Dosimetric study of the plan quality and dose to organs at risk on tangential breast treatments using the Halcyon linac

Abstract Purpose To investigate the plan quality and doses to the heart, contralateral breast (CB), ipsilateral lung (IL), and contralateral lung (CL) in tangential breast treatments using the Halcyon linac with megavoltage setup fields. Methods Radiotherapy treatment plans with tangential beams from 25 breast cancer patients previously treated on a C‐arm linac were replanned for Halcyon. Thirteen corresponded to right‐sided breasts and 12 to left‐sided breasts, all with a dose prescription of 50 Gy in 25 fractions. Plans were created with the following setup imaging techniques: low‐dose (LD) MVCBCT, high‐quality (HQ) MVCBCT, LD‐MV and HQ‐MV pairs and the imaging dose was included in the plans. Plan quality metric values for the lumpectomy cavity, whole‐breast and doses to the organs at risk (OARs) were measured and compared with those from the original plans. Results No significant differences in plan quality were observed between the original and Halcyon plans. An increase in the mean dose (Mean) for all the organs was observed for the Halcyon plans. For right‐sided plans, the accumulated Mean over the 25 fractions in the C‐arm plans was 0.4 ± 0.3, 0.2 ± 0.2, 5.4 ± 1.3, and 0.1 ± 0.1 Gy for the heart, CB, IL, and CL, respectively, while values in the MVCBCT‐LD Halcyon plans were 1.2 ± 0.2, 0.6 ± 0.1, 6.5 ± 1.4, and 0.4 ± 0.1 Gy, respectively. For left‐sided treatments, Mean in the original plans was 0.9 ± 0.2, 0.1 ± 0.0, 4.2 ± 1.2, and 0.0 ± 0.0 Gy, while for the MVCBCT‐LD Halcyon plans values were 1.9 ± 0.2, 0.6 ± 0.2, 5.1 ± 1.2, and 0.5 ± 0.2 Gy, respectively. Conclusions Plan quality for breast treatments using Halcyon is similar to the quality for a 6 MV, C‐arm plan. For treatments using megavoltage setup fields, the dose contribution to OARs from the imaging fields can be equal or higher than the dose from treatment fields.


1.A | Breast cancer
Breast cancer is the cancer with the highest incidence in women in the US after skin cancer. 1 The American Cancer Society estimated 266 120 new cases in women in 2018, which was about 30% of the new female cancer cases, excluding skin cancer. 2 During 2010-2014, in the US, the median age at diagnosis for breast cancer patients was 62 yr. 3 Breast cancer treatment options depend on the stage of the disease, and include mastectomy, breast conserving surgery (BCS), chemotherapy, and radiation. For stages I and II, approximately 60% of the treated patients undergo radiation therapy, 3 which may be administered as external beam therapy, brachytherapy, or a combination of them. can be significant, and needs to be evaluated. The American Association of Physicists in Medicine (AAPM) Task Group 75 recommended that the total dose should be evaluated patient-by-patient, assessing individual exposure risk and also recommended to optimize the imaging protocols to decrease the dose to OARs. 4 For breast cancer patients, reducing the dose to OARs from radiotherapy treatments is important as the 10-year survival probability for diagnosed stage I and II patients treated with radiation after breast-conserving therapy is higher than 60%. 5 Several studies have reported dose to the OARs from the different modalities available for radiation delivery including three dimensional conformal therapy (3D-CRT), 6,7 tomotherapy, [8][9][10][11][12][13] static-field intensity-modulated radiation therapy (IMRT) 10,12,14,15 and volumetric arc therapy (VMAT). 8,10,[15][16][17][18] 1.C | Halcyon The Halcyon TM system is a bore-enclosed, jawless linac using a 6 MV flattening-filter free (FFF) beam, where beam modulation is achieved by means of a stacked and staggered dual-layer multi-leaf collimator (MLC). Halcyon treatments are 100% image-guided, allowing a faster turnaround due to its high efficiency while ensuring accurate setup. Another distinctive feature of the Halcyon is that it does not have light field, optical distance indicator or lasers at the treatment isocenter. Instead, it has a virtual isocenter out of the bore identifiable by external lasers. In a typical treatment, the patient is setup using the virtual isocenter lasers, and then the couch is moved into the bore to the treatment isocenter and patient setup is  This study was performed for 25 breast plans, 12 left-sided, and 13 right-sided, corresponding to patients with stages I and II, without nodal involvement, undergoing radiation after breast conservative surgery. Patients had a chest wall separation of less than 22 cm so that 6 MV beams were appropriate for planning. The dose prescription was 50 Gy in 25 fractions using tangential 6 MV photon beams and a boost of 10 Gy in 5 fractions with electron beams (6 or 9 MeV). Patients were treated with a Varian 21EX linac with a Millennium 120 MLC (Varian Medical Systems, Palo Alto, CA, USA). The tangential photon plans were replanned for the Halcyon system to compare plan quality and dose to OARs.

2.A.2 | Simulation and contouring
Patient simulation was performed with a GE CT scanner available at our radiotherapy department. The slice thickness was 2.5 mm and the scans covered from the mid-abdomen to the clavicle with the  Alto, CA, USA) for contouring and planning. The physician contoured the lumpectomy cavity and reviewed the contours for OARs: ipsilateral lung (IL), contralateral lung (CL), and heart. For this study, treatment breast and contralateral breast (CB) were also contoured including all the palpable breast and setting the borders to exclude the chest wall and 5 mm from the skin. The medial border was set to the mid-sternum and the caudal limit to 2 cm below the inframammary line.

2.A.3 | C-arm planning
For the original plans, using the 21EX linac, the tangential fields were defined using the linac jaws. The superior limit was set to the head of the clavicle and the inferior limit to 2 cm below the inframammary line, the medial margin was set to the mid sternum, and the anterior limit included a 2 cm flash from the contour of the body. Gantry angles were set such that the posterior edges of the opposing fields were coplanar and preventing the fields to extend more than 2 cm into the lungs. Dose was calculated with a grid size of 0.25 cm using the Anisotropic Analytical Algorithm (AAA), version 13.6. The irregular surface compensator tool was used for both fields, and hot spots larger than 105% were cleared by using the fluence editor available in Eclipse. For left-sided treatments, the heart was spared by adding a heart block defined by the MLC. Figure 1 shows the workflow we used to generate tangential breast treatments with Halcyon. The planning process starts by selecting one of the imaging techniques available and the field size for the imaging beams. The dose contribution from these imaging fields will be included in the total dose reported by the treatment planning system. The second step is to check and correct the positioning of the couch. When a plan is created for Halcyon, the couch is positioned automatically by Eclipse but when the breast board is used, the automatic positioning of the couch may be inaccurate. This task is performed in the axial view and verified on the sagittal view using a lung window leveling as seen in Fig. 1. The next step is to define the isocenter, which can be moved directly on the axial view. A constraint for Halcyon is that the position of the isocenter is limited by the lateral range of the couch inside of the bore. This poses a challenge when planning for larger patients, as the isocenter would need to be displaced more laterally. If the position of the isocenter is such that the couch collides with the bore, the bore will appear highlighted on the axial view and the system will not perform any dose calculation. Once the isocenter is set without collisions, the beam angle can be adjusted on the axial view and the collimator angle value can be entered manually while checking it on the beam's eye view with the body contour turned on. The field sizes are adjusted on the beam's eye view, setting the superior limit to the head of the clavicle and the inferior limit to 2 cm below the inframammary line, the medial margin to the mid sternum, and the anterior limit including a flash from the contour of the body. The skin flash will be adjusted on a later step. A similar approach is done for the opposing field, ensuring that gantry angles are set such that the posterior edges of the opposing fields are coplanar and preventing the fields to extend more than 2 cm into the lungs. At our clinic, the beam configuration is checked and modified if needed by the radiation oncologist to ensure appropriate coverage. The oncologist provides the contouring for the lumpectomy and other OARs are contoured as well. The first dose calculation is performed with the initial beam configuration and then, the irregular surface compensator is inserted for each beam with a penetration depth of 50%. At this point the flash is incorporated, by measuring the transmission factor of a point near the edge of the body contour and assigning that value (usually between 0.75 and 0.90) to the surrounding area 2 cm beyond the most anterior point of the body. Finally, the fluence editor is used to F I G . 1. Workflow for tangential whole breast treatments in the Halcyon TM system, steps performed by the Radiation Oncologist is indicated with an asterix. reduce hotspots larger than 105% of the prescription dose and the plans are reviewed by the radiation oncologist, who checks the coverage, normalization, and dose to OARs.

2.A.4 | Halcyon planning
In this work, four Halcyon plans, one for every imaging technique available were created for each patient.
The Halcyon plans had the same isocenter and beam configuration as the C-arm plans and the tangential fields were defined using the MLC with the same aperture as the C-arm plans. The field sizes for the imaging fields were (

2.B | Plan quality evaluation
The total number of monitor units (MUs) was compared for C-arm and Halcyon plans. Dose volume histogram (DVH) metrics naming followed TG-263 guidelines 19   technique was used. This can be explained considering that the T A B L E 2 Dose to organs at risk in C-arm plans and Halcyon plans for right-sided treatments. Stated values indicate mean ± SD and statistical analysis was performed using the two-sample Kolmogorov-Smirnov test (at the 0.01 level).  | 63 MV HQ plans but not for the MV/MV LD plan. CL Mean also increased significantly, at the P < 0.001 level, for all the Halcyon plans.

4.C | Dose to OARs
Halcyon plans had higher dose values to the OARs than C-arm plans for most of the metrics evaluated in this work. In particular higher doses to the CB are a concern as it has been reported that for women more than 40 yr old, mean doses to the CB higher than In the case of the heart, the probability of major coronary events increase linearly with the mean dose to the heart by 7.4% per Gy. 25 Taylor et al, 26 performed a systematic review of published doses to the heart from breast cancer radiation therapy and reported that for treatments that did not included the internal mammary nodes, the mean dose to the heart was 3.4 ± 0.2 Gy and 5.6 ± 0.4 Gy for tangential treatments and IMRT, respectively. In this study, for Halcyon plans using MVCBCT LD fields, the mean dose to the heart was

4.D.2 | Use of nonionizing imaging techniques
SGRT has been successfully used to setup and monitor linac-based stereotactic radiosurgery and deep inspiration breath-hold treatments. [28][29][30] At our institution SGRT has been successfully implemented for setting up patients treated on the brain area with the Halcyon linac, reducing the residual rotational errors before the IGRT fields and the total treatment time. 31 Surface-guided radiotherapy could be used for breast patients in Halcyon for daily setup. To fully take advantage of this approach, the user should have the option to skip the imaging fields, however, this would require a site-specific modification in the workflow by the manufacturer.

4.D.3 | Angled MV ports
Dose from the imaging fields may also be reduced with site-specific modifications by the manufacturer, such as MV images at arbitrary angles. Tangential images at the treatment angles are part of the setup technique at our clinic using the C-arm linac for breast patients as they allow the visualization of the breast and minimize the dose to the OARs, Fig. 6

CONFLI CT OF INTEREST
We have no relevant conflict of interest to disclose.