Stereotactic body radiotherapy (SBRT) is a specialized radiation therapy modality employed to treat specific lung malignancies, typically encompassing early-stage non-small-cell lung cancer (NSCLC) or small lung metastases originating from other primary cancers [1-4]. In contrast to conventional radiation therapy, SBRT administers precisely targeted, high-dose radiation to tumors, effectively minimizing exposure to adjacent healthy tissues. This precision targeting allows for a more effective treatment with reduced side effects. Recently, SBRT has been delivered via intensity-modulated radiotherapy (IMRT) and volumetric-modulated radiotherapy (VMAT) [5-7], optimizing the dose distribution to closely conform to the tumor while maintaining a sharp dose spillage (gradient), thereby enhancing the protection of organs at risk (OAR).

In Chonnam National University Hwasun Hospital, we have adopted the IMRT and VMAT techniques for SBRT treatments, employing a conventional C-arm linear accelerator (LINAC), which uses a 6-MV flattening filter-free (6MV-FFF) photon beam along with a high-definition multileaf collimator (HD-MLC) for lung SBRT applications. The 6MV-FFF beam offers several advantages, such as a higher dose rate of 1,400 monitor unit (MU)/min, reduced out-of-field scatter dose, and improved target dose coverage, especially at the tumor–lung interface [8]. The HD-MLC (Varian Medical Systems, Palo Alto, CA, USA) has a 2.5-mm leaf width and can provide significant dosimetric benefits compared with a conventional 5-mm leaf MLC, particularly for lung lesion SBRT [9].

The Halcyon LINAC (Varian Medical Systems) has gained widespread adoption in the field of IMRT because of its numerous advantages, such as rapid delivery at 4 rpm with an impressive dose rate of 800 MU/min, a dual-layer stacked and staggered MLC, and an essential daily cone-beam computed tomography (CBCT)-based image-guided radiation therapy (IGRT) workflow. The characteristics and advantages of Halcyon in IMRT and VMAT have been reviewed by several scholars [10-13].

The advantages of Halcyon, including its precision in IMRT procedures and reduced treatment time, allow it to be effectively applied to SBRT in practice. Despite the 6MV-FFF dose rate in the Halcyon beam being 800 MU/min, which is lower than that of the 6MV-FFF in the C-arm LINAC, the treatment time does not experience a significant increase, attributable to the high-speed (5.0 cm/sec) movement of the MLC and gantry rotation (4 rpm) in Halcyon. The dual-layer stacked and staggered MLC configuration in Halcyon achieves an effective width of 5 mm at the isocenter while maintaining exceptionally low leakage and transmission levels [14]. This shows that Halcyon is a viable option for SBRT, considering its higher accuracy in tumor target localization, improved target dose coverage, and reduced treatment time. Consequently, numerous studies have been conducted on SBRT using Halcyon [15,16].

In this study, the dosimetric characteristics of lung SBRT plans using Halcyon were analyzed to investigate its applicability. We analyzed the key dosimetric parameters calculated in Halcyon SBRT plans and compared them with the parameters of C-arm LINAC plans with HD-MLC. Although many studies have demonstrated that smaller MLC leaf widths have higher plan quality for SBRT than larger MLC leaf widths [17,18], our investigation assesses the suitability of Halcyon for lung SBRT. We evaluate how Halcyon’s characteristics and advantages in IMRT and VMAT are reflected in lung SBRT plans and confirm its clinical applicability to lung SBRT through comparison with existing results.

1. Lung stereotactic body radiotherapy plans

A total of 10 patients who were previously treated to 4,800 cGy in four fractions for early-stage I–II NSCLC were selected for this study. Table 1 presents the prescription doses, fractions, and tumor volumes for the planning target volume (PTV) of the patients. When the dose per fraction was calculated, a dose of 1,200 cGy per treatment was irradiated. We created SBRT plans for each patient using both a C-arm LINAC and a Halcyon LINAC based on PTV. The C-arm LINAC used was a Trilogy Tx with an HD-MLC, and a 6MV-FFF photon beam was used for SBRT plans. We employed the RapidArc technique with four half-arcs for the SBRT plans on both Trilogy Tx and Halcyon. All SBRT plans were generated using the Eclipse (version 16.0) planning system (Varian Medical Systems), and final dose calculations were performed using the AcurosXB algorithm.

Table 1 . Prescription doses and fractions and measured volumes of PTV and OAR constraints for lung stereotactic body radiotherapy.

Patient	Prescription dose	PTV volume (cm3)	OAR constraints (Gy)
A	12 Gy×4 fraction	13.37	Cord<15, bronchus<30, chest wall<40
B	12 Gy×4 fraction	7.36	Cord<5, bronchus<10, chest wall<40
C	12 Gy×4 fraction	21.59	Cord<5, esophagus<15
D	12 Gy×4 fraction	14.48	Cord<5, esophagus<10, bronchus<10
E	12 Gy×4 fraction	4.33	Cord<4, bronchus<10, chest wall<40
F	12 Gy×4 fraction	9.09	Cord<12, esophagus<8, chest wall<40
G	12 Gy×4 fraction	7.33	Cord<5, great vessel<13, chest wall<40
H	12 Gy×4 fraction	22.97	Bronchus<15, chest wall<40
I	12 Gy×4 fraction	33.53	Cord<13, esophagus<13, chest wall<30, bronchus<20
J	12 Gy×4 fraction	22.55	Chest wall<40, lung Dmean<20

PTV, planning target volume; OAR, organs at risk..

Concerning the PTV dose constraint, we ensured that at least 95% of the volume received 95% or more of the prescribed dose. In addition, we performed an optimization process to meet all various constraints for OAR for each patient (Table 1). Fig. 1 depicts dose distribution examples for lung SBRT plans created with Halcyon (a) and Trilogy Tx (b).

Figure 1. Examples of dose distribution of lung stereotactic body radiotherapy plans in (a) Halcyon and (b) Trilogy Tx.

2. Plan comparison and statistical analysis

We evaluated the SBRT plans by comparing the calculated dosimetric parameters between Trilogy Tx and Halcyon. Specifically, we analyzed the dose conformity index (CI), homogeneity index (HI), global maximum dose (D_{0.1 cc}), and minimum dose covering 95% of PTV (D_95%) to assess the suitability of PTV dose distribution [19]. CI is the PTV volume enclosed by 100% prescription dose divided by the PTV volume, and HI is the D_2%/D_98% of the PTV. We calculated the gradient measure as the difference between the equivalent sphere radius of the prescription and 50% prescription doses to assess the dose falloff sharpness [20]. This measure allows us to evaluate dose sparing in normal lung tissue.

Maximum doses to 0.03 cc for the spinal cord, bronchus, chest wall, and esophagus and the mean dose of the lung were calculated to evaluate Halcyon’s plan quality. Plan complexity was evaluated based on the modulation factor (MU/cGy) [20]. The total number of MU and treatment beam-on time were compared to analyze the efficiency of Halcyon in terms of clinical treatment application.

The statistical difference of the calculated parameters between Halcyon and Trilogy Tx was analyzed based on P-values calculated using the paired t-test.

1. Target coverage and intermediate dose falloff

The mean values of D_95% were 47.06±0.84 and 47.35±0.79 Gy for Halcyon and Trilogy Tx, respectively. Those of D_0.1cc were 52.39±1.74 and 51.59±1.48 Gy for Halcyon and Trilogy Tx, respectively. Those of CI were 0.91±0.08 and 0.93±0.09 for Halcyon and Trilogy Tx, respectively. Those of HI were 1.12±0.06 and 1.10±0.04 for Halcyon and Trilogy Tx, respectively. The mean gradient values were 1.06±0.10 and 1.01±0.11 cm for Halcyon and Trilogy Tx, respectively. These results indicate that Trilogy Tx achieved better target dose coverage and falloff than Halcyon.

Table 2 shows the calculated P-values for the differences between Halcyon and Trilogy Tx, which were 0.030, 0.004, 0.188, 0.006, and 0.039 for D_95%, D_{0.1 cc}, CI, HI, and gradient, respectively. These findings demonstrate that Trilogy Tx yielded statistically significant improvements over Halcyon in target dose distribution, except for CI.

Table 2 . Comparison of target coverage and dose spillage between Halcyon and Trilogy Tx plans for 10 lung stereotactic body radiotherapy patients.

Parameter	Halcyon	Trilogy Tx	P-value
D95% (Gy)	47.06±0.84	47.35±0.79	0.030
Dmax to 0.1 cc (Gy)	52.39±1.74	51.59±1.48	0.004
Conformity index	0.91±0.08	0.93±0.09	0.188
Homogeneity index	1.12±0.06	1.10±0.04	0.006
Dose gradient (cm)	1.06±0.10	1.01±0.11	0.039

2. Comparison of organs at risk dose

For Halcyon and Trilogy Tx, the average maximum doses of 0.03 cc for the spinal cord were 7.84±3.34 and 7.07±2.92 Gy, respectively. The values were 9.95±5.19 and 10.94±6.63 Gy for the bronchus, respectively. The values were 38.36±6.64 and 37.72±6.14 Gy, respectively, for the chest wall. The values were 8.49±2.80 and 8.50±2.99 Gy, respectively, for the esophagus. Halcyon and Trilogy Tx had average lung mean dose values of 2.17±0.92 and 2.24±1.01 Gy, respectively.

When considering the OAR dose, no significant difference was observed between the two treatment plans. Figs. 2 and 3 confirm the absence of a statistically significant difference.

Figure 2. Comparison of average maximum dose to 0.03 cc in organs at risk (OAR) between Halcyon and Trilogy Tx.

Figure 3. Comparison of average lung mean dose between Halcyon and Trilogy Tx.

3. Efficiency of treatment delivery

Table 3 shows a comparison of dose delivery efficacy on both treatment machines, which was evaluated by the total number of MUs, beam-on time, and modulation factor. The average total number of MUs was 3,678.4±473.3 and 3,408.1±503.2 in Halcyon and Trilogy Tx, respectively. The average beam-on times were 4.60±0.59 and 2.71±0.18 minutes in Halcyon and Trilogy Tx, respectively. The average modulation factors were 3.07±0.39 and 2.84±0.42 in Halcyon and Trilogy Tx, respectively. The calculated P-values for the differences between Halcyon and Trilogy Tx were 0.008, 2.78×10⁻⁷, and 0.008 for the total number of MUs, beam-on time, and modulation factor, respectively. These findings indicate that Trilogy Tx was more efficient than Halcyon in the lung SBRT beam delivery process.

Table 3 . Comparison of treatment delivery parameters between Halcyon and Trilogy Tx plans for 10 lung stereotactic body radiotherapy patients.

Parameter	Halcyon	Trilogy Tx	P-value
Total no. of MUs	3,678.4±473.3	3,408.1±503.2	0.008
Modulation factor (MU/cGy)	3.07±0.39	2.84±0.42	0.008
Beam-on time (min)	4.60±0.59	2.71±0.18	2.78×10−7

In this study, we evaluated the plan quality and treatment delivery efficiency of lung SBRT using Halcyon by comparing it with SBRT plans using Trilogy Tx.

In terms of target dose coverage and spillage, Trilogy Tx demonstrated significant superiority over Halcyon. The results suggest that the HD-MLC with a 0.25-cm leaf width is more suitable for achieving precise target dose distribution in SBRT than the Halcyon dual-layer MLC with a practical width of 0.5 cm, especially for treating small-volume tumor targets. These results align with the findings of Tanyi et al. [9] and Wu et al. [17], who demonstrated the advantages of using HD-MLC in achieving precise SBRT plans with high-dose irradiation.

Although there was a significant difference in target dose distribution, there was no significant difference in OAR dose distribution between Halcyon and Trilogy Tx because both treatment plans were designed to meet OAR constraints during the VMAT optimization process, ensuring that the optimization requirements based on maximum dose values were successfully met. In essence, generating a target dose distribution involves shaping the prescribed dose to match the target volume. Therefore, finer MLC widths, such as HD-MLC, improve dose distribution. However, the OAR dose values, which are evaluated based on maximum dose constraints, did not significantly differ between both treatment machines unless the constraint dose values were set under highly impractical conditions.

In terms of treatment delivery efficiency, Trilogy Tx outperformed Halcyon, with lower average values in the total number of MUs, modulation factor, and beam-on time. Statistically significant differences were observed between the two plans in beam delivery efficiency, including the total number of MUs, beam-on time, and modulation factor. These differences are primarily attributed to the dose rate disparity between the two machines. In Trilogy Tx, the 6MV-FFF dose rate is 1,400 MU/min, which is more than 1.7 times higher than the 800 MU/min rate in Halcyon. Although the total number of MUs did not vary significantly, the difference in dose rate had a substantial impact on the actual beam-on time. However, considering the IGRT process involving CBCT performed before actual treatment, relying solely on the beam-on time value to assess treatment efficiency has limitations because of the rapid IGRT performance time of Halcyon. In essence, given the advantages of the rapid IGRT process and the overall treatment efficiency, Halcyon is not expected to introduce significant delays in the entire SBRT process.

In this study, considering that Halcyon cannot rotate the treatment table of C-arm LINAC, we compared the dose distributions of both treatment machines using a VMAT plan that considered only the four half-arcs of coplanar fields, excluding noncoplanar techniques. Although the C-arm LINAC allows for noncoplanar plans, there is a greater potential for plan improvement in terms of tumor target dose distribution and OAR dose reduction. Further, the C-arm LINAC offers the capability for respiratory-gated treatment, enabling the reduction of tumor target volume in areas with significant motion. This feature provides an additional advantage by reducing both the OAR dose and overall treated volume.

Considering the above advantages and findings of this study, we confirmed that Trilogy Tx was superior to Halcyon in terms of dose distribution in lung SBRT. However, from the perspective of achieving the required dose distribution goal, there will be no major problems in applying Halcyon to lung SBRT. Therefore, when applying Halcyon to lung SBRT, it seems reasonable to apply it only to lesions where the range of tumor motion is small and the respiratory-gated treatment method does not need to be applied.

There are difficulties in determining the clinical application of the recently invented Halcyon based solely on the findings of previous studies, although this study’s methodology to compare the SBRT plans of Halcyon and Trilogy Tx is not significantly different from previous studies. In fact, there were some similarities between the results of this study and those of Pokhrel et al. [20], but there were differences in the total number of MU, modulation factor, and CI results. Therefore, before implementing the Halcyon system as a new equipment for lung SBRT, a preliminary process of conducting dose characteristic studies and analyzing the treatment procedures should be performed in hospitals.

After analyzing the lung SBRT plan created using a Halcyon system, it was found to be inferior to a C-arm LINAC equipped with HD-MLC in terms of tumor target dose coverage and spillage. Nevertheless, it was feasible to achieve a dose distribution that met SBRT plan requirements, with no significant differences observed in satisfying OAR dose constraints.

The findings confirm the viability of using the Halcyon system as a suitable alternative for performing lung SBRT, instead of a C-arm LINAC equipped with HD-MLC. However, in lesions where the respiratory tumor motion is large, it is necessary to increase the PTV setting accuracy and minutely analyze the applicability of the Halcyon system as the PTV size increases.

This study was financially supported by Chonnam National University (Grant number: 2022-2617).

The authors have nothing to disclose.

All relevant data are within the paper and its Supporting Information files.

Conceptualization: Shinhaeng Cho, Ju-Young Song. Data curation: Ick Joon Cho, Yong Hyub Kim. Formal analysis: Jae-Uk Jeong, Mee Sun Yoon. Funding acquisition: Ju-Young Song. Investigation: Taek-Keun Nam, Sung-Ja Ahn, Shinhaeng Cho, Ju-Young Song. Methodology: Shinhaeng Cho, Ju-Young Song. Project administration: Ju-Young Song. Resources: Ju-Young Song. Software: Shinhaeng Cho, Ju-Young Song. Supervision: Shinhaeng Cho, Ju-Young Song. Validation: Ju-Young Song. Visualization: Shinhaeng Cho. Writing – original draft: Shinhaeng Cho, Ju-Young Song. Writing – review & editing: Shinhaeng Cho, Ju-Young Song.