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Year : 2021  |  Volume : 17  |  Issue : 5  |  Page : 1165-1171

Dosimetry study of nasopharyngeal carcinoma based on Halcyon accelerator fixed-field intensity-modulated radiation therapy

Department of Radiotherapy, Shandong Cancer Hospital and Institute, Shandong First Medical University and Shandong Academy of Medical Sciences, Jinan 250117, Shandong Province, China

Date of Submission21-Jun-2021
Date of Acceptance30-Sep-2021
Date of Web Publication27-Nov-2021

Correspondence Address:
Yong Yin
Department of Radiotherapy, Shandong Cancer Hospital and Institute, Shandong First Medical University and Shandong Academy of Medical Sciences, Jinan 250117, Shandong Province
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/jcrt.jcrt_992_21

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 > Abstract 

Objective: Halcyon accelerator applies the flattening filter (FF)-free mode instead of the lead gate and FF treatment mode for traditional C-type accelerators. We aimed at comparing and analyzing the quality and delivery of nasopharyngeal carcinoma (NPC) plans between Halcyon and VitalBeam (VB) accelerators in fixed-field intensity-modulated radiation therapy (IMRT).
Methods: The IMRT plans for thirty patients with NPC who had received radiotherapy were optimized using the VB (Plan VB) and Halcyon (Plan H) accelerators. Quality assurance verification was then conducted. The dose coverage of the planning target volume (PTV) and organs at risk (OARs), monitor units (MUs), and delivery time were analyzed for each plan.
Results: All PTV and OAR indexes of Plan H and Plan VB met the clinical requirements. In the exposure dose of bilateral optic nerves between Plan H and Plan VB, no difference was found. The maximum dose of the lens, brainstem, spinal cord were 1.13 Gy, 1.36 Gy, 1.35 Gy, 2.82 Gy lower than the plan using VB , and the mean dose of the parotid glands were 3.82 Gy, 5.56 Gy lower than the plan using VB respectively, and an insignificant difference was found in the brainstem (P > 0.05). The MU for Plan H (22.92 ± 1.58 Gy) was higher than that for Plan VB (19.69 ± 4.52 Gy), and the difference was significant (P < 0.05).
Conclusions: The treatment plans designed by Halcyon can meet clinical requirements with better protection for OARs and show advantages over VB in the dosimetry of NPC IMRT plans.

Keywords: Delivery efficiency, Halcyon accelerator, intensity-modulated radiation therapy, nasopharyngeal carcinoma, VitalBeam accelerator

How to cite this article:
Li K, Li C, Zhu T, Chen J, Cao W, Yin Y, Ma C. Dosimetry study of nasopharyngeal carcinoma based on Halcyon accelerator fixed-field intensity-modulated radiation therapy. J Can Res Ther 2021;17:1165-71

How to cite this URL:
Li K, Li C, Zhu T, Chen J, Cao W, Yin Y, Ma C. Dosimetry study of nasopharyngeal carcinoma based on Halcyon accelerator fixed-field intensity-modulated radiation therapy. J Can Res Ther [serial online] 2021 [cited 2022 May 26];17:1165-71. Available from: https://www.cancerjournal.net/text.asp?2021/17/5/1165/331316

 > Introduction Top

Nasopharyngeal carcinoma (NPC) is one of the most common malignancies in South China and is mainly managed using radiotherapy.[1] The 5-year overall survival of NPC patients has already reached 86.3% due to the advent of intensity-modulated radiotherapy (IMRT) and concurrent chemotherapy (CCT).[2] Nevertheless, treatment-related late toxicities are still common, especially in those receiving CCT. The incidence of late toxicity was reported to be as high as 70.8%. Moreover, the incidence of Grade ¾ late toxicity was nearly 11.4%.[3],[4] Due to the large target area, involving many organs, and high complexity, high-precision treatment technology must be used. With the development and progress of precision radiotherapy and intelligent radiotherapy, modulated radiation therapy has slowly become the standard treatment for NPC, achieving better dose coverage to protect normal tissue and significantly improving local control.[5],[6],[7],[8]

Currently, intensity modulation technology enables adjusting the accelerator's multileaf collimator (MLC) motion to meet the dose distribution requirements for clinical treatment.[9],[10],[11] Thus, the rays in the traditional IMRT flattening filter (FF) mode are not fully used.[12] A new straight-through jawless treatment delivery system was introduced by Varian Medical Systems (Palo Alto, CA), trademarked Halcyon. This system introduced a new dual-layer MLC design to fulfill the jawless configuration while simultaneously providing sufficient beam attenuation and shape modulation.

The new Halcyon accelerator by Varian introduces 29 pairs of upper-layer MLC and 28 pairs of lower-layer MLC. Halcyon reduces the setting of the jaws of a traditional accelerator, and the lower blade reduces the leakage and transmission between blades by moving half of the blade width toward parallel motion.[13] The Halcyon accelerator system's actual dual-layer MLC leakage rate is only 0.7%, much lower than that of the VitalBeam (VB) accelerator with 1.5%, and can provide a motion speed of 5.0 cm/s, which is twice that of the VB accelerator.[14],[15] At present, the Halcyon accelerator system only supports 6 MV FF-free (FFF) modes for treatment, which can increase the output usage of photon beam.

Because of the unique structure of the Halcyon accelerator, its clinical application value warrants investigation. The VB accelerator also has the FFF mode; however, the advantages and disadvantages of the Halcyon compared to VB are unknown. Some studies[16],[17] have reported that it can meet the clinical requirements of H and N cancer, but does Halcyon still have advantages over the latest C-arm accelerator VB? This study analyzed the planning quality and delivery efficiency of Halcyon and VB in treating NPC IMRT.

 > Objects and Methods Top


Thirty patients with NPC who had received IMRT in our hospital from January 2019 to October 2019 were randomly selected for this study. The ICRU Report Nos. 50 and 62 were referred to define the target. GTV was divided into primary foci GTV (GTV-P) and lymph nodes GTV (GTV-N). CTV was divided into high-risk volume (CTV1) and low-risk area CTV2. Planning target volume (PTV) = PTV-G (GTV + 5 mm) + PTV-C1 (CTV1 + 3 mm) + PTV-C2 (CTV2 + 3 mm). The mean ± standard deviation of the PTV was 727.06 ± 180.56 cc. The prescription dose was 70 Gy, with a single dose of 2 Gy for 35 fractions. All delineations were performed by only a single medical doctor.


The VB features a 6-MV FF mode and a 6-MV FFF mode. The beam-shaping collimation comprises upper and lower jaws with a jaw that follows the MLC. The VB can deliver a 6-MV FF beam at 400 monitor unit (MU)/min and a 6-MV FFF beam at 800 MU/min and is equipped with a Millennium MLC with a 14.5-cm limit on overtravel of the leaves. The Millennium MLC has 120 leaves, with a maximum leaf speed of 2.5 cm/s. The central 80 leaves and outer 40 leaves have projection leaf widths of 0.5 cm and 1.0 cm, respectively, at the isocenter. The leaf has an 8.0-cm radius of curvature with a 6.5-cm height. Leaf transmission is reduced by jaw and MLC. The Halcyon (Varian Medical Systems, Palo Alto, CA, USA) accelerator can deliver a 6-MV FFF beam with a maximum dose rate of 800 MU/min. The device is configured with a dual-layer MLC (SX1 mode). The proximal bank pairs have 58 leaves, and the distal bank pairs have 56 leaves with a standard width of 1 cm at the isocenter. Leaf transmission is reduced by stacked and staggered leaves. Each leaf can reach across the entire 28-cm field and has a 23.4-cm radius of curvature with a 7.7-cm height.

Treatment planning

All plans were designed by the same experienced physicist and designed by the Eclipse V15.5 planning system of Varian Medical Systems (Palo Alto, CA). The plans designed by VB accelerator with 6-MV FF/FFF mode were named as Plan VB (FF/FFF), and were a coplanar fixed nine-field equipartition IMRT plan with gantry angles of 0°, 40°, 80°, 120°, 160°, 240°, 280°, and 320°, in which the appropriate angles of collimators were chosen to avoid irradiation of the lens. Using a photon optimization algorithm (Photon Optimizer, PO; V15.5) and an anisotropic analysis algorithm (Anisotropic Analytical Algorithm, AAA; V15.5), the PTVs and organs at risk (OARs) of thirty patients were reintroduced into the Varian V15.5 planning system, and the plans were redesigned by the Halcyon platform and defined as Plan H (setting the low MV cone-beam computed tomography (CBCT) and scanning the upper boundary below the lens). The gantry and collimator angle, optimization, analysis algorithm, and prescription dose were the same as those for Plan VB in the 6-MV FFF mode. All the plans were normalized by 95% of the prescribed dose as the 100% PTV. Because the Halcyon accelerator is completely dependent on CBCT for positioning verification, the megavolt radiation dose has a certain cumulative effect on the endangered organs. Therefore, in order to explore the dose accumulation of 5 MU MV CBCT on OARs, Plan H without normalization was defined as Plan H norm without CBCT dose was defined as Plan HC.

Planning evaluation

The dose–volume histograms (DVHs) of the target and OARs and the RT plan files were exported from the two plans of the planning system. For the dosimetric evaluation, the following indexes were calculated: the maximum and minimum doses, represented by the doses received by 2% (D2) and 98% (D98) of the target volume, respectively. In addition, the homogeneity index (HI) and conformity index (CI) of the target were calculated as follows:




For the OARs, the maximum dose (Dmax) of the lens, optic nerves, brainstem, spinal cord were reported. The mean dose (Dmean) and the volumes of the parotids receiving more than 10, 20, and 30 Gy (V10, V20, and V30) were reported. The total MU of all the plans was counted. In this study, the limit of endangered organs is referred to in [Table 1].
Table 1: The clinical requirements of organs at risk

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Delivery verification

All the plans were verified using Varian portal dosimetry. The composite fluence of all fields in one plan was evaluated using gamma-index analysis (3%/3 mm) with a 10% lower dose exclusion threshold. The delivery time was timed to simulate the actual treatment time of the patient.

Statistical analysis

Any differences among the thirty patients according to the VB and Halcyon accelerators were analyzed by SPSS 21.0 (SPSS Inc., Chicago, IL, USA). The paired sample t-test was chosen to compare two plans, and the nonparametric Friedman test was chosen for the pairwise comparison of multiple groups of data, where P < 0.05 was considered statistically significant.

 > Results Top

All NPC IMRT plans designed and normalized using the Halcyon and VB accelerators met the clinical requirements regarding the PTV coverage and radiation dose of OARs.

Planning target volume coverage

As indicated in [Table 2], the D2 and D98 of Plan H were slightly lower than those of Plan VB (FF) and Plan VB (FFF), and insignificant differences were discovered among the plans in the D2, D98, HI, and CI of the PTV (P > 0.05) except the D2 of Plan H (75.90 ± 0.32 Gy) with Plan VB (FF) (76.14 ± 0.47 Gy) and Plan VB (FFF) (76.24 ± 0.38 Gy). Therefore, the dose distributions of the three plans for a typical patient in this study are indicated in [Figure 1], and the DVHs of the three plans for the same patient are indicated in [Figure 2].
Table 2: Dose of the planning target volume index of intensity-modulated radiation therapy plans for thirty cases of nasopharyngeal carcinoma

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Figure 1: Dose distributions of the three plans for a representative patient. The images show Halcyon and VitalBeam accelerators from the top row to the bottom row. The structures outlined in green are the nasopharyngeal carcinoma planning target volumes

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Figure 2: The dose–volume histograms of the planning target volume and organs at risk for VitalBeam (black solid for flattening filter and blue solid for flattening filter free) and Halcyon (red solid)

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The table shows the dose index of the PTV and statistical analysis values for each plan group.

Dose of organs at risk

Regarding OARs [Table 3] and [Figure 2], insignificant differences were found between the plans in the maximum dose of the optic nerves (P > 0.05) and brainstem (P > 0.05). The clinical requirement for the lens was relatively high. Halcyon exhibited better protective ability for the left and right lens than VB (FF) (P < 0.05), which was reduced using 1.22 Gy and 1.26 Gy, respectively. VB and FFF showed a lower dose than FF (P < 0.05) but a higher dose than Halcyon (P > 0.05). For parallel organs, the mean doses of the left and right parotid glands in Plan VB (FF) were 28.42 ± 3.03 Gy and 28.69 ± 3.17 Gy, while those in Plan H were 24.73 ± 3.33 Gy and 24.73 ± 3.59 Gy, respectively, and the V10, V20, and V30 indexes of the parotid glands in Plan H were reduced significantly (P < 0.05) compared to Plan VB (FF). Plan VB (FFF) minimally reduced the dose of the parotid glands compared with Plan VB (FF); however, Plan H showed a significantly lower dose than Plan VB (FFF), 2.89 Gy versus 3.26 Gy (P < 0.05).
Table 3: Dose of the organ at risk of intensity-modulated radiation therapy plans for thirty cases of nasopharyngeal carcinoma

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Low MV CBCT had a specific effect on the dose of OARs. All the dose indexes of OARs [Table 4] of Plan HC were lower than those of Plan H norm, and an insignificant difference was found between them in the mean dose of the left parotid gland (P > 0.05).
Table 4: Dose of organs at risk of the Halcyon plans for thirty cases of nasopharyngeal carcinoma with or without MV cone-beam computed tomography

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The table shows the dose index of OARs and statistical analysis values for each plan group.

The table shows the dose of OARs and statistical analysis values for plans with or without MV CBCT.

Monitor units

MU [indicated in [Table 5]] of Plan H (2291.64 ± 158.20) was higher than that of Plan VB (FF) (1969.06 ± 452.10), with an average increase of 16.38% (P < 0.05). However, Plan VB (FFF) showed the highest MU at 2687.63 ± 523.87.
Table 5: Monitor unit of intensity-modulated radiation therapy plans for thirty cases of nasopharyngeal carcinoma

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The table shows the MU and statistical analysis values for each plan group.

Dosimetry verification

The verification pass rate [indicated in [Table 6]] of Plan H was slightly better than those of Plan VB (FF) (P > 0.05) and Plan VB (FFF) (P > 0.05), and γ analysis (3 mm; 3%) revealed rates of 99.68% ± 0.54%, 99.13% ± 0.94%, and 99.34% ± 0.67%, respectively. The plan delivery time of Plan H was significantly lower than that of Plan VB (FF), improving the delivery efficiency of the plan by 56.88% (P < 0.05). Plan VB (FFF) also reduced the delivery time to 50.71% (P > 0.05).
Table 6: Comparison of the verification of intensity-modulated radiation therapy plans for thirty cases of nasopharyngeal carcinoma

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The table shows the pass rate, delivery time, and statistical analysis values for each plan group.

 > Discussion Top

Varian's new linear accelerator – Halcyon (V1.0) – was evaluated, which can generate clinically acceptable, high-quality treatment plans. The accelerator breaks the design of the traditional C-arm, puts the accelerator head inside the casing, and uses a slip ring structure to accelerate the rotation of the frame. Catalina Riley et al.[16] discovered that Halcyon can meet the requirements of clinical radiotherapy for head-and-neck tumors. Chengqiang Li et al.[18] showed that the IMRT plan for cervical cancer designed by the Halcyon accelerator has the advantages of higher efficiency and dose consistency than the Trilogy accelerator. The statistics of the number of fields show that the Halcyon has lower scattering and leakage, which were also confirmed by Li et al.[19] Furthermore, Jia et al.[20] reported that the 6-MV FFF mode could protect OARs better than the mode for treating NPC.

When the VB accelerator performs dynamic intensity regulation, it retains the fixed mode of the traditional accelerator and increases the mode of jaw tracking. Jaw tracking indicates that while the MLC is moving, the jaw in the X and Y directions automatically adjusts the irradiation range according to the size of the subfield. Yu Murakami et al.[21] showed that VMAT using jaw-tracking technology in the FFF mode was statistically significant for reducing lung SBRT plan OAR dose (the lung V20 was 1.2%, and that of all other OARs was <1%). Li Cheng et al.[22] pointed out that the dynamic intensity regulation plan by jaw tracking is less affected by radiation leakage of the MLC and closed section, low dose of OARs, and higher verification pass rate than the fixed intensity regulation plan by jaw tracking. This finding is consistent with the conclusion of Yao et al.[23]

To be more suitable for clinical practice, this study used the FF and FFF modes of VB and the FFF mode of Halcyon for comparative analysis. The dose of the target and organs met the clinical requirements in Plan H, Plan VB (FF), and Plan VB (FFF). The D2 and D98 of Plan H were slightly lower than those of Plan VB (FF) and Plan VB (FFF), but no significant difference was found between the plans. Furthermore, Halcyon showed advantages in protecting OARs. The maximum radiotherapy dose in the left and right crystalline lens in Plan VB (FFF) was significantly lower than those in Plan VB (FF). A previously published study reported that the eye's crystalline lens is susceptible to radiation damage. Therefore, the lower radiation triggered by the FFF mode might reduce the radiation risk to the left and right crystalline lens. The dose to the brainstem in Plan VB (FFF) was decreased by 1.08 Gy compared with that in Plan VB (FF), which may be caused by the more rapid drop in the axis dose, reduced volume dose, reduced beams, and decreased radiation dose of the MLC in the FFF mode. However, the dose was reduced more with Halcyon than VB in the FFF mode, likely because of the low scattering and leakage by the dual-MLC. Although FFF increases the MU of Halcyon, the treatment time is reduced because of the high dose rate. Finally, all plans with EPID configured on the machine were verified, and the pass rate showed no difference between the plans. Considering that the CBCT of Halcyon is of a megavolt class, the normalization will modify the segment weight to make the dose distribution more uniform. However, the 5-MU MV CBCT is also involved in this series of calculations, and the results cannot fully explain the effect. Thus, the Plan HC group was set up to explore the effect of MV CBCT on the dose. Normalization will homogenize the target dose and cause a change in field dosimetry.[24] Therefore, to evaluate the dose of MV CBCT on OARs, they were made nonnormalized, and the results showed that 6-MV CBCT produces sufficient doses in OARs.

Compared to the traditional FF mode, the FFF mode removes the jaw, has soft radiation, less scattering, and leakage, and causes a lower dose around the radiation field.[16] Thus, the movement of the MLC can achieve the dose distribution of clinical requirements, improve the beam usage rate, and increase the MU. Furthermore, the dose rate can effectively strengthen the working efficiency of the accelerator, reduce the treatment time of patients and the uncertainty of target movement, improve the operation efficiency of accelerators, and reduce the extra radiation in patients. Compared Plan FF and Plan FFF, it was found that FFF improves the dose of PTV and OARs.

Overall, compared with the NPC IMRT plan designed by the VB accelerator, the Halcyon accelerator can meet clinical requirements, has better protection of OARs, can improve delivery efficiency, can reduce treatment time, and may increase the treatment accuracy.

Financial support and sponsorship


Conflicts of interest

There are no conflicts of interest.

 > References Top

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Riley C, Cox C, Graham S, Havran H, Kramer B, Netherton T, et al. Varian Halcyon dosimetric comparison for multiarc VMAT prostate and headandneck cancers. Med Dosim 2018,44:S0958-3947(18)30084-0.  Back to cited text no. 16
Michiels S, Poels K, Crijns W, Delombaerde L, De Roover R, Vanstraelen B, et al. Volumetric modulated arc therapy of head-and-neck cancer on a fast-rotating O-ring linac: Plan quality and delivery time comparison with a C-arm linac. Radiother Oncol 2018;128:479-84.  Back to cited text no. 17
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  [Figure 1], [Figure 2]

  [Table 1], [Table 2], [Table 3], [Table 4], [Table 5], [Table 6]


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