Treatment planning comparison of volumetric modulated arc therapy with the trilogy and the Halcyon for bilateral breast cancer

In radiotherapy for breast cancer, high position accuracy is essential to prevent under-dose in the target and excessive irradiation to OARs. Image guidance must be taken before each patient is treated on the Halcyon. Compared to two-dimensional (2D) position verification, three-dimensional (3D) position verification like CBCT can more accurately measure 3D vector changes and observe the body position rotation [15, 16]. For breast tissue, CBCT is very useful because it can provide 3D soft tissue and bony anatomy information and can be compared with the planning CT to assess the accuracy of the setting [16, 17]. Rossi et al. [18] reported that CBCT matching is recommended when breast/chest wall patients were treated with the VMAT technique. However, CBCT may be associated with more additional dose and time-consuming than orthogonal planar images. On Halcyon, MV CBCT imaging process take only 15 s. It's beneficial for patients. In C-arm accelerators such as the Trilogy, kilovoltage (KV) CBCT can be used to correct the patient’s position if 3D position verification is necessary. In Halcyon 1.0, MV CBCT was used for image-guided radiotherapy (IGRT). Compared to KV imaging, MV imaging has some advantages, such as identical isocenter as the treatment beam and no metal artifacts. Compared with KV imaging, the main disadvantages of MV imaging are higher dose and lower image quality. About Halcyon’s MV CBCT, Malajovich et al. [19] reported that the highest tissue dose of MV CBCT ranges from 2 to 7 cGy per fraction in different treatment sites, which is equivalent to the fractional dose of KV CBCT during breast and pelvic IGRT application, and MV CBCT images of Halcyon is able to identify different soft tissues and lack of metal-induced artifacts. On Halcyon, two different dose image mode could be selected to apply the CBCT. Compared to the low-dose mode, the high-quality imaging mode does not provide material advantages [19]. We found that high-quality CBCT plans increased the OARs doses compared to low-dose CBCT plans. Therefore, we inferred that low‐dose MV CBCT was the optimal setup-field mode. Flores‐Martinez et al. [6] compared four different setup-field plans for unilateral breast cancer, and in their opinion, low-dose MV CBCT was the most suitable technique for patients treated on the Halcyon. Their results agreed with ours. On Halcyon, the dose of the setup field is incorporated in the calculation of the planned dose, and the irradiated doses to the target and the OARs are also reflected in the total plan dose, which is more intuitive.

Partial-arc plans on the Trilogy showed more dosimetric advantage, especially in low-dose volumes of the heart, left ventricle, and lungs and the mean doses of the heart, lungs, and liver. This is because while designing a partial-arc plan, it is possible to artificially choose the arc degree that irradiates less OAR volume. Rotating the collimator angle and fixing the jaw can further reduce the influence of the leakage between the MLC on the dose of the OARs, which can minimize the dose of the OARs. Boman et al. [11] compared the dosimetric differences between the whole and partial arc VMAT plans of unilateral breast cancer, including regional lymph node irradiation, and the results showed that partial-arc plans significantly reduced the dose of the ipsilateral lung and the V₅ of the heart but increased the V₅ of the contralateral breast. The result is similar to ours, but the cases in our study are bilateral breast cancer, which does not involve the dose of the contralateral breast. Comparing the two plans of the Halcyon, the results were contrary to the Trilogy’s, and the whole-arc plans showed better dosimetry. For the PTV, in addition to the mean dose, the whole-arc plan was better than partial-arc plans in terms of the maximum dose, minimum dose, conformity and uniformity. For OARs, partial-arc plans increased the doses to the heart, LV, LAD, and lungs. The results showed that partial-arc plans have no advantage for the Halcyon, which may be related to the jawless setting and the fewer arc degrees. According to the results described above, when designing a treatment plan for bilateral breast cancer, we can choose a more suitable arc setting according to the corresponding LINAC.

Based on the results described above, we mainly compared the dosimetric differences between the Halcyon's whole-arc plan and the Trilogy's two plans. All plans met clinical requirements. For the PTV, apart from the mean dose, there were no significant differences in other dosimetric parameters between the two whole-arc plans on the Halcyon and the Trilogy. Compared with the T-8arc plan, H-4arc showed worse D_mean, D₂ and V₁₀₇, and better D₉₈ and CI of the PTV. In short, the plans of the two machines were comparable in terms of target dose.

Darby et al. [20] found a linear relationship between the mean dose of the heart and the incidence of ischaemic heart disease, and the incidence increased by 7.4% for each 1-Gy increase in the mean dose. Therefore, the mean dose of the heart is often used as a reference for cardiac toxicity. However, the dose to the cardiac substructure also needs to be considered in radiotherapy. Some studies believe that the LAD and LV are important parts of the heart in the context of radiation-induced heart disease [21‐23]. In this study, for low-dose irradiated volumes of the heart and LV, partial-arc plans on the Trilogy showed the lowest values, while for high-dose irradiated volumes, the whole-arc plans on the Halcyon showed the lowest values. For the mean dose of the heart, the partial-arc plans on the Trilogy showed the lowest value, and the whole-arc plans on the Halcyon showed the highest values. This may be related to the additional radiation dose to the heart from each MV CBCT scan. For all dosimetric parameters of the LAD, there is no significant difference between whole-arc plans on the Halcyon and the two plans on the Trilogy.

For lungs, the partial-arc plans on the Trilogy reduced the low-dose irradiated volume (V₅, V₁₀) and the mean dose but increased the high-dose irradiated volume (V₄₀). There was no significant difference in the comparison of the V₂₀ and V₃₀ in the lungs between all plans of the two LINACs. For all dosimetric parameters of the lungs, there was no significant difference between the two whole-arc plans on the two LINACs. Fiorentino et al. [24] retrospectively analysed the VMAT plans of 16 patients with bilateral breast cancer. For lungs, the average values of the D_mean, V₅ and V₂₀ were 11.8 ± 2.3 Gy, 78.9 ± 15.3% and 15.7 ± 5%, respectively. No acute and late complications above grade 2 were observed during the 24 months of follow-up. In our study, the mean dose, V₅ and V₂₀ of the lungs in all plans were lower than in their study.

The plans on the Halcyon increased the skin’s dose compared to the two plans on the Trilogy. O’Grady et al. [25] found that 6 X FFF fields on the Halcyon increased the superficial dose compared to FF fields for breast cancer radiotherapy, as demonstrated by in vivo measurements, phantom measurements, and planning comparisons. Their results were the same as ours. Because the rays are softened after the flattening filter is removed, the 6 MV X-rays in FFF mode are equivalent to lower-energy rays, resulting in a shallower depth of the dose build-up area, thereby increasing the superficial dose. Studies on the Monte Carlo (MC) have shown that the influence of contamination electrons in the FFF mode is greater, which further leads to an increase in surface dose [26, 27]. Barsky et al. [5] retrospectively analysed 34 breast cancer cases treated on the Halcyon, and the results showed that breast cancer cases were well tolerated on Halcyon; additionally, the acute toxicity was comparable to the published reports using conventional LINACs. The difference from our study is that they used tangential fields instead of the VMAT in our study. On the Halcyon, the effect of VMAT on the skin dose needs more prospective clinical studies to be proved.

In contrast to Halcyon, KV CBCT could be used for image-guided on Trilogy. Although the dose of KV CBCT is less than that of MV CBCT, it is not negligible. The difference with Halcyon is that the imaging dose of Trilogy is not calculated into the entire plan. So, in fact, the dose difference between the two machines may vary slightly. The whole-arc plans on Halcyon reduced the number of MUs compared to whole-arc plans on Trilogy and showed similar MUs compared to partial-arc plans on Trilogy. On Halcyon, the imaging dose is integrated into plans and thus it is contributing to the PTV and OAR dose. However, in Trilogy the imaging dose is not accounted for plans, what might explain the lower MUs required on Halcyon.

Previous studies have demonstrated that the Halcyon could reduce the treatment time significantly compared with conventional LINACs [3‐7]. Our study proved the same results in radiotherapy for bilateral breast cancer. This may be related to the Halcyon's faster gantry and MLC speed and higher dose rate. The shortening of the treatment time can reduce intra-fraction movement, improve patient comfort, and increase machine throughput.

An issue that needs to be addressed is the correction of rotation errors. On C-arm accelerators like Trilogy, we can use the six degree of freedom (6-DoF) couch to correct the rotation errors. The combination of 6-DoF couch and CBCT can correct the translational and rotational setup errors and improve the positioning accuracy obviously [28, 29]. However, on O-ring accelerators like Halcyon, the installed 3-DoF couch allows only for correction of translational shifts. On the Halcyon, if large rotation errors are found, time-consuming repositioning may be required to improve the positioning accuracy. This drawback might significantly compromise the advantage of faster treatment plan delivery with Halcyon compared to Trilogy. Therefore, in order to reduce the positioning time and improve the positioning accuracy, it is necessary to improve the immobilization equipment or adopt online 3D IGRT equipment, such as surface imaging. 3D surface imaging system is a quick and non-invasive method to assist the patient in setting up, which could improve the accuracy and speed of patient positioning in breast cancer radiotherapy [30, 31]. On Halcyon, 3D surface imaging is especially suitable, which could correct rotation errors in patient positioning and monitor patient movement during beam delivery [32]. The combination of 3D surface imaging and daily CBCT may greatly improve treatment accuracy and reduce positioning time, which needs more research to prove in the future.

Many studies [33‐35] have shown that deep inspiration breath hold (DIBH) can significantly reduce the radiation dose to the heart and lungs for breast cancer radiotherapy. However, the technique requires patients to hold their breath for a certain amount of time. Minimizing the treatment time is beneficial to patients with DIBH. The combination of Halcyon and VMAT can further reduce the treatment time and expand the application range of DIBH. The robustness of VMAT plans under respiratory motion or other influences which might change form or position of PTV needs to be considered. Field-in-field and fixed-field intensity-modulated radiation therapy can reduce the influence of respiratory movement and skin deformation or swelling during treatment by opening the MLC outside the skin or adding skin flash. When designing VMAT plans, we could increase the robustness of the plan by adding a virtual bolus on the skin [11, 36, 37].

VMAT plans on the Halcyon can meet the clinical requirements in radiotherapy for bilateral breast cancer. Low-dose MV CBCT can be selected as an optimal setup mode. For Halcyon, the whole-arc plans are better than the partial-arc plans. While the Halcyon increases the mean dose to the heart and the dose to the skin compared with the conventional LINAC, it is comparable to the Trilogy in doses to the cardiac substructure and other OARs and significantly reduces the delivery time.