Dosimetry and treatment efficiency of SBRT using TaiChiB radiotherapy system for two-lung lesions with one overlapping organs at risk

Abstract

Purpose:

This study aims to assess the dosimetry and treatment efficiency of TaiChiB-based Stereotactic Body Radiotherapy (SBRT) plans applying to treat two-lung lesions with one overlapping organs at risk.

Methods:

For four retrospective patients diagnosed with two-lung lesions each patient, four treatment plans were designed including Plan _Edge, TaiChiB linac-based, RGS-based, and a linac-RGS hybrid (Plan _TCLinac, Plan _TCRGS, and Plan _TCHybrid). Dosimetric metrics and beam-on time were employed to evaluate and compare the TaiChiB-based plans against Plan _Edge.

Results:

For Conformity Index (CI), Plan _TCRGS outperformed all other plans with an average CI of 1.06, as opposed to Plan _Edge′s 1.33. Similarly, for R_50 %, Plan _TCRGS was superior with an average R_50 % of 3.79, better than Plan _Edge′s 4.28. In terms of D₂ _cm, Plan _TCRGS also led with an average of 48.48%, compared to Plan _Edge′s 56.25%. For organ at risk (OAR) sparing, Plan _TCRGS often displayed the lowest dosimetric values, notably for the spinal cord (D_max 5.92 Gy) and lungs (D_1500cc 1.00 Gy, D_1000cc 2.61 Gy, V₁₀ _Gy 15.14%). However, its high D_max values for the heart and great vessels sometimes exceeded safety thresholds. Plan _TCHybrid presented a balanced approach, showing doses comparable to or better than Plan _Edge without crossing safety limits. In terms of beam-on time, Plan _TCLinac emerged as the most efficient treatment option in three out of four cases, followed closely by Plan _Edge in one case. Plan _TCRGS, despite its dosimetric advantages, was the least efficient, recording notably longer beam-on times, with a peak at 33.28 minutes in Case 2.

Conclusion:

For patients with two-lung lesions treated by SBRT whose one lesion overlaps with OARs, the Plan _TCHybrid delivered by TaiChiB digital radiotherapy system can be recommended as a clinical option.

Keywords

SBRT TaiChiB two-target lung lesions rotating gamma system Edge linac hybrid plan

1 Introduction

Stereotactic body radiotherapy (SBRT) is a widely adopted clinical technique for tumor treatment that delivers high-precision ablative therapy to tumors, sparing the surrounding organs at risk (OARs) [1]. Numerous studies have attested to SBRT’s efficacy, which rivals that of surgery in early-stage non-small cell lung cancer (NSCLC) patients, making it the preferred option for those unable to undergo surgery [2 –5]. Moreover, SBRT serves as an effective local control strategy for patients with small metastatic tumors [6].

Traditionally, lung SBRT plans have been delivered using linac. However, with the progression in four-dimensional (4D) technology and the enhancement of image guidance devices, the rotating gamma system (RGS) [7] has expanded its applications from exclusively treating head targets to addressing targets throughout the body. In 1999, the body-specific RGS was pioneered in China [7]. Owing to its outstanding dosimetric performance with focused beams, the RGS has been extensively adopted and validated as an effective treatment modality for patients with lung cancer [7, 8].

Due to differences in physical and mechanical designs, as well as radiation properties, dose distributions in linac-based treatment plans are markedly different from those in RGS-based plans. The RGS can achieve a sharper dose gradient, which leads to reduced doses to organs-at-risk (OARs) and notably higher hotspots within the target, in contrast to linac plans [9 –12]. When the planning target volume (PTV) does not overlap with organs requiring dose maximum (D_max) constraints, the RGS is more adept at sparing OARs and should be the preferred choice. However, it is challenged to reduce the maximum organ dose when the organ overlaps with the PTV [10]. The optimal machine for SBRT treatment in patients with multiple lung lesions can be ascertained based on the anatomical positioning of each lesion. Consequently, utilizing a singular machine type might not be suitable for all patients. In cases where certain lesions are best treated with linac and others with RGS, a hybrid plan integrating both linac and RGS is necessitated. Nonetheless, even when a medical center possesses both linac and RGS capabilities, simultaneous treatments using both machines can introduce intricacies and inconvenience to the treatment protocol. This not only increases the staff workload but also becomes unrealistic for institutions with high patient volumes.

The Edge^TM linac (Varian Medical Systems, Palo Alto, CA) stands as a prevalent device equipped with image-guidance, predominantly employed for SBRT treatments. The TaiChiB digital radiotherapy system (OUR United Corporation, Xi’an, China) represents an innovative piece of radiotherapy equipment. This system integrated a linac system, an RGS system, and a kV image guidance system all within its gantry. TaiChiB offers versatility, allowing patients to undergo treatment using either the linac, the RGS, or a combination of both. Furthermore, its associated treatment planning system, the RT Pro TPS, possesses the ability to generate linac-based and RGS-based treatment plans, as well as hybrid strategies that capitalize on the strengths of both the linac and RGS.

In this work, we compared the SBRT treatment plans generated for the Edge linac and TaiChiB system for patients with two-lung lesions, one of which overlaps with OAR. These treatment plans include treatments with Edge linac, with TaiChiB linac, with TaiChiB RGS, and with both TaiChiB linac and RGS. Dosimetric comparison and beam-on time evaluation were performed to explore the dosimetric advantage and feasibility of TaiChiB system for lung lesion treatment, and to provide forthcoming experiment with useful theoretical background.

2 Materials and methods

2.1 Patients

This study performed a retrospective analysis on eleven cancer patients with two-lung lesions who underwent SBRT treatment by Edge accelerator at XX Hospital from June 2019 to October 2020. Among these patients, four individuals with one lesion overlapping critical organs were included in the experimental phase of this study. The age range of the patients was between 58 to 73 years. Tumor volumes varied from 0.59cc to 18.46cc, and the prescribed dose ranged from 30–60 Gy, delivered in 3–8 fractions. Table 1 provides detailed information about the participants, and Fig. 1 illustrates the anatomical locations of the tumors. All the enrolled patients had completed their radiotherapy and signed informed consent. This study received approval from the local Ethics Committee (Reference Number: XX).

Table 1
Clinical characteristics of the enrolled patients

Case 1 2 3 4

Gender Female Male Male Female

Age (years) 73 58 58 66

Number of lesions 2 2 2 2

Location of target LU + LL RM + metastases LU RU

Type Peripheral + Central Central + metastases Peripheral Peripheral

ITV volume(cc) 3.52 + 2.05 17.95 + 18.46 1.21 + 0.59 6.57 + 2.25

PTV volume(cc) 16.92 + 11.64 63.71 + 67.39 8.72 + 6.37 23.14 + 12.34

Dose (Gy)/Fraction 50/5 30/5 30/3 60/8

Organs overlapping with PTV One lesion: large blood vessels and heart One lesion: large blood vessels, heart and trachea One lesion: large vessels One lesion: large blood vessels, heart and trachea

Case	1	2	3	4
Gender	Female	Male	Male	Female
Age (years)	73	58	58	66
Number of lesions	2	2	2	2
Location of target	LU + LL	RM + metastases	LU	RU
Type	Peripheral + Central	Central + metastases	Peripheral	Peripheral
ITV volume(cc)	3.52 + 2.05	17.95 + 18.46	1.21 + 0.59	6.57 + 2.25
PTV volume(cc)	16.92 + 11.64	63.71 + 67.39	8.72 + 6.37	23.14 + 12.34
Dose (Gy)/Fraction	50/5	30/5	30/3	60/8
Organs overlapping with PTV	One lesion: large blood vessels and heart	One lesion: large blood vessels, heart and trachea	One lesion: large vessels	One lesion: large blood vessels, heart and trachea

LU: Left upper lobe, LL: Left lower lobe, RU: Right upper lobe, RM: Right middle lobe.

Fig. 1

CT images showing the anatomical location of tumors in four patients.

Patients were scanned using the Siemens Somatom Definition AS CT Scanner System (Siemens Healthcare, Erlangen, Germany) to acquire both free-breathing CT and 4DCT image sets. Expert radiation oncologists delineated all targets on the MIM Maestro Station (MIM Vista Corp, Cleveland, OH). The gross tumor volume (GTV) was defined across all ten phases of the 4DCT. Subsequently, these ten GTVs were merged to form the internal target volume (ITV). The planned target volume (PTV) was obtained by expanding the ITV by 0.5 cm in all three dimensions. An independent radiation oncologist reviewed and approved all delineations prior to their use in treatment planning.

2.2 Treatment Planning

Treatment plans were generated using the patient’s average CT. Every plan was designed by an experienced physicist, unaware of the planning procedures for other techniques, and subsequently reviewed by an independent senior physicist. For all treatment plans, PTV coverage by the prescription dose was normalized to encompass 99% of the volume. The criteria for OAR acceptance adhered to guidelines set by the Radiation Oncology Working Group (RTOG), the American Association of Physicists in Medicine (AAPM), and relevant references [13 –17].

The Edge treatment plan (Plan _Edge) was planned to use the auto-planning module of the Pinnacle TPS (V9.10, Philips Radiation Oncology Systems, Fitchburg, WI, USA) for an Edgetrademark linac equipped with a high-definition HD 120 multileaf collimator (MLC)trademark. The HD 120 MLCtrademark comprises 120 leaves, with a projected leaf width at the isocenter plane of 2.5 mm for the central 32 pairs and 5.0 mm for the 14 pairs on either side [18]. The planning approach echoed methods from our preceding research [19, 20]. All plans utilized ten or more 6 MV fields, with gantry angle intervals set at either 15 or 20 degrees between fields. Adjustments were made to collimator and couch angles based on individual requirements. Dose calculations employed the collapsed cone convolution (CCC) algorithm with a grid size of 1.0 mm.

Treatment plans for the TaiChiB system are designed for each patient and are evaluated together with the Plan _Edge. Figure 2 illustrates the concept layout and machine profile of the TaiChiB system. Figure 2.a depicts the TaiChiB concept layout, [21] while Fig. 2.b presents the machine profile of TaiChiB. The TaiChiB digital radiotherapy system is an innovative teletherapy device, combining a linac with a rotating gamma radiosurgery system, all encased within a slip ring gantry. The linac unit has FFF beams with a 6 MV energy, a peak dose rate of 1400cGy/min, and a maximum treatment field of 40×40 cm. Central MLC leaves are 5 mm wide, while those on the peripheries measure 10 mm in width. The rotating gamma radiosurgery unit houses 18 Co-60 sources, converging at the isocenter, paired with adjustable collimators. These sources are organized in two fan-shaped, non-coplanar rows in the superior-inferior orientation. Each source can be fitted with one of seven collimator sizes, ranging from 6 to 35 mm in diameter (6, 9, 12, 16, 2, 25, and 35 mm). This unit can execute 18 non-coplanar arcs through the rotating gantry. Its radiological isocenter precision is within 0.5 mm. Upon initial installation, it offers a dose rate of 3.5 Gy/min at the center of a 16 cm diameter water-equivalent spherical phantom at the isocenter. Both two units can deliver continuous arc therapy with more than 360 degrees relying on the slip ring gantry.

Fig. 2

Details of the concept layout and machine profile of TaiChiB system. (a) TaiChiB concept layout [21]. Linac, gamma, and kV x-ray are installed in a donut ring gantry with slip ring power supply and share the same IsoCenter. (b) TaiChiB machine profile. TaiChiB is a novel teletherapy device combined linear accelerator, rotating gamma radiosurgery system, MV EPID panel and kV image system within an enclosed slip ring gantry, EPID panel has its own track, can move to linac or gamma opposite position for QA to the above system. (A) Plan _TChybrid (B) Plan _TCRGS (C) Plan _TCLinac (D) Plan _Edge.

Due to its novel system structure, the TaiChiB digital radiotherapy system can administer conventional radiotherapy with the linac, SBRT or stereotactic radiosurgery (SRS) using the gamma radiosurgery system, or a hybrid treatment employing both units. This hybrid approach allows for the delivery of varied radiation modalities to a single patient. The RT Pro TPS (V2.0.1.4750, OUR United Corporation, Xi’an, Shanxi, China) is employed to generate treatment plans for the TaiChiB system. Beyond supporting intensity-modulated radiotherapy (IMRT) and volumetric modulated arc therapy (VMAT) planning for linac, as well as SRS/SBRT planning for the rotating gamma radiosurgery system, the RT Pro TPS can also produce multi-modality plans that blend the delivery capabilities of both the linac and RGS.

For the TaiChiB digital radiotherapy system, three distinct treatment plans were generated for each patient. These encompass: (1) a linear accelerator-based plan (Plan _TCLinac), (2) a plan employing the RGS (Plan _TCRGS), and (3) a composite approach (Plan _TCHybrid) wherein one lesion, which overlapped with the OARs, was treatment via the linac, while another lesion was addressed utilizing RGS. The planning methods are described below.

The Plan _TCLinac employs the VMAT technique, utilizing either a full or partial arc with control points spaced at two-degree intervals for each specific lesion. All treatment plans are planned in a coplanar fashion, with the collimator angle being adjusted to the individual situation of each case. A field margin of 0.1 cm was used to achieve a sharp dose gradient external to the PTV. Both the Direct Machine Parameter Optimization (DMPO) and the Collapsed Cone Convolution (CCC) algorithms were used for plan optimization and dose calculation. The dose grid resolution was set at 1.0 mm.

The Plan _TCRGS was planned using partial arcs and an inverse optimization method. Prior to this, suitable collimator sizes, arc spans, and the number of shots were determined for each PTV. Subsequently, parameters including coverage, selectivity, gradient, and beam-on time were defined for plan optimization. The prescription dose, represented by the 50% isodose line, was configured to cover 99% of the PTV volume. The RayTracing algorithm was employed for dose calculation, utilizing a dose grid resolution of 1.0 mm.

The Plan _TCHybrid strategy aims to administer linac therapy to targets that overlap with organs constrained by maximum dose thresholds, thereby mitigating radiation-associated risks. Concurrently, RGS therapy is applied to other targets to achieve a sharp dose gradient. Initially, an RGS-specific plan is planned, followed by the creation of a linac-based plan. This latter plan is formulated with the RGS plan serving as the foundational dose background. Such an approach ensures the incorporation and superposition of doses from the RGS plan during the optimization of the linac-based plan. Consequently, the aggregate dose of the Plan _TCHybrid can be efficiently assessed. The grid used for dose calculations maintains a resolution of 1.0 mm.

2.3 Evaluation Tools

Plans employing various techniques were assessed based on metrics delineated in the guidelines from RTOG 0915 [13], RTOG 0813 [17], and AAPM Report 101 [15].

For the dosimetric assessment of the PTV, the metrics encompassed the RTOG Conformity Index (CI), RTOG Gradient Index R_50 %, and D₂ _cm.

CI is computed as $CI = V_{Rx} / V_{T}$ (1)

where V_Rx is the volume covered by prescription dose, and V_T is the PTV volume. CI ranges from 0-1, and CI = 1 indicates the best conformability.

R_50 % is calculated as $R_{50 %} = V_{50 % Rx} / V_{T}$ (2)

where V_50 %Rx is the volume receiving half the prescription dose. A lower R_50 % represents a faster dose falloff in normal tissue from the target.

D₂ _cm is defined as the maximum dose (in % of the prescribed dose) at 2 cm away from PTV in all directions.

The dosimetric metrics of OARs and the recommended safe dose threshold [13 –17] are listed in Table 2, where D _x _cc represents the dose covering x cc tissue, and V _x _Gy represents the percentage volume that receives a dose level of at least x Gy.

Table 2

Metrics used in different plans and the recommended safe dose threshold

OARs	Volume	Threshold dose (Gy)	Max dose (Gy)
Spinal Cord	<0.25cc	22.5	30
	<0.5cc	13.5	–
Total Lung	<1500cc	12.5	–
	<1000cc	13.5	–
	<25%	10	–
	<10%	20	–
Esophagus	<5cc	19.5	52.5
Heart	<15cc	32	52.5
Trachea and Large Bronchus	<4cc	16.5	52.5
Great Vessels	<10cc	47	52.5
Chest Wall	<30cc	30	–

2.4 Beam-on time estimation

The beam-on time was used as the metric to gauge the treatment efficacy across diverse platforms. For Plan _Edge, the beam-on time was derived from the total MU quotient with the dose rate. For Plan _TCLinac, it was extrapolated from the predetermined duration allocated for each arc. The beam-on time for RGS was manually recorded by the physicist. For the Plan _TCHybrid, the beam-on time constituted the aggregate of the times from both plans.

3 Results

3.1 Case Example

Figure 3 demonstrates the dose distribution of a single case example for three different treatment modalities. Plan _TCHybrid and Plan _TCRGS have focused the dose above 20 Gy (indicated by the sky-blue area) into a smaller region compared to Plan _Edge.

Fig. 3

Axial, coronal, and sagittal views of the dose distribution of the example case to visually demonstrate the differences among (A) Plan _TChybrid, (B) Plan _TCRGS, (C) Plan _TCLinac, and (D) Plan _Edge. The solid lines represent PTV.

3.2 Dosimetric Evaluations for PTV

Table 3 provides a comparison of the dosimetric outcomes for the PTV across TaiChiB-based treatment plans. In the comparative analysis of the various treatment plans against Plan _Edge, the following observations were made. For CI, Plan _TCRGS consistently outperformed Plan _Edge, with an average CI of 1.06 compared to Plan _Edge′s 1.33. Plan _TCHybrid and Plan _TCLinac showed competitive CIs, averaging 1.12 and 1.19, respectively, but still higher than Plan _TCRGS. For R_50 %, all TaiChiB-based plans bettered Plan _Edge′s average R_50 % value of 4.28. Plan _TCRGS emerged as the best with an average R_50 % of 3.79. Plan _TCHybrid and Plan _TCLinac followed closely, averaging 4.04 and 4.12, respectively. For D₂ _cm, Plan _TCRGS was superior with an average of 48.48%, compared to Plan _Edge′s 56.25%. Plan _TCHybrid and Plan _TCLinac trailed but still performed better than Plan _Edge, with averages of 51.06% and 53.09%, respectively.

Table 3
Evaluations of different plans for PTV

Case Parameters Plan _TCHybrid Plan _TCRGS Plan _TCLinac Plan _Edge

CI 1.09 1.05 1.10 1.28

Case 1 R_50 % 4.54 4.21 4.55 4.84

D₂ _cm (% of D_P) 47.90 45.92 51.46 52.62

CI 1.13 1.01 1.28 1.31

Case 2 R_50 % 3.65 3.34 3.75 3.78

D₂ _cm (% of D_P) 62.13 59.97 62.10 66.17

CI 1.14 1.10 1.15 1.47

Case 3 R_50 % 4.19 3.84 4.33 4.39

D₂ _cm (% of D_P) 44.67 39.00 48.73 47.40

CI 1.11 1.08 1.24 1.27

Case 4 R_50 % 3.77 3.76 3.83 4.09

D₂ _cm (% of D_P) 49.53 49.02 50.05 58.82

Average CI 1.12 1.06 1.19 1.33

R50% 4.04 3.79 4.12 4.28

D2 cm (% of DP) 51.06 48.48 53.09 56.25

Case	Parameters	Plan _TCHybrid	Plan _TCRGS	Plan _TCLinac	Plan _Edge
	CI	1.09	1.05	1.10	1.28
Case 1	R_50 %	4.54	4.21	4.55	4.84
	D₂ _cm (% of D_P)	47.90	45.92	51.46	52.62
	CI	1.13	1.01	1.28	1.31
Case 2	R_50 %	3.65	3.34	3.75	3.78
	D₂ _cm (% of D_P)	62.13	59.97	62.10	66.17
	CI	1.14	1.10	1.15	1.47
Case 3	R_50 %	4.19	3.84	4.33	4.39
	D₂ _cm (% of D_P)	44.67	39.00	48.73	47.40
	CI	1.11	1.08	1.24	1.27
Case 4	R_50 %	3.77	3.76	3.83	4.09
	D₂ _cm (% of D_P)	49.53	49.02	50.05	58.82
Average	CI	1.12	1.06	1.19	1.33
	R50%	4.04	3.79	4.12	4.28
	D2 cm (% of DP)	51.06	48.48	53.09	56.25

D_P: Prescription dose.

In summary, Plan _TCRGS showed the best performance in all parameters followed by Plan _TCHybrid. Both were superior to Plan _Edge in terms of CI, R_50 %, and D₂ _cm. Plan _TCLinac, while less optimal than Plan _TCRGS and Plan _TCHybrid, still showed competitive results when compared to Plan _Edge.

3.3 Dosimetric Evaluations for OARs

Table 4 outlines a comprehensive assessment of the dosimetric parameters for OARs among the four treatment plans: Plan _TCHybrid, Plan _TCRGS, Plan _TCLinac, and Plan _Edge.

Table 4
Evaluations of different plans for OARs

Case Parameters Plan _TCHybrid Plan _TCRGS Plan _TCLinac Plan _Edge

Spinal Cord Dmax 8.39 7.42 11.74 13.17

Case1 D0.35cc (Gy) 7.61 6.5 10.35 10.91

D1.2cc (Gy) 7.08 5.9 9.38 9.4

Total Lung D1500cc(Gy) 0.47 0.4 0.47 0.68

D1000cc (Gy) 1.55 1.59 2.05 2.8

V10 Gy (%) 14.48 13.48 15.22 17.73

esophagus D5cc(Gy) 9.65 9.21 10.87 13.58

Dmax 19.04 17.65 19.94 21.42

Heart D15cc(Gy) 22.96 20.19 22.19 24.32

Dmax(Gy) 65.58x 90.29x 66.65x 66.79x

Trachea and large Bronchus D4cc(Gy) 1.24 2.68 1.33 2.69

Dmax(Gy) 31.47 27.9 30.58 33.06

Great Vessels D10cc(Gy) 25.94 22.7 24.77 27.32

Dmax(Gy) 63.72 84.88x 64.12 65.76

Chest Wall D30cc(Gy) 15.56 13.44 16.35 18.88

Spinal Cord Dmax 5.77 6.04 6.81 7.82

Case2 D0.35cc (Gy) 5.34 5.43 5.81 5.9

D1.2cc (Gy) 5.09 5.11 5.42 6.39

Total Lung D1500cc(Gy) 1.21 1.07 1.29 1.72

D1000cc (Gy) 2.05 1.83 1.98 2.71

V10 Gy (%) 13.13 14.19 13.63 21.02

esophagus D5cc(Gy) 4.25 4.69 4.59 5.82

Dmax 8.83 9.47 9.01 10.79

Heart D15cc(Gy) 15.24 15.4 15.14 17.99

Dmax(Gy) 36.38 51.64x 37.6 37.63

Trachea and large Bronchus D4cc(Gy) 13.86 16.05 14.51 16.05

Dmax(Gy) 34.91 53.16x 36.99 38.56

Great Vessels D10cc(Gy) 25.84 26.58 27.31 31.41

Dmax(Gy) 37.67 60.03x 38.53 40.47

Chest Wall D30cc(Gy) 9.38 9.04 9.82 12.77

Spinal Cord Dmax 4.58 3.75 4.25 5.59

Case3 D0.35cc (Gy) 4.07 3.34 3.7 4.8

D1.2cc (Gy) 3.68 2.29 3.26 4.2

Total Lung D1500cc(Gy) 0.19 0.15 0.2 0.28

D1000cc (Gy) 0.92 0.84 0.98 1.28

V10 Gy (%) 2.77 2.57 3.11 4.01

esophagus D5cc(Gy) 5.55 4.87 7.09 7.18

Dmax 10.91 9.58 13.33 11.86

Heart D15cc(Gy) 0.48 0.68 0.4 0.37

Dmax(Gy) 1.57 1.51 1.58 6.26

Trachea and large Bronchus D4cc(Gy) 6.56 5.75 8.6 7.71

Dmax(Gy) 10.79 9.02 13.59 12.41

Great Vessels D10cc(Gy) 14.12 14.03 15.17 16.29

Dmax(Gy) 39.25 53.75x 40.01 44.84

Chest Wall D30cc(Gy) 6.06 5.36 7.42 7.95

Spinal Cord Dmax 6.92 6.45 7.47 9.38

Case4 D0.35cc (Gy) 6.17 5.77 6.53 6.93

D1.2cc (Gy) 5.24 4.84 5.53 6.47

Total Lung D1500cc(Gy) 0.86 0.82 1.4 1.33

D1000cc (Gy) 2.81 2.77 3.02 3.66

V10 Gy (%) 15.58 14.95 17.77 17.8

esophagus D5cc(Gy) 9.13 7.64 9.59 11.36

Dmax 17.07 13.4 16.5 16.76

Heart D15cc(Gy) 30.8 37.39 33.8 36.41

Dmax(Gy) 54.47 79.84x 52.9 62.83

Trachea and large Bronchus D4cc(Gy) 12.93 14.2 13.28 15.43

Dmax(Gy) 20.41 24.5 22.89 24.25

Great Vessels D10cc(Gy) 35.98 41.89 39.77 42.57

Dmax(Gy) 41.4 66.96x 43.93 52.83

Chest Wall D30cc(Gy) 22.06 23.11 22.47 26.01

Spinal Cord Dmax 6.42 5.92 7.57 8.99

Average D0.35cc (Gy) 5.80 5.26 6.60 7.14

D1.2cc (Gy) 5.27 4.54 5.90 6.62

Total Lung D1500cc(Gy) 0.68 0.61 0.84 1.00

D1000cc (Gy) 1.83 1.76 2.01 2.61

V10 Gy (%) 11.49 11.30 12.43 15.14

esophagus D5cc(Gy) 7.15 6.60 8.04 9.49

Dmax 13.96 12.53 14.70 15.21

Heart D15cc(Gy) 17.37 18.42 17.88 19.77

Dmax(Gy) 39.50 55.82 39.68 43.38

Trachea and large Bronchus D4cc(Gy) 8.65 9.67 9.43 10.47

Dmax(Gy) 24.40 28.65 26.01 27.07

Great Vessels D10cc(Gy) 25.47 26.30 26.76 29.40

Dmax(Gy) 45.51 66.41 46.65 50.98

Chest Wall D30cc(Gy) 13.27 12.74 14.02 16.40

Case		Parameters	Plan _TCHybrid	Plan _TCRGS	Plan _TCLinac	Plan _Edge
	Spinal Cord	Dmax	8.39	7.42	11.74	13.17
Case1		D0.35cc (Gy)	7.61	6.5	10.35	10.91
		D1.2cc (Gy)	7.08	5.9	9.38	9.4
	Total Lung	D1500cc(Gy)	0.47	0.4	0.47	0.68
		D1000cc (Gy)	1.55	1.59	2.05	2.8
		V10 Gy (%)	14.48	13.48	15.22	17.73
	esophagus	D5cc(Gy)	9.65	9.21	10.87	13.58
		Dmax	19.04	17.65	19.94	21.42
	Heart	D15cc(Gy)	22.96	20.19	22.19	24.32
		Dmax(Gy)	65.58x	90.29x	66.65x	66.79x
	Trachea and large Bronchus	D4cc(Gy)	1.24	2.68	1.33	2.69
		Dmax(Gy)	31.47	27.9	30.58	33.06
	Great Vessels	D10cc(Gy)	25.94	22.7	24.77	27.32
		Dmax(Gy)	63.72	84.88x	64.12	65.76
	Chest Wall	D30cc(Gy)	15.56	13.44	16.35	18.88
	Spinal Cord	Dmax	5.77	6.04	6.81	7.82
Case2		D0.35cc (Gy)	5.34	5.43	5.81	5.9
		D1.2cc (Gy)	5.09	5.11	5.42	6.39
	Total Lung	D1500cc(Gy)	1.21	1.07	1.29	1.72
		D1000cc (Gy)	2.05	1.83	1.98	2.71
		V10 Gy (%)	13.13	14.19	13.63	21.02
	esophagus	D5cc(Gy)	4.25	4.69	4.59	5.82
		Dmax	8.83	9.47	9.01	10.79
	Heart	D15cc(Gy)	15.24	15.4	15.14	17.99
		Dmax(Gy)	36.38	51.64x	37.6	37.63
	Trachea and large Bronchus	D4cc(Gy)	13.86	16.05	14.51	16.05
		Dmax(Gy)	34.91	53.16x	36.99	38.56
	Great Vessels	D10cc(Gy)	25.84	26.58	27.31	31.41
		Dmax(Gy)	37.67	60.03x	38.53	40.47
	Chest Wall	D30cc(Gy)	9.38	9.04	9.82	12.77
	Spinal Cord	Dmax	4.58	3.75	4.25	5.59
Case3		D0.35cc (Gy)	4.07	3.34	3.7	4.8
		D1.2cc (Gy)	3.68	2.29	3.26	4.2
	Total Lung	D1500cc(Gy)	0.19	0.15	0.2	0.28
		D1000cc (Gy)	0.92	0.84	0.98	1.28
		V10 Gy (%)	2.77	2.57	3.11	4.01
	esophagus	D5cc(Gy)	5.55	4.87	7.09	7.18
		Dmax	10.91	9.58	13.33	11.86
	Heart	D15cc(Gy)	0.48	0.68	0.4	0.37
		Dmax(Gy)	1.57	1.51	1.58	6.26
	Trachea and large Bronchus	D4cc(Gy)	6.56	5.75	8.6	7.71
		Dmax(Gy)	10.79	9.02	13.59	12.41
	Great Vessels	D10cc(Gy)	14.12	14.03	15.17	16.29
		Dmax(Gy)	39.25	53.75x	40.01	44.84
	Chest Wall	D30cc(Gy)	6.06	5.36	7.42	7.95
	Spinal Cord	Dmax	6.92	6.45	7.47	9.38
Case4		D0.35cc (Gy)	6.17	5.77	6.53	6.93
		D1.2cc (Gy)	5.24	4.84	5.53	6.47
	Total Lung	D1500cc(Gy)	0.86	0.82	1.4	1.33
		D1000cc (Gy)	2.81	2.77	3.02	3.66
		V10 Gy (%)	15.58	14.95	17.77	17.8
	esophagus	D5cc(Gy)	9.13	7.64	9.59	11.36
		Dmax	17.07	13.4	16.5	16.76
	Heart	D15cc(Gy)	30.8	37.39	33.8	36.41
		Dmax(Gy)	54.47	79.84x	52.9	62.83
	Trachea and large Bronchus	D4cc(Gy)	12.93	14.2	13.28	15.43
		Dmax(Gy)	20.41	24.5	22.89	24.25
	Great Vessels	D10cc(Gy)	35.98	41.89	39.77	42.57
		Dmax(Gy)	41.4	66.96x	43.93	52.83
	Chest Wall	D30cc(Gy)	22.06	23.11	22.47	26.01
	Spinal Cord	Dmax	6.42	5.92	7.57	8.99
Average		D0.35cc (Gy)	5.80	5.26	6.60	7.14
		D1.2cc (Gy)	5.27	4.54	5.90	6.62
	Total Lung	D1500cc(Gy)	0.68	0.61	0.84	1.00
		D1000cc (Gy)	1.83	1.76	2.01	2.61
		V10 Gy (%)	11.49	11.30	12.43	15.14
	esophagus	D5cc(Gy)	7.15	6.60	8.04	9.49
		Dmax	13.96	12.53	14.70	15.21
	Heart	D15cc(Gy)	17.37	18.42	17.88	19.77
		Dmax(Gy)	39.50	55.82	39.68	43.38
	Trachea and large Bronchus	D4cc(Gy)	8.65	9.67	9.43	10.47
		Dmax(Gy)	24.40	28.65	26.01	27.07
	Great Vessels	D10cc(Gy)	25.47	26.30	26.76	29.40
		Dmax(Gy)	45.51	66.41	46.65	50.98
	Chest Wall	D30cc(Gy)	13.27	12.74	14.02	16.40

x: Failed (above the safe dose threshold recommended by relevant guidelines).

For the spinal cord, Plan _TCRGS consistently exhibited the best D_max values, followed by Plan _TCHybrid, across all cases. These metrics were invariably lower than those observed in Plan _Edge. Regarding the total lung, Plan _TCRGS and Plan _TCHybrid generally had lower dose parameters (D_1500cc, D_1000cc, and V₁₀ _Gy) than Plan _Edge, indicating better OAR sparing. In terms of the esophagus, Plan _TCHybrid and Plan _TCRGS showed preferable D_5cc and D_max values, compared to Plan _Edge, suggesting better dose distribution and safety. For the heart, the D_max values in the Plan _TCRGS were notably higher and even exceeded the tolerance in some cases, whereas Plan _TCHybrid achieved acceptable OAR doses that were not higher than those in Plan _Edge. Regarding the trachea and large bronchus, both Plan _TCHybrid and Plan _TCRGS demonstrated lower D_4cc and D_max values compared to Plan _Edge, which implies better OAR protection. For great vessels, Plan _TCRGS displayed the hightest D_max values, exceeding the safety dose threshold in many instances. Plan _TCHybrid and Plan _TCLinac were generally in line or better than Plan _Edge. For the chest wall, all plans except Plan _Edge keep D_30cc below the recommended guidelines.

When analyzing based on the average values, Plan _TCRGS recorded the lowest D_max to the spinal cord, at 5.92 Gy, compared to 8.99 Gy in Plan _Edge, which was the highest. Plan Edge had a D_1500cc of 1.00 Gy and a D_1000cc of 2.61 Gy, which were the highest among all plans. It also had the highest lung V₁₀ _Gy, at 15.14%. The esophagus, in the case of Plan _Edge, was subject to a D_max of 15.21 Gy, which was higher than any other plans but not remarkably so. The D_5cc in Plan _Edge was also the highest, at 9.49 Gy. Plan _TCRGS resulted in the highest D_max to the heart, at 55.82 Gy, which was significantly greater than 43.38 Gy for Plan _Edge. However, Plan _Edge had the highest D_15cc at 19.77 Gy. For trachea and large bronchus, Plan _Edge was also associated with a higher D_max (27.07 Gy) and D_4cc (10.47 Gy). The D_max to the great vessels was highest in Plan _TCRGS (66.41 Gy), considerably greater than in Plan _Edge (50.98 Gy). However, Plan _Edge had the highest D_10cc of 29.40 Gy. The chest wall was exposed to the highest radiation in Plan _Edge, with a D_30cc of 16.40 Gy.

In summary, Plan _TCRGS often showed the best dosimetric outcomes for OARs but sometimes exceeded the safety dose threshold, rendering it clinically unusable. Plan _TCHybrid consistently exhibited OAR doses that were either comparable to or better than the clinically approved Plan _Edge, making it a viable alternative for treatment planning.

3.4 Treatment efficiency estimation

Table 5 tabulates the beam-on time of different techniques for the four patients. The Plan _TCRGS consistently recorded the longest beam-on time across all cases, with its duration being notably higher than the other plans. Particularly, in Case 2, it reached up to 33.28 minutes, marking it as the worst performer in terms of beam-on time. In contrast, Plan _TCLinac consistently showed the shortest beam-on time, emerging as the most time-efficient plan in three out of four cases (Cases 1, 3, and 4). However, in Case 2, Plan _Edge slightly outperformed Plan _TCLinac with a beam-on time of 2.75 minutes, making it the best for that specific case. Plan _TCHybrid′s beam-on times were intermediate, being significantly shorter than Plan _TCRGS but considerably longer than both Plan _TCLinac and Plan _Edge. While it may not offer the same level of efficiency as Plan _TCLinac, it still maintains a reasonable balance between treatment duration and dosimetric outcomes.

Based on all the above results, it can be concluded that the relative trends of the evaluation metrics are generally consistent across the four cases for the four plans.

Table 5
Beam-on Time (min) comparison of different plans in this study

Case Plan _TCHybrid Plan _TCRGS Plan _TCLinac Plan _Edge

Case 1 10.24 21.50 2.44 2.52

Case 2 19.74 33.28 2.85 2.75

Case 3 6.35 16.72 1.67 1.85

Case 4 8.41 14.81 1.06 1.12

Case	Plan _TCHybrid	Plan _TCRGS	Plan _TCLinac	Plan _Edge
Case 1	10.24	21.50	2.44	2.52
Case 2	19.74	33.28	2.85	2.75
Case 3	6.35	16.72	1.67	1.85
Case 4	8.41	14.81	1.06	1.12

4 Discussion

In this study, we performed a comprehensive evaluation of the dosimetry and beam-on time for Plan _TCRGS, Plan _TCLinac, and Plan _TCHybrid, all delivered through the innovative TaiChiB digital radiation system, which combines both accelerator and RGS technologies into a single platform. This evaluation was against the clinically appoved Plan _Edge and pertinent guidelines [13 , 17]. The cohort consisted of patients exhibiting two-lung lesions amenable to SBRT, with one of the targets overlapping with OARs. Our findings underscore that the TaiChiB-based plans were either superior to or similar Plan _Edge in metrics such as CI, R_50 %, and D₂ _cm, and met the clinical requirements. Notably, Plan _TCLinac and Plan _TCHybrid displayed better or comparable dose for most OARs compared with Plan _Edge. However, Plan _TCRGS had the D_max for organs like the heart, major vessels, or trachea (when overlapping with PTV) exceeding their prescribed tolerances. Overall, given equivalent PTV dose coverage and clinically acceptable OAR doses, the TaiChiB-based Plan _TCHybrid emerged as the optimal in OAR sparing, while maintaining acceptable beam-on time. To our knowledge, this is the first dosimetric report of the TaiChiB-based SBRT plans for patients with two-lung tumors (one overlapping OAR), marking a pivotal stride towards the clinical application of hybrid equipment integrating linac and RGS. Yet, the real-world clinical ramifications of these findings await further exploration and validation.

The TaiChiB Radiotherapy System is a novel device, and currently, there is limited research on such equipment. This study falls under exploratory research. While comparative studies on different SBRT techniques are not uncommon, there is a notable scarcity of research on hybrid plans for multi-lesion lung cancer SBRT using integrated devices. This study provides data support for the clinical use of this new equipment and offers new insights for its development.

In the dosimetric comparison for the target (refer to Table 3), Plan _TCRGS exhibited the highest conformity across all four cases, with Plan _TCHybrid closely following. In contrast, the linac-based plans (specifically Plan _TCLinac and Plan _Edge) showed the least conformity. The superior conformity of Plan _RGS can be attributed to its mechanical design. The RGS system delivers dose conformally to the target, utilizing a greater number of non-coplanar focusing beams compared to linac-based plans. Previous studies have also indicated that Plan _RGS offers a sharper dose gradient in comparison to linac-based plans, which aligns with findings from recent reports [22, 23]. It’s worth noting that even though the MLC leaf of the TaiChiB Linac is broader than that of the Edge, Plan _TCLinac still surpasses Plan _Edge in performance. This could be attributed to differences in planning techniques; Plan _Edge employed fixed-field IMRT (ff-IMRT), while Plan _TCLinac used VMAT with enhanced modulation.

In terms of OAR-sparing, both Plan _TCHybrid and Plan _TCLinac either surpassed or matched the performance of Plan _Edge. While RGS can produce a sharp dose gradient, its hotspots within the PTV often exceed double the prescribed dose [22, 23]. This can lead to unsafe D_max values in organs that overlap with the PTV, such as the heart, large blood vessels, and esophagus. Such outcomes might heighten the risk of complications from radiotherapy in patients. On the other hand, Plan _TCHybrid, delivered by the integrated TaichiB device, serves as a hybrid treatment plan of both RGS and linac. This approach harnesses the combined benefits of the lower D_max values from linac and the sharp dose gradient from RGS.

Although linac-based plans didn’t show a dosimetric advantage in this study, they had the shortest beam-on times among all plans (Table 5). The beam-on time for Plan _TCRGS was significantly longer than the two linac-based plans, perhaps due to its more complex planning with more non-coplanar beams targeting lesions. In contrast, linac-based plans had far fewer fields. In addition, the beam-on time of Plan _TCHybrid was only a third of that for Plan _TCRGS, minimizing dose uncertainty during patient treatment.

This study’s findings suggest that patients with two-target lung lesions (one overlapping with OARs) can benefit from treatments utilizing two devices. However, few institutions possess both an accelerator and a gamma knife. The duplication of tasks such as appointment scheduling, planning design, positioning, and position verification not only amplifies the time and manpower demands for medical staff and patients but also compromises patient comfort and dose delivery precision. The TaiChiB digital radiotherapy system addresses these challenges by integrating accelerator and gamma knife treatments within a single apparatus. The data from this study could bolster the clinical adoption of such devices in the future.

Here are some limitations and prospects of this research. Firstly, as a retrospective study, only four patients were enrolled. The limited number of samples was primarily due to the challenges associated with data collection in this particular field of dual-target lung cancer located in different positions. While the limited dataset may affect the generalizability of our findings, it is crucial to note that we have identified the trends and insights that contribute to the value of integrated devices in tumor treatment. Future research efforts will focus on expanding the dataset to include a larger and more diverse sample, thereby enhancing the robustness and applicability of our conclusions. Then, it is necessary to consider whether the dosimetric and biological differences of those techniques in this study have clinically tangible impacts, which would require multi-institutional prospective clinical trials with long-term follow-up. Finally, this study aimed to evaluate the dosimetric results and beam-on times of three TaiChiB-based SBRT plans for patients with multi-lung lesions. It provides a benchmark for understanding the superiority of one technique over another. Factors such as dosimetry, delivery efficiency and clinical situation must be evaluated comprehensively before selecting the most appropriate lung SBRT method.

5 Conclusion

In SBRT-treated patients with two lung lesions and one overlapping OAR, Plan _TCRGS offers better dosimetric results than Plan _Edge but raises efficiency and safety concerns. While Plan _TCLinac is more efficient, its dosimetric quality is unremarkable. Plan _TCHybrid generated by the TaiChiB system combines linac and RGS strengths, ensuring OAR safety and efficient treatment. Thus, Plan _TCHybrid could be the preferred option for these patients.

Ethics approval and consent to participate

The study is a retrospective study. When the study began, all selected patients signed informed consents and completed radiotherapy. Ethical standards and patients’ confidentiality were ensured and in line with regulations of the local institutional review board and data safety laws. This study was approved by the Ethics Committee of Shanghai Chest Hospital (the committee’s reference Number: KS1863).

Consent for publication

All authors of the manuscript have read and agreed to its content and are accountable for all aspects of the accuracy and integrity of the manuscript.

Availability of data and material

The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.

Conflict of interest statement

We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work.

Funding

Nurture projects for basic research of Shanghai Chest Hospital, Grant/Award Number: 2022YNJCQ09.

General Program of National Natural Science Foundation of China, Grant/Award Number: 12375346.

Authors’ contributions

DYH, YS: data collection, statistical analysis, writing and revising the manuscript. HC, HW, HLG, AHF: statistical analysis and revising the manuscript. YH, YL, ZJS: patient administration, and critical revision of the manuscript. QK, ZYX: study design, critical revision of the manuscript and funds collection. All authors gave final approval of the version to be published.

Footnotes

Acknowledgments

The authors thank the staff from OUR United Corporation for their technical support.

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Dosimetry and treatment efficiency of SBRT using TaiChiB radiotherapy system for two-lung lesions with one overlapping organs at risk

Abstract

Purpose:

Methods:

Results:

Conclusion:

Keywords

1 Introduction

2 Materials and methods

2.1 Patients

3 Results

3.1 Case Example

Table 5 Beam-on Time (min) comparison of different plans in this study Case Plan TCHybrid Plan TCRGS Plan TCLinac Plan Edge Case 1 10.24 21.50 2.44 2.52 Case 2 19.74 33.28 2.85 2.75 Case 3 6.35 16.72 1.67 1.85 Case 4 8.41 14.81 1.06 1.12

5 Conclusion

Ethics approval and consent to participate

Consent for publication

Availability of data and material

Conflict of interest statement

Funding

Authors’ contributions

Footnotes

Acknowledgments

References

Table 5
Beam-on Time (min) comparison of different plans in this study

Case Plan _TCHybrid Plan _TCRGS Plan _TCLinac Plan _Edge

Case 1 10.24 21.50 2.44 2.52

Case 2 19.74 33.28 2.85 2.75

Case 3 6.35 16.72 1.67 1.85

Case 4 8.41 14.81 1.06 1.12