The brain is a common site of metastasis for various malignant tumors, and the vast majority of intracranial tumors are brain metastases (BM).
Radiation therapy, as one of the important treatment methods, can effectively prolong the survival period of patients and improve the quality of life of patients with brain metastases.
With the continuous development and iteration of radiation therapy technology, radiotherapy for brain metastases has also ushered in innovation: after the advent of volume rotation intensity modulation (VMAT) technology, Stereotactic Radiosurgery (SRS) achieved using this technology can more effectively protect normal brain tissue outside the target area and reduce patient side effects compared to whole brain radiotherapy.
The HyperArc "super arc knife" from Varian Medical has outstanding performance in the treatment of brain metastases. From October 2020 to February 2023, the Radiotherapy Center of Jinshazhou Hospital of Guangzhou University of Traditional Chinese Medicine completed 300 cases of brain tumor radiotherapy using HyperArc "super arc knife" on TrueBeam linear accelerator, gaining valuable and rich clinical planning and treatment implementation experience. The following is the sharing from its clinical team.

What is HyperArc?

HyperArc "Super Arc Knife" is a single center non coplanar VMAT SRS treatment technology based on linear accelerator launched by Varian Medical, used to treat single or multiple intracranial lesions. It has the characteristics of high dose in the target area, rapid drop of dose outside the target area, and high automation of HyperArc planning process.
For physicists, HyperArc can achieve more efficient and accurate planning and design, such as one click automatic arc placement, automatic optimization of small nose angle, automatic addition of Lower conditions for each target area, automatic switching from NTO to SRS NTO, and so on.
For technicians, HyperArc's treatment process is safe and efficient: all beam exits can be completed with just one click, and there is no need to enter the machine room for bed rotation during the treatment period, making it more convenient.

HyperArc Program Design Advantages

Varian Medical's radiation therapy planning system platform, Eclipse, has a module specifically designed for HyperArc planning.
The HyperArc program uses 6 MV-FFF energy at a dose rate of 1400 MU/min. The HyperArc program supports automatic field placement: 1 full arc (bed angle 0 °) and 3 non coplanar half arcs (bed angle 315 °, 45 °, 90 °, respectively).
The starting angle and bed angle of the HyperArc plan are the same, but the angle of the small machine head is different. This is because HyperArc technology has a collimator angle optimization function, which automatically optimizes the optimal small nose angle based on the distribution of metastatic tumors in each plan, forming the so-called "island effect".
Island effect: When multiple metastatic tumors are irradiated, the multi leaf collimator (MLC) can cover as much normal brain tissue as possible, so as to avoid the situation shown in Figure 1 as much as possible; When multiple target areas are irradiated, the normal brain tissue between the two target areas cannot be obstructed
The optimization function of the collimator angle can play its role, allowing the plan to reduce the dose of normal brain tissue and simultaneously irradiate multiple metastatic tumors, thereby improving treatment efficiency.
In addition, the HyperArc plan optimization process has SRS NTO (Normal Tissue Objective) function, which is significantly better than the traditional auxiliary structure "ring", allowing the dose line outside the target area to quickly drop. The HA program has also introduced the Automatic Low Dose Objective (ALDO) tool, which can customize the lower target of the target area for the HyperArc program to automatically achieve 98% target coverage.

Figure 1: The "Island Effect" Problem of MLC

Figure 2 Collimator angle optimization function

HyperArc Plan Design Tips Sharing

1. Sketch of thermoplastic film

As shown in Figure 3, the body of the HyperArc program must outline the thermoplastic film dedicated to HyperArc.

Figure 3 BODY Sketching of HyperArc Plan

The reason for doing so is that the CT value of this membrane is relatively high, and if not outlined, it will cause an error of approximately 14.4% in dose calculation.
As shown in the DVH images of Figure 4 and Figure 5 (the lines with the same color represent the same GTV structure, the lines with triangles indicate that BODY does not include thermoplastic film, and the lines with squares indicate that BODY includes thermoplastic film), it can be seen that there is a significant dose error in drawing HyperArc specific thermoplastic film, and there may be differences depending on the location of the target area.

Figure 4: The currently selected GTV4 outlines the HyperArc of thermoplastic film for BODY_ TB Plan

Figure 5: The currently selected GTV4 is a HyperArc with BODY and no thermoplastic film drawn_ NoM Plan

2.Sketch the independent structure of the target area

In the process of delineating target areas, it is important to always remember to use as many structures as there are, and not to use the same structure to delineate all target areas.
For example, if there are three brain metastases, they should be delineated using GTV1, GTV2, and GTV3 respectively, rather than outlining the three brain metastases on the same GTV structure.
Under the premise of consistent planning conditions, through the comparison between Figure 6 and Figure 7, it can be seen that the HyperArc plan in Figure 6 outlines each target area separately, and adds lower components to each target area for optimization, resulting in a better distribution of dose lines in the plan, with 50% of prescription dose lines being disconnected; And the target area merging and delineation plan in Figure 7, 50% of the prescription dose line cannot be disconnected.

Figure 6: Independently delineate each target area and add lower optimization for each structure

Figure 7: Merge and sketch all target areas to the same structure, add lower optimization

3.Processing with a large number of PTVs

When there are a large number of PTVs in the plan, it may be difficult to consider that each PTV can have a 100% prescription dose line covering at least 95% of the volume. In this case, the ALDO function in Figure 8 can be used to automatically achieve a coverage rate of 98% for each PTV.

Figure 8 Use of ALDO function

Alternatively, it is possible to consider using dose lines to generate a replenishment structure and participate in optimization. However, the HyperArc plan does not allow the addition of new PTVs after optimization to continue participating in optimization. Therefore, we can take advantage of this by creating a replenishment structure in advance, as shown in Figure 9, to participate in the creation and optimization of the HyperArc plan.

Figure 9: Participated in the creation of HyperArc plan by creating a replenishment structure in advance for insufficient coverage of certain target areas

HyperArc Program Case Sharing

Patient 1

Multiple brain metastases from lung cancer (5 cases), with a prescription of 48Gy/12 times. The clinical treatment time is approximately 6 minutes, including CBCT and beam therapy.
The following figures show the planned dose distribution of HyperArc, non coplanar VMAT, radioknife, and Tomo:
(Note: All plans are normalized to 100% of the prescription dose line covering 95% of the target volume, with a maximum point dose of 130%)

Figure 10 HyperArc planned dose distribution for Patient 1

Figure 11 Non coplanar VMAT planned dose distribution for Patient 1

Figure 12 Planned dose distribution of radioknife for patient 1

Figure 13 Tomo Plan Dose Distribution for Patient 1

(Note: The closer the CI value is to 1, the better the conformability of the 100% prescription dose line is. The smaller the GI value, the faster the dose drops outside the target area.)

Table 1 Comparison of HyperArc, nCO-VMAT, CK, and Tomo plans in Patient 1

Patient 2

Single brain metastasis of lung cancer (1), with a prescription dose of 40Gy/10f. The clinical treatment time is about 6 minutes for CBCT and beam out treatment.

Figure 14 Patient 2 Target Area

As shown in the dose distribution and DVH situation in Figures 15 and 16, the target area has good conformability, and the dose line outside the target area drops quickly. It only takes 5mm to drop to 50% of the prescribed dose.

Figure 15 Dose distribution map for Patient 2

Figure 16 Patient 2 DVH diagram

Patient 3

Multiple brain metastases from lung cancer (29 cases), with a prescription of 56Gy/14 times and a prescription of 42Gy/14 times. The clinical treatment time is about 6 minutes for CBCT and beam out treatment.

Figure 20 Patient 3 Target Area

The HyperArc plan can achieve different prescription doses for different target areas in a plan, and can also achieve some target areas to be done according to SRS, and some target areas to be done according to uniform doses within the target area.

Figure 21 Patient 3 DVH diagram

In summary, HyperArc is a safe and efficient SRS treatment technology that can achieve satisfactory results in the treatment of single and multiple brain metastases in terms of CI, GI, and exit time.

Epilogue

Obviously, the Varian HyperArc "super arc knife" treatment technology not only makes stereotactic radiotherapy for multiple brain lesions more concise and efficient, but also greatly improves the convenience of clinical operations. This is also why it is widely favored by medical institutions worldwide. I believe that with the further application of HyperArc, the prognosis and quality of life of brain metastases will be more effectively improved.