Precise target localization is an important goal in the treatment of cervical cancer. Uncertainties may stem from both inter- and intra-fractional motion, with inter-fractional motion potentially exerting a more significant influence [14]. Inter-fractional motion can occur due to the tumor and surrounding organ movements, deformations, and volume changes; therefore, to ensure high precision, the rational application of margins to the CTV is a crucial approach [8]. Meanwhile, IMRT is an essential tool for effectively addressing and minimizing uncertainties during treatment [15].

In this study, we evaluated the PTV motion during RT for cervical cancer with daily iCBCT. Previous studies have paid more attention to the uniform margin of the CTV [9]. Our study found that the motions of the target volume were different in six directions. The change in the CTV-C volume was larger than that of the CTV-U. According to the T stage, the CTV-C and CTV-U of patients with stage T1 showed greater motion. In addition, the results showed that the movements of the cervix and uterine corpus were different. A uniform 15-mm margin would have missed some of the CTV-U and CTV-C volumes. Thus, personal margins during RT for cervical cancer are necessary.

Different image-guided systems for RT have been applied in the study of the ITV for cervical cancer, including magnetic resonance imaging (MRI) and cone beam computed tomography (CBCT) [16]. Chan et al. [14] reported the movement of the external cervix oscillated between 10 mm and 15 mm during EBRT with weekly MRI scans, suggesting the utilization of daily images to maximize the advantages of high-precision radiation in patients with cervical cancer. Langerak et al. [17] conducted a comparison between the planning CT scans of cervical cancer and daily CBCT to identify variations. They observed mean shifts of 0.4 mm, 1.0 mm, and − 3.9 mm in the RL, AP, and SI directions, respectively. However, the CBCT images exhibited suboptimal quality, which was attributed to scattering and artifacts [18]. As a technological advancement, iCBCT attains uniform imaging with reduced noise and enhanced quality [19]. To our knowledge, there is limited research using iCBCT images to investigate the motion of the target area in cervical cancer.

In addition, several studies have focused on the movements of the uterine corpus and cervix. The average displacements documented for the uterine corpus varied between 3.3 mm and 14.2 mm in the AP direction, 6.1 mm to 9.5 mm in the SI direction, and 0.7 mm to 6.5 mm laterally [20]. As for the cervix, the amplitude of the mean inter-fractional movements ranged from 2.4 mm to 16 mm in the AP direction, 1.5 mm to 8 mm in the SI direction, and 0.3 mm to 10 mm laterally [21]. Taylor et al. [22] demonstrated that the uterus is prone to significant displacements whereas cervical movement is comparatively less pronounced. This is similar to the results of our study. Due to bladder and rectum filling, large movements occur in the SI and AP directions compared to the LR direction [23]. Researchers propose an asymmetrical margin with the CTV–PTV margin of the uterus and cervix [22]; however, the disadvantage of this strategy was insufficient target coverage [24]. Therefore, further research is required.

Tumor regression has the potential to significantly impact the positioning of organs and structures during RT for cervical cancer [20, 25]. Beth et al. [7] investigated tumor regression and cervical mobility in the context of concurrent chemoradiotherapy for cervical cancer. Their study revealed that the average cervical volumes before and after receiving 45 Gy of EBRT were 97.0 cc and 31.9 cc, respectively, indicating a mean volume reduction of 62.3%. The regression observed in their research resulted in a change in the position of the cervix. In our study, the mean reduction in the CTV-C and CTV-U volumes on iCBCT images was 44.3% and 20.3%, respectively. Notably, the mean change in the CTV-C volume was larger than that for the CTV-U. In addition to tumor regression, the T stage of the tumor may exert an influence on the mobility of the target area. To our knowledge, this study is the first to use daily iCBCT imaging to evaluate the relationship between changes in the target and tumor stage. Different T stages of the tumor represent the relationship between the lesion and surrounding organs [26]. In patients with stage T1, the variation in the CTV-C and CTV-U was greater, especially in the AP and LR directions. One reason may be that the carcinoma is strictly confined to the cervix, rectum, and bladder in stage T1, having a significant impact on the target volume [27].

Some studies involving patients with cervical cancer have explored the inter-fractional motion of the cervix using imaging modalities such as computed tomography, MRI, and portal imaging with fiducial markers [14, 28, 29]. Motion was assessed either directly at the cervix (center of mass, cervical boundaries) or by employing fiducial markers as a surrogate for cervical motion [21]. We developed a novel procedure using Python to automatically analyze the daily iCBCT images and understand the boundaries of the CTV expansion. In comparison to previous studies, our research offers a more convenient and accurate means of assessing the mobility of the CTV in cervical cancer.

The requirement for uniform large margins to accommodate the movement of the target volume results in acute gastrointestinal and genitourinary toxicity [30]. Applying asymmetric planning margins around subregions of the CTV is a potential approach to reducing margins [9, 31]. A reduction in margin implies a decreased dose of the OARs, and the incidence of adverse reactions exhibited a downward trend [32]. Advancements in technology have led to the implementation of adaptive radiotherapy (ART), which could tailor treatment to these changes and provide more benefits [31]. Previous studies have shown the dosimetric benefits of reduced margins in comparison to larger conventional margins through daily ART in pelvic cancers [33]. Meanwhile, Wang et al. [32] found that PTV margins could reduce to 5 mm by daily online ART, which significantly decreases the dose to critical organs at risk and potentially lead to a lower incidence of acute toxicity in cervical cancer.

Nevertheless, this study has several limitations. While our study entailed a meticulous analysis of a substantial volume of iCBCT scans, the patient sample size was limited, leading to corresponding imprecision in our estimates. In addition, the disadvantage of iCBCT is poor soft tissue display, which has some influence on contouring. Finally, our novel program can be further confirmed by more data analysis.