Gamma-rays produced by cobalt-60 have a low average energy and exhibit a steep dose falloff. Our study demonstrated that Plans of Gamma Knife can achieve lower GI, volumes receiving10Gy and 5 Gy, as well as lung doses compared to VMAT and IMRT plans. In SBRT for lung tumors, Gamma Knife reduces the low-dose areas surrounding the PTVs and minimizes the dose of healthy lung tissue. Previous studies have demonstrated that Plans of Gamma Knife for SRS of brain lesions achieve lower GI compared to accelerator-based dynamic conformal arc plans. Additionally, Plans of Gamma Knife minimize the low-dose areas surrounding the target volumes and reduce the volume of healthy brain tissue receiving radiation29,30. Cao et al.18 compared various techniques applied to SRS for brain tumors, and the results indicated that Gamma Knife had the optimal GI, with the lowest V5Gy in normal brain tissues for small volume tumors (less than 20 cm³). In studies comparing the dose between Gamma Knife and Tomotherapy plans, Wu et al.22 reported that Tomotherapy plans for pancreatic tumors exhibited a larger volume of low-dose areas compared to Plans of Gamma Knife. Zhu et al.23 demonstrated that in the treatment for liver tumor, Tomotherapy can reduce V25-V40 of the liver but it increases V5-V10 of the liver. The study by Cao et al.20 demonstrates that the GI and the volume enclosed by the 10% prescription dose line for the Gamma Knife are significantly lower than those associated with Tomotherapy and conventional linear accelerators.Additionally, our study demonstrates that Plans of Gamma Knife can decrease maximum doses of OARs that do not overlap with the target volumes, including the esophagus, spinal cord, and heart. Balik et al.31 demonstrated that VMAT plans can reduce the maximum dose of the optic nerve compared to Plans of Gamma Knife in SRS for pituitary adenoma and vestibular schwannoma. However, in the study of Kim et al.32, no significant difference was observed in the doses of the cochlea and brain stem between VMAT plans and Plans of Gamma Knife. Wu et al.22 demonstrated that Tomotherapy plans can effectively decrease the maximum doses of the duodenum and stomach in pancreatic tumors compared to Plans of Gamma Knife. Likewise, Zhu et al.23 found that Tomotherapy plans can reduce the doses of the spinal cord, stomach, and left kidney in liver tumors compared to Plans of Gamma Knife. In the study conducted by Cao et al.20, the Plans of Gamma Knife resulted in the lowest mean dose to both the left and right kidneys, as opposed to the conventional linear accelerator and Tomotherapy, however, it yielded the highest maximum dose to the small bowel. The variation in results may be attributed to the fact that accelerator-based VMAT and IMRT plans allow for better adherence to dose restrictions of OARs through the motion of MLCs. However, Plans of Gamma Knife can achieve a steep dose falloff, which might counterbalance the advantage of OARs in VMAT and IMRT plans due to changes in distance from the PTVs.In Plans of Gamma Knife, both PTVs and GTVs are capable of achieving higher maximum and mean doses. Additionally, Plans of Gamma Knife deliver higher minimum doses to the GTVs and exhibit enhanced values of V150, V100, D5, and D2 for the PTVs. In SBRT, high doses within the target volumes is generally deemed acceptable and clinically advantageous, particularly when sparing functional normal tissue1. There is a disparity in the definition of isodose lines. Gamma Knife plans typically define the prescription dose at the 50% isodose line, allowing the Dmax to reach up to twice the prescription dose. In contrast, linac systems, equipped with MLCs, can precisely shape the beam to achieve superior dose uniformity. Linac plans generally do not impose strict constraints on the relationship between Dmax and the prescription dose, providing greater flexibility in dose distribution.The V100 and D98 for PTVs in Gamma Knife plans are lower than those observed in VMAT and IMRT plans. This discrepancy is primarily attributed to the sharp penumbra effect of the Gamma Knife, which can reduce the dose at the periphery of the PTVs, potentially leading to insufficient dose delivery at the target’s edges. Although the Gamma Knife’s penumbra is typically sharper and more precise than that of linear accelerators, this precision can sometimes result in inadequate dose coverage at the periphery of the target. Interestingly, there was no significant difference observed in the minimum dose of PTV between Gamma Knife plans and accelerator-based plans. However, the algorithms utilized in various treatment planning systems (TPS) differ. For instance, Monaco employs the X-ray Voxel Monte Carlo (XVMC) algorithm, which is considered most appropriate for lung tumor SBRT calculations1. In contrast, the RT Pro TPS uses Fast Photonalgorithm. Both algorithms incorporate heterogeneity correction. In our study, Monaco allows the inclusion of normal lung tissue with negligible density into the PTV, potentially resulting in a minimum dose as low as zero. This feature, however, is not present in the RT Pro TPS, which may account for some of the observed differences in dose distribution.As we know, dose optimization using inverse planning strategy is generally believed to enable accelerator-based VMAT and IMRT plans to achieve optimal conformity. Wu et al.22 and Zhu et al.23 have reported that Tomotherapy plans exhibit superior conformity to lesions when compared with Plans of Gamma Knife. In our study, we found no significant difference in CI between Gamma Knife and VMAT plans, or between Gamma Knife and IMRT plans. Two potential reasons may explain this: (1) Uniform spherical lesions in SBRT are inherently easier to achieve good conformity, thus the advantage of VMAT and IMRT plans in achieving conformity was not evident. (2) In Plans of Gamma Knife, the use of numerous small-diameter collimators can improve conformity but significantly prolong treatment time. HI is a crucial parameter for plan evaluation, and it is higher in VMAT and IMRT plans compared to Plans of Gamma Knife. However, in SRS/SBRT, accepting dose heterogeneity within the target volumes is permissible to achieve a steep dose falloff outside the target volumes1.Compared to Gamma Knife, accelerator-based VMAT and IMRT plans showed a significant reduction in delivery time, enhancing treatment efficiency. However, the extended treatment time in Plans of Gamma Knife brings certain challenges. Prolonged periods of immobilization during treatment may cause discomfort to the patient. Moreover, it is challenging to avoid intrafraction errors, leading to dose uncertainty.When comparing VMAT and IMRT plans, IMRT plans resulted in a lower mean dose of the ipsilateral lung compared to VMAT plans. However, there were no significant differences in V20 and V5. Previous studies have demonstrated that in conventional radiation therapy for lung tumors, the potential increase in low-dose areas in healthy lung tissue caused by VMAT plans is a concern33,34. As a result, conventional radiotherapy for lung tumors typically opts for static irradiation, with the beams setup to shorten the penetration path of the radiation in healthy lung tissues as much as possible. Nevertheless, SBRT poses even greater demands. To achieve optimal conformity, steep dose falloff, and avoid “hot spots” on the skin, it is recommended to use multiple fields in IMRT plans for SBRT. Prioritizing the distance of the radiation path from normal lung tissue is not necessary. The choice of irradiation technique exerts a limited influence on lung dose.Additionally, VMAT plans demonstrated a reduction in the maximum dose to the spinal cord compared to IMRT plans. No significant differences were observed in the doses to other OARs. Furthermore, VMAT plans achieved higher D95 values compared to IMRT plans. There were no significant differences in other parameters of the PTVs and GTVs. The superior conformity of IMRT plans over VMAT plans contradicts previous assumptions. It’s crucial to note that the performance of both IMRT and VMAT plans is heavily reliant on the optimization functions and the specific characteristics of the tumor and surrounding normal tissues. As a result, it’s feasible that IMRT could outperform VMAT in terms of conformity in certain instances, despite VMAT generally being presumed to offer better or at least equal conformity34,35,36. Overall, both VMAT and IMRT plans can achieve comparable dose distributions in SBRT. When comparing delivery time and MUs, VMAT was found to be more complex but resulted in reduced treatment time.The current study has several limitations. The most notable limitation is the small sample size, which may affect the generalizability of our findings. Additionally, the study was conducted under specific experimental conditions that may not fully represent all clinical scenarios. Furthermore, the dose distribution can be influenced by the plan designer’s subjective factors and the settings of optimization parameters, which could potentially bias the comparison results between different techniques. To address these limitations, future research should aim to include a larger, more diverse sample to enhance the statistical power and generalizability of the results. Incorporating a wider range of variables and refining the experimental design could provide deeper insights into the observed phenomena. Moreover, in SBRT for lung tumors, the impact of respiratory motion cannot be overlooked. The randomness of positioning CT scans can cause tumor location deviations, resulting in underdosage or excess radiation to normal tissues4. Therefore, the implementation of respiratory motion management measures is recommended1. While this study did not thoroughly investigate the influence of respiratory motion on treatment precision and dose distribution, but only attempted to reduce target underdosage caused by respiratory motion by directly expanding the GTV. Common respiratory motion management techniques in accelerator include active breathing control (ABC), gating, tracking, abdominal pressure, and 4DCT4. However, the prolonged duration of Gamma Knife treatment renders it unsuitable for the implementation of ABC. Moreover, the Gamma Knife equipment does not meet the necessary requirements for the implementation of gating and tracking. Furthermore, the limited treatment space and prolonged treatment time of Gamma Knife equipment pose challenges for the implementation of abdominal pressure. Therefore, the use of 4DCT is recommended for precise tumor localization during various respiratory phases in the treatment of Gamma Knife.The dosimetric differences observed between Gamma Knife and accelerator-based SBRT techniques have significant clinical implications. Gamma Knife SBRT exhibits superior dose falloff characteristics, which may enhance local tumor control by delivering higher doses to the tumor while sparing surrounding healthy tissue. This advantage could potentially reduce the risk of local recurrence, especially for tumors near critical structures. Additionally, the reduced low-dose exposure to normal lung tissue with Gamma Knife SBRT may lower the incidence of radiation-induced pneumonitis and other pulmonary toxicities, benefiting patients with compromised lung function. However, the longer treatment duration associated with Gamma Knife SBRT poses challenges, including increased patient discomfort and potential intrafraction motion, which could impact dose delivery accuracy. In contrast, accelerator-based techniques such as IMRT and VMAT offer comparable dose distribution characteristics and may be preferable when treatment efficiency is a priority. These techniques can deliver effective doses within shorter treatment times, reducing patient discomfort and motion-related uncertainties. Therefore, personalized treatment planning is essential. Patients with centrally located tumors might benefit more from Gamma Knife SBRT due to its precision and steep dose gradients, which help protect nearby critical structures. However, for peripheral tumors, accelerator-based techniques such as IMRT and VMAT might be more suitable as they can deliver effective doses within shorter treatment times, reducing patient discomfort and motion-related uncertainties. Additionally, patients with significant comorbidities, such as cardiovascular or pulmonary diseases, may also benefit from shorter treatment durations provided by accelerator-based SBRT to minimize the burden of the treatment process. Further research with long-term follow-up is needed to fully understand the impact of these dosimetric differences on overall survival, late toxicities, and quality of life. By carefully considering these factors, clinicians can optimize SBRT for lung tumors, enhancing both efficacy and patient safety.
