf collimators. The adjacent leaf pairs of these collimators rapidly open and close to achieve robust modulation capability. Therefore, IMRT and TD exhibit distinct characteristics in radiotherapy.

Tang et al. [12] compared the application of TD and fixed-field IMRT (ff-IMRT) in breast cancer radiotherapy. However, their prescribed doses were 50.4 Gy in 28 fractions for the PTV and 60.2 Gy in 28 fractions for the PTV boost. Zeverino et al. [13] also compared TD with IMRT, but their prescribed dose was 42.4 Gy in 16 fractions for the PTV, with no PTV boost. Squires et al. [14] directly compared TD with IMRT; however, the prescribed dose was 50 Gy in 25 fractions, and the irradiation fields used in the study consisted of two tangent fields. Our study is the first to compare IMRT and TD for HFRT-SIB after breast-conserving surgery with left breast cancer.

In TD, collimators are required to prevent radiation leakage and protect normal tissues. It can adopt field widths (FW) of 1.0 cm, 2.5 cm, and 5.0 cm. The larger the FW, the more limited the planned modulation capability becomes, but the treatment time is shortened. When employing TD for breast cancer radiotherapy, Kadhim et al. [15] set the FW at 5 cm, resulting in shorter treatment times, whereas other studies [12, 14] set the FW at 2.5 cm, yielding better plan quality. This study compared TD (FW = 5 cm) with TD (FW = 2.5 cm) to determine a reasonable FW setting for HFRT-SIB.

Radiotherapy is an essential and important component of breast cancer comprehensive treatment [7]. Study has shown that the increase in coronary events is related to the mean heart dose (7.4 Gy) occurring within 5 years of radiotherapy [19]. IMRT and VMAT have been reported to minimize the high dose to the lung and heart. DIBH gating is an additional selection technology in breast cancer radiotherapy [20]. It has confirmed a reduction of 25–67% and 20–73% in mean heart doses and mean LAD doses by using DIBH in breast cancer radiotherapy [21]. However, some patients cannot tolerate DIBH. Although patients have undergone training, 29% of 72 patients cannot complete a DIBH CT scan [8]. Therefore, our research investigated the dosimetric comparison of TD and IMRT in left-sided breast cancer with free breathing.

HI for 49.5 Gy and 43.5 Gy in TD (FW = 2.5 cm) and TD (FW = 5 cm) were all slightly lower or equal to IMRT. This suggested that TD achieved superior dose uniformity. This finding aligned with previous studies [14, 15]. Research indicated that inhomogeneity within the target volume can increase the incidence of acute adverse events and negatively impact cosmetic outcomes in adjuvant breast cancer radiotherapy [22]. TD technology offers better dose uniformity, which is more beneficial to patients in terms of reducing acute radiotherapy reactions and improving skin cosmetic outcomes.

TD exhibited a lower CI than IMRT. Fiandra et al. [23] indicated that the poorer the target conformity, the greater the volume of OARs adjacent to the target that receive high radiation doses. This study also showed similar results. TD (FW = 5 cm) and TD (FW = 2.5 cm) demonstrated superior performance in terms of low-dose exposure to the lung and heart but were slightly superior to IMRT regarding high-dose exposure, as shown in Table 3. However, the high-dose volume for TD (FW = 5 cm) and TD (FW = 2.5 cm) remained within the dose-limiting range, having a minimal negative impact on patients’ radiation treatment.

V₅, V₁₀, V₂₀, V₃₀, and D_mean for the left lung and heart were all lower in TD than in IMRT, except for V₂₀ of the heart was higher in TD (FW = 5 cm) than IMRT. They are shown in Table 3. D_max and D_mean of CA in TD (FW = 5 cm) and TD (FW = 2.5 cm) were all lower than IMRT. A study has shown that V₅, V₂₀, and D_mean of the lung were associated with radiation-induced lung injury [24]. During radiotherapy for left-sided breast cancer, due to the proximity of the target area to the heart, ischemic events may be induced because the coronary artery and heart are irradiated [25]. Mireştean et al. [26] also indicated that special attention should be paid in the future to limiting the toxic effects of radiotherapy on the heart and coronary arteries in HFRT. In this conclusion, TD can offer better protection to the lungs and heart.

Since the total dose of HFRT for breast cancer was slightly smaller, V₃₀ of the lung and heart was smaller than that of CFRT. In the Pinnacle planning system, EUD (equivalent uniform dose) considers the corresponding volume values at various dose points (such as 5 Gy, 10 Gy, 20 Gy, and 30 Gy), minimizing V₅, V₂₀, and V₃₀. However, this function is not available in the TD planning system. TD planners should pay more attention to it and increase its proportion, which would further reduce the value of V₃₀ relatively.

When comparing TD (FW = 5 cm) and TD (FW = 2.5 cm), minimal differences were observed in terms of homogeneity and conformity of the target and protection of the left lung, and LAD, with the latter being slightly superior. D_max of the contralateral breast were 2.700 Gy and 2.615 Gy in TD (FW = 5 cm) and TD (FW = 2.5 cm), while D_mean of the contralateral breast were 0.285 Gy and 0.240 Gy. Although D_max and D_mean of the contralateral breast in TD (FW = 5 cm) were slightly higher, the differences were minimal. When creating a tomotherapy plan with TD (FW = 5 cm), the beam angles can be adjusted to reduce the dose received by the contralateral breast. However, this approach may compromise the plan quality for the target volume and increase the dose delivered to other OARs. However, in our study, the doses received by the OAR were far below the constraint values, and there was still room for reducing the dose to the contralateral breast.

Fields et al. [27] indicated that increasing the value of FW can reduce dose hotspots outside the target volume. In this study, there was no significant improvement in the maximum dose to OARs outside the target volume when comparing TD (FW = 5 cm) with the other two. A possible distinction could be that the technique used in the literature was Tomotherapy Direct–Three Dimensional Conformal Radiation Therapy (TD-3DCRT), whereas in this study, the technique employed was TD with intensity-modulated.

However, the average treatment time for TD (FW = 5 cm) was 5 min, while for TD (FW = 2.5 cm), it was 9 min. TD (FW = 5 cm) can significantly reduce treatment time while having minimal impact on plan quality. Therefore, in general, when employing TD technology for HFRT-SIB, FW can be chosen as 5 cm.

TD technology can better protect cardiopulmonary function without compromising the quality of the target area. In general, when employing TD technology for HFRT-SIB, FW can be chosen as 5 cm.

TD, TomoDirect; IMRT, intensity-modulated radiotherapy; HFRT-SIB, hypofractionated radiotherapy with simulation integrated boost; OAR, organs at risk; Vx, The percentage of receiving the x value of dose; FW, field width; D_max, maximum dose; D_mean, mean dose; CFRT, conventional fractionated radiotherapy; HFRT, hypofractionated radiotherapy; VMAT, volumetric modulated arc therapy; HT, helical tomotherapy; CT, computed tomography; PGTV, planning gross target volume; CTV, clinical target volume; PTV, planning target volume; CA, coronary artery; CI, conformity index; HI, homogeneity index; PTV-PGTV, the area included in the PTV but not directly encompassed by the PGTV; EUD, equal uniform dose; TD-3DCRT, Tomotherapy Direct–Three Dimensional Conformal Radiation Therapy.

The datasets generated and analyzed during the present study are available from the corresponding author upon reasonable request.

YNB designed the study; JW performed the investigation; JXG, ZHL, and JGZ collected the data; LHW and XLW contributed to the data analysis. All authors contributed to editorial changes in the manuscript. All authors read and approved the final manuscript. All authors have participated sufficiently in the work and agreed to be accountable for all aspects of the work.

This study was approved by the ethics committee of the Affiliated Hospital of Inner Mongolia Medical University and ID: KY2025162. All procedures performed in studies involving human participants were in accordance with the ethical standards of the institutional and/or national research committee and with the 1964 Helsinki Declaration and its later amendments or comparable ethical standards. Informed consent was waived by the ethics committee of The Affiliated Hospital of Inner Mongolia Medical University due to the retrospective nature of the study.

Not applicable.

This research received no external funding.

The authors declare no conflict of interest.