Dosimetric comparison of gamma knife and linear accelerator (VMAT and IMRT) plans of SBRT of Lung tumours | Scientific Reports

Scientific Reports volume 14, Article number: 22949 (2024) Cite this article

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This study evaluates dosimetric differences in Stereotactic Body Radiation Therapy (SBRT) for lung tumors using plans of Gamma Knife, and Volumetric Modulated Arc Therapy (VMAT), Intensity-Modulated Radiation Therapy (IMRT) plans based on Linear Accelerator, aiming to inform the reader of appropriate treatment strategy selection. Ten patients with 23 lung tumor lesions treated with SBRT at Zhongshan Hospital of Dalian University were analyzed. Plans of Gamma Knife, and VMAT, IMRT plans based on Linear Accelerator were created for each lesion, totaling 18 plans per type. Lesions were treated with 30–50 Gy in 5–10 fractions. Dosimetric parameters, including gradient index (GI), heterogeneity index (HI), conformity index (CI), and doses to the plan target volumes (PTVs), the gross tumor volumes (GTVs) and organs at risk (OARs) were compared. Plans of Gamma Knife showed superior HI and GI, higher PTV and GTV doses, and reduced doses to the ipsilateral and contralateral lungs, esophagus, spinal cord, and heart compared to VMAT and IMRT plans (p < 0.05). However, Plans of Gamma Knife required longer delivery times. When comparing VMAT and IMRT plans, VMAT plans had shorter delivery times than IMRT plans, but required more monitor units (MUs). Additionally, IMRT plans delivered a lower mean dose to the ipsilateral lung compared to VMAT plans. Gamma Knife SBRT plans achieves steeper dose falloff and minimizes radiation to normal lung tissue compared to VMAT and IMRT plans, but with longer delivery times. VMAT and IMRT plans displayed similar dose distributions for lung SBRT.

Stereotactic Body Radiotherapy (SBRT) originated from Stereotactic Radiosurgery (SRS). Combining high single doses, fewer fractions, small-field irradiation, stereotactic fixation devices, and Image-Guided Radiotherapy (IGRT) enables a steep dose falloff and a higher equivalent biological dose. Compared to traditional radiotherapy, SRS/SBRT significantly reduces the economic burden on patients1. Traditionally used for intracranial lesions, SRS employs invasive stereotactic head frame to ensure high precision in single-session radiotherapy2. With advancements in IGRT, tools like thermoplastic masks and vacuum bags are now adopted for patient immobilization. These non-invasive, reusable tools facilitate fractionated treatments3. Furthermore, the development of stereotactic body frames has increasingly met the requirements for precision, comfort, and repeatability in immobilization for extracranial tumors. 4D Computed Tomography (4DCT) can be used to monitor tumor motion associated with respiration, thereby enhancing the precision of tumor delineation4. The use of high dose rate photon beams, particularly in flattening filter free (FFF) mode, has been shown to improve treatment efficiency and reduce beam on time, which are particularly beneficial for SBRT delivery with high fractional prescription doses5. Additionally, the integration of 6D couches allows for precise patient positioning and motion management during treatment delivery, These couches allow for corrections in both translational and rotational positioning errors, improving patient setup accuracy6. Moreover, advancements in imaging technologies, such as four-dimensional cone-beam computed tomography (4D-CBCT), have further revolutionized the field of radiotherapy, enabling clinicians to monitor and adapt treatment plans in real-time7. Optical surface monitoring systems (OSMS) can provide accurate, real-time patient positioning and motion detection without additional radiation exposure8. The aforementioned developments in radiotherapy technology have laid a solid foundation for the application of SBRT in treating lung tumors.

SBRT has been extensively used to treat early-stage lung cancers and metastatic lung tumors, and it has been reported to achieve good treatment efficacy with minimal severe toxicity9,10,11,12,13. Currently, SBRT can be performed using various equipment, such as linear accelerators, Gamma Knife, Tomotherapy, or Cyberknife1. In linear accelerator-based SBRT, intensity-modulated radiation therapy (IMRT) and volumetric modulated arc therapy (VMAT) are two commonly used techniques that utilize inverse treatment planning and multileaf collimators (MLCs) movements to achieve complex dose distribution. VMAT has been shown to provide superior dose conformity and homogeneity while significantly reducing treatment time compared to IMRT. VMAT achieves this by continuously modulating the dose rate and gantry speed during treatment delivery14,15. Nonetheless, VMAT can lead to increased low-dose exposure to surrounding healthy tissues16. IMRT, on the other hand, offers excellent sparing of organs at risk (OARs) through its capability to deliver highly conformal dose distributions. However, IMRT involves longer treatment times and more complex planning processes14,15. Furthermore, the Gamma Knife employs gamma-rays with an average energy of 1.25 MeV from Cobalt-60 sources, ensuring high output stability. Additionally, the Gamma Knife offers high precision and steep dose gradients, minimizing exposure to surrounding normal tissues. At present, the integration of a rotational gantry within Gamma Knife system allows for flexible selection of irradiation angles, accommodating larger or complex-shaped tumor volumes17. This enhances dose uniformity and minimizes radiation exposure to surrounding healthy tissue. However, the treatment duration of Gamma Knife can be relatively long, which poses challenges for managing potential errors and ensuring consistent accuracy throughout the procedure. These dosimetric advantages and disadvantages highlight the need for a comprehensive comparison to optimize treatment efficacy and safety, thereby motivating the current study.

Although numerous studies have compared the dosimetric profiles of Gamma Knife and linear accelerators in the treatment of intracranial, liver, and pancreatic tumors 3 cm in longest diameter. J. Neurosurg.132, 1024–1032 (2019)." href="#ref-CR18" id="ref-link-section-d62960932e589">18, 14 cm(3)) recurrent glioblastomas. J. Radiosurg SBRT. 7, 233–243 (2021)." href="#ref-CR19" id="ref-link-section-d62960932e589_1">19,20,21,22,23, there remains a notable scarcity of research focused on the dosimetric comparison of SBRT techniques in the context of lung tumors. Furthermore, while VMAT and IMRT have been extensively analyzed and compared across various cancer sites14,15, there is a paucity of studies that directly address their dosimetric performance and clinical outcomes in lung SBRT. This study aims to compare the dosimetric differences among accelerator-based VMAT, IMRT, and Gamma Knife SBRT plans for lung tumor treatment. The findings will serve as a reference for selecting the most appropriate treatment strategies.

Ten patients who underwent SBRT for lung tumors at Zhongshan Hospital Affiliated with Dalian University between January 2022 and August 2023 were included in the study. A total of 23 lesions were included in the analysis. For all 23 lesions, treatment plans were designed using VMAT (VMAT_SBRT), IMRT (IMRT_SBRT), and Gamma Knife (γ_SBRT), respectively. Adjacent lesions for a patient were combined into a single plan. As a result, a total of 54 plans were obtained, including 18 VMAT_SBRT plans, 18 IMRT_SBRT plans, and 18 γ_SBRT plans. All lesions were treated with a total dose of 30–50 Gy delivered in 5–10 fractions. The research was approved by the Institutional Review Board of Zhongshan Hospital affiliated with Dalian University. All research was performed in accordance with relevant guidelines/regulations, and informed consent was obtained from all participants.

Thermoplastic masks, with or without vacuum bags, were used for patient immobilization. The patient is positioned in a supine position with their arms extended above the head and hands clasping the opposite elbow. Enhanced scanning was performed using a 4-row spiral Toshiba computer tomography (CT) machine (Toshiba Medical Systems Corporation, Japan), and images were acquired during free breathing. The scanning parameters were set as follows: tube voltage of 120 kV, tube current of 150 mA, and an image layer thickness of 2.5 mm. Images were transferred to the treatment planning system (TPS) in DICOM (digital imaging and communications in medicine) format to define the target volumes and organs at risk (OARs). The radiotherapy physicians delineated the GTV on the lung window of the planning CT images. The PTV was then generated by adding a 0.3–1.0 cm margin to the GTV. The definition of target volumes followed the guidelines outlined in the ICRU Report 8324. The organs at risk (OARs) considered in this study included the ipsilateral lung, contralateral lung, esophagus, heart, and spinal cord.

In this study, γ_SBRT plans were executed using the CybeRay stereotactic radiotherapy system (DAYI, China), which is equipped with 13 cobalt-60 radiation sources and features a source-to-axis distance of 60.8 cm. The system offers a range of seven collimators with diameters of 0.6 cm, 0.9 cm, 1.2 cm, 1.6 cm, 2.0 cm, 2.5 cm, and 3.5 cm, thereby allowing precise targeting of tumors of varying sizes. Additionally, the integration of a rotational gantry capable of a -245 to + 245degree rotation facilitates the flexible selection of irradiation angles, thereby accommodating larger or irregularly shaped tumor volumes. The Synergy accelerator (Elekta, Sweden) was utilized for delivering the VMAT_SBRT and IMRT_SBRT plans, employing Agility TM MLCs with a leaf width of 5 mm at the isocenter. Regarding the planning systems, the VMAT_SBRT and IMRT_SBRT plans were designed using the Monaco treatment planning system (V6.00.11, Elekta, Sweden), while the γ_SBRT plans were developed using the RT Pro TPS (V1.2.3, DAYI, China).

The planning CT scans, target volumes, and organs at risk (OARs) of all patients were imported into Monaco and RT Pro TPS, respectively. The VMAT_SBRT and IMRT_SBRT plans were designed using an inverse planning approach, employing 6MV X-rays with a maximum dose rate of 600 MU/min. The VMAT_SBRT plans utilized 1–2 coplanar full or partial arcs, while the IMRT_SBRT plans employed 6–9 equally distributed fields. Monaco employs the X-ray Voxel Monte Carlo (XVMC) algorithm, deemed most appropriate for lung tumor SBRT calculations1. In our study, The dosimetric objectives for VMAT_SBRT and IMRT_SBRT plans were as follows: (1) ensuring that ≥ 98% of the PTV received the 100% prescription dose, (2) limiting the Dmax of the PTV to 150% of the prescription dose, and (3) minimizing the dose to OARs.

γ_SBRT plans utilized coplanar arcs, with appropriate angles chosen to optimize dose distribution and avoid collision. RT Pro TPS uses Fast Photon algorithm. In our study, we employed a forward planning approach for the design of the Gamma Knife treatment plans. This involved manually setting the position, the size of the collimator, the gantry rotation angles, and the weights of each shot to meet the desired dose distribution for the target and surrounding tissues. The dosimetric objectives for γ_SBRT plans were as follows: (1) ensuring that ≥ 98% of the PTV received the 100% of the prescription dose, (2) ensuring that 100% of the PTV received ≥ 90% of the prescription dose, and (3) optimizing the conformity of the PTV.

Both algorithms incorporate heterogeneity correction, which is important for lung tumor dose calculation. A dose calculation grid size of 2 mm was used for all three types of plans. The dose constraints for the OARs were based on Timmerman Table25.

A variety of parameters were considered for comparison between the PTVs and GTVs, including GI, HI, CI, Dmax, Dmean, Dmin, D98, D95, D5, D2, V150, V100, and V90. Formulas (1)–(3) were used to define GI, HI, and CI26,27,28.

V50% represents the volume covered by the 50% prescription dose, VPTV represents the volume of the PTV, D5 represents to the dose received by 5% of the PTV, D95 represents the dose received by 95% of the PTV, and V100% represents the volume covered by the 100% prescription dose.

The GI should ideally be below 3.0, reflecting a sharp dose fall-off. Minor deviations are acceptable between 3.0 and 4.0, but a GI greater than 4.0 suggests a gradual dose fall-off, which may result in higher doses to surrounding healthy tissues. For HI, the ideal value is also 1.0, representing perfect homogeneity. Minor deviations are acceptable within the range of 1.0 to 1.2, while an HI greater than 1.2 indicates significant heterogeneity, raising concerns regarding dose uniformity within the target. The optimal value for CI is 1.0, signifying perfect conformity. Minor deviations are considered acceptable within the range of 0.9 to 1.1. However, a CI less than 0.9 or greater than 1.1 indicates significant deviations and potential issues in treatment planning26,27,28.

The dose distribution in low dose areas was assessed using, V10Gy (cc), and V5Gy (cc). For the OARs, comparisons were made for the Dmean (cGy), V20 (%), and V5 (%) of the ipsilateral lung, as well as the Dmean (cGy), V5 (%) of the contralateral lung, and the Dmax (cGy) of the esophagus, spinal cord, and heart. In this study, the volume of the ipsilateral lung is defined as the volume of the ipsilateral lung tissue minus the PTV. This definition allows for a more accurate assessment of the lung tissue at risk, excluding the area directly targeted by the treatment. The MUs of VMAT_SBRT and IMRT_SBRT plans, as well as the treatment delivery time (s) of all three types of plans, were compared. The treatment delivery time refers to the time taken for beam or arc delivery in VMAT_SBRT and IMRT_SBRT plans, and the time required for source activation in γ_SBRT plans, excluding time for imaging and patient positioning.

Statistical analysis was performed using IBM SPSS Statistics 19.0. Descriptive characteristics of all parameters for all three types of plans are presented. The Wilcoxon Signed Rank Test was performed to compare the differences between γ_SBRT and VMAT_SBRT plans, γ_SBRT and IMRT_SBRT plans, as well as VMAT_SBRT and IMRT_SBRT plans, respectively. Statistical significance was determined using a threshold p-value of 0.05.

The median age of all patients was 70 years (range: 36–76), with 8 males and 2 females. Four patients were diagnosed with primary lung cancer, while six patients were diagnosed with metastatic lung tumors. The analysis included a total of 23 lesions. The median PTV volume was 8.76 (range: 2.34–56.20) cm3, and the median diameter was 2.71 (range: 1.73–6.85) mm. A total of 54 plans were obtained, all of which met the clinical requirements. Table 1 shows the summary of plans and PTVs. Dosimetric parameters were acquired from the dose-volume histograms (DVHs).

The γ_SBRT plans exhibited a lower GI compared to both VMAT_SBRT and IMRT_SBRT plans (p = 0.000 and p = 0.000), while the HI was higher for γ_SBRT plans compared to VMAT_SBRT and IMRT_SBRT (p = 0.000 and p = 0.000). The CI of γ_SBRT plans was significantly higher than that of VMAT_SBRT plans (p = 0.016), but there was no statistically significant difference in CI between γ_SBRT and IMRT_SBRT plans (p = 0.145).

The Dmax (p = 0.000 and p = 0.000 of the PTVs, p = 0.000 and p = 0.000 of the GTVs) and Dmean (p = 0.000 and p = 0.000 of the PTVs, p = 0.000 and p = 0.000 of the GTVs) of the PTVs and GTVs in γ_SBRT plans were significantly higher compared to the VMAT_SBRT and IMRT_SBRT plans. The γ_SBRT plans obtained higher Dmin for the GTVs than the VMAT_SBRT and IMRT_SBRT plans (p = 0.002 and p = 0.001). There was no statistically significant difference in the Dmin of the PTVs between γ_SBRT and VMAT_SBRT plans, or between γ_SBRT and IMRT_SBRT plans (p = 0.605 and p = 0.484). The D98 of the PTVs in γ_SBRT plans was found to be lower than that in VMAT_SBRT and IMRT_SBRT plans (p = 0.003 and p = 0.024). Additionally, the D5 (p = 0.000 and p = 0.000) and D2 (p = 0.000 and p = 0.000) of the PTVs were higher in the γ_SBRT plans compared to the VMAT_SBRT and IMRT_SBRT plans. There was no statistically significant difference in D95 of the PTVs between γ_SBRT and VMAT_SBRT plans, or between γ_SBRT and IMRT_SBRT plans (p = 0.543 and p = 0.128).

Additionally, The V150 (p = 0.000 and p = 0.000) and V90 (p = 0.018 and p = 0.028) of the PTVs in γ_SBRT plans were higher than those in VMAT_SBRT and IMRT_SBRT plans, as well as the V100 (p = 0.001 and p = 0.002) of the PTVs in γ_SBRT plans was lower than that in VMAT_SBRT and IMRT_SBRT plans.

In the comparison between VMAT_SBRT and IMRT_SBRT plans, the CI of IMRT_SBRT plans was significantly higher than that of VMAT_SBRT plans (p = 0.015). Additionally, the Dmean and D95 of the PTVs in VMAT_SBRT plans was found to be significantly higher than that in IMRT_SBRT plans (p = 0.045 and p = 0.017). There was no statistically significant difference in the Dmax (p = 0.274) and Dmin (p = 0.888) of the PTVs, Dmax (p = 0.236), Dmean (p = 0.648), and Dmin (p = 0.855) of the GTVs between VMAT_SBRT and IMRT_SBRT plans. As well as, there was no statistically significant difference in the GI (p = 0.845), HI (p = 0.910), D98 (p = 0.073), D5 (p = 0.346), D2 (p = 0.346), V150 (p = 1.000), V100 (p = 0.279), V90 (p = 0.066) of the PTVs between VMAT_SBRT and IMRT_SBRT plans. Table 2 shows the dosimetric differences of the PTVs and GTVs among γ_SBRT, VMAT_SBRT, and IMRT_SBRT plans.

The γ_SBRT plans demonstrated lower V10Gy (p = 0.000 and p = 0.000), V5Gy (p = 0.000 and p = 0.000), and V50% (p = 0.000 and p = 0.003) compared to the VMAT_SBRT and IMRT_SBRT plans. There was no statistically significant difference in V10Gy (p = 0.557), V5Gy (p = 0.811), and V50% (p = 0.711) between VMAT_SBRT and IMRT_SBRT plans. Figure 1 shows the transverse-section dose distribution in maximum diameter plane among γ_SBRT, VMAT_SBRT, and IMRT_SBRT plans of two patients. Table 3 shows the differences of dose distribution in low dose areas among γ_SBRT, VMAT_SBRT, and IMRT_SBRT plans.

shows the transverse-section dose distribution in maximum diameter plane among γ_SBRT, VMAT_SBRT, and IMRT_SBRT plans of two patients. (a): Patient 1’s dose distribution of γ-SBRT plan, with a prescription dose of 36 Gy; (b): Patient 1’s dose distribution of VMAT-SBRT plan, with a prescription dose of 36 Gy; (c): Patient 1’s dose distribution of IMRT-SBRT plan, with a prescription dose of 36 Gy; (d): Patient 2’s dose distribution of γ-SBRT plan, with a prescription dose of 50 Gy; (e): Patient 2’s dose distribution of VMAT-SBRT plan, with a prescription dose of 50 Gy; (f): Patient 2’s dose distribution of IMRT-SBRT plan, with a prescription dose of 50 Gy.

The γ_SBRT plans demonstrated lower Dmean (p = 0.000 and p = 0.000), V20 (p = 0.005 and p = 0.000), and V5 (p = 0.001 and p = 0.002) for the ipsilateral lung compared to the VMAT_SBRT and IMRT_SBRT plans. Similarly, the γ_SBRT plans exhibited lower Dmean (p = 0.000 and p = 0.000) and V5 (p = 0.017 and p = 0.010) for the contralateral lung compared to the VMAT_SBRT and IMRT_SBRT plans. The VMAT_SBRT plans showed a higher Dmean for the ipsilateral lung compared to the IMRT_SBRT plans (p = 0.009). There was no statistically significant difference in the V20 (p = 0.071) and V5 (p = 0.459) for the ipsilateral lung, Dmean (p = 0.184) and V5 (p = 0.401) for the contralateral lung between VMAT_SBRT and IMRT_SBRT plans. Furthermore, the γ_SBRT plans exhibited lower Dmax for the esophagus (p = 0.001 and p = 0.000), spinal cord (p = 0.048 and p = 0.012), and heart (p = 0.002 and p = 0.002) compared to the VMAT_SBRT and IMRT_SBRT plans. The VMAT_SBRT plans demonstrated lower Dmax for the spinal cord (p = 0.012) and esophagus (p = 0.035) compared to the IMRT_SBRT plans. There was no statistically significant difference in the Dmax for the heart (p = 0.306) between VMAT_SBRT and IMRT_SBRT plans. Table 4 shows the differences in dose of OARs among γ_SBRT, VMAT_SBRT, and IMRT_SBRT Plans.

The delivery time of SBRT plans was significantly shorter than that of VMAT_SBRT and IMRT_SBRT plans (p = 0.000 and p = 0.000). A comparison between VMAT_SBRT and IMRT_SBRT plans revealed that VMAT_SBRT plans had a longer delivery time compared to IMRT_SBRT plans (p = 0.004). However, VMAT_SBRT plans showed an increase in MUs compared to IMRT_SBRT plans (p = 0.006). Table 5 shows the differences of treatment delivery efficiency among γ_SBRT, VMAT_SBRT, and IMRT_SBRT Plans.

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. 3 cm in longest diameter. J. Neurosurg.132, 1024–1032 (2019)." href="/articles/s41598-024-74397-2#ref-CR18" id="ref-link-section-d62960932e5547">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.

In conclusion, both Gamma Knife and accelerator-based SBRT techniques can meet the clinical requirements for pulmonary SBRT treatment. Gamma Knife SBRT plans exhibit superior dose falloff characteristics outside the target volumes, potentially reducing low-dose areas in the vicinity of the target volumes and normal lung tissue. However, this advantage is accompanied by a notable increase in treatment duration, which may increase the uncertainty of dose delivery and complicate intrafraction error management and respiratory motion management.

Therefore, Gamma Knife SBRT may be more suitable for cases where minimizing radiation exposure to surrounding healthy tissue is critical. On the other hand, accelerator-based techniques such as IMRT and VMAT offer comparable dose distribution characteristics and may be preferable when treatment efficiency is a priority. It is crucial to analyze each case individually to determine the most appropriate technique, taking into account factors such as tumor location, patient condition, and specific clinical objectives.

The clinical relevance of these findings lies in the ability to tailor SBRT treatment plans to the specific needs of lung tumor patients, balancing between minimizing radiation exposure and optimizing treatment duration. This study’s results could potentially impact clinical decision-making by providing a basis for selecting the most appropriate SBRT technique, thereby improving patient outcomes and treatment efficiency.

Author contributions.

Wenyue Duan was responsible for designing the research protocol, implementing the study, and drafting the manuscript. Huajian Wu, Yanmei Zhu, Genghao Zhao, Chuanhao Zhang, Jianing Jiang, and Zhijun Fan were responsible for carrying out the research, collecting, organizing, and analyzing the data. Zhe Wang was in charge of the project design and guidance, as well as the review and editing of the manuscript. Ruoyu Wang provided the research concept and supervised the project.

Funding

Scientific and Technological Projects in Liaoning Province (2021JH1/10400051); Dalian High-Level Talent Project (2021RD02); National Key Research and Development Program of China (2022YFC2407104).

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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Department of Radiotherapy, Affiliated Zhongshan Hospital of Dalian University, Dalian, P. R. China

Wenyue Duan & Jianing Jiang

Department of Medical Oncology, Affiliated Zhongshan Hospital of Dalian University, Dalian, P. R. China

Huajian Wu, Yanmei Zhu, Genghao Zhao, Chuanhao Zhang, Zhijun Fan, Zhe Wang & Ruoyu Wang

Graduate School of Dalian Medical University, Dalian, P. R. China

Chuanhao Zhang

The Key Laboratory of Biomarker High Throughput Screening and Target Translation of Breast and Gastrointestinal Tumor, Dalian University, Dalian, P. R. China

Wenyue Duan, Huajian Wu, Yanmei Zhu, Genghao Zhao, Jianing Jiang, Zhijun Fan, Zhe Wang & Ruoyu Wang

The Key Laboratory of Radioactive Particles and Thermal Precision Therapy, Dalian University, Dalian, P. R. China

Wenyue Duan, Huajian Wu, Yanmei Zhu, Genghao Zhao, Jianing Jiang, Zhijun Fan, Zhe Wang & Ruoyu Wang

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Correspondence to Zhe Wang or Ruoyu Wang.

The authors declare no competing interests.

This study was approved by the Institutional Review Board of Zhongshan Hospital affiliated with Dalian University.

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Duan, W., Wu, H., Zhu, Y. et al. Dosimetric comparison of gamma knife and linear accelerator (VMAT and IMRT) plans of SBRT of Lung tumours. Sci Rep 14, 22949 (2024). https://doi.org/10.1038/s41598-024-74397-2

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Received: 24 June 2024

Accepted: 25 September 2024

Published: 03 October 2024

DOI: https://doi.org/10.1038/s41598-024-74397-2

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