What is RGSC?

Varian's latest Respiratory Gating for Scanners (RGSC) system collects a patient's respiratory waveform curve through infrared reflection, then transmits respiratory information to the Computed Tomography (CT) system and automatically synchronizes it with the patient's radiation therapy planning program. After completing the plan design, Simply apply the patient's respiratory curve recorded by RGSC on the treatment preparation interface to use TrueBeam linear accelerator for respiratory gating treatment.

Ct simulation

The patient's position is fixed by raising their hands and lying on their back on foam, with a 5mm thick compensation film placed on the chest wall, or by using a thermoplastic film to fix the chest. Place the reflector block of the RGSC system at the patient's xiphoid position, select the gating type as DIBH scanning on the RGSC system, and then train the patient to perform DIBH with nose inhalation, ensuring that the DIBH time of the patient is at least 25 seconds before completing the DIBH CT scan at once. In order for the patient's chest to expand sufficiently, the distance between the heart and the chest wall increases. As shown in Figure 2, the peak amplitude value of a patient's free breathing curve is 0.3cm. Set the threshold amplitude of DIBH to 0.9cm (2-3 times the peak amplitude value of the free breathing waveform) and maintain it between 0.9cm and 1.1cm (breathing amplitude controlled within 2-3mm error). After the RGSC system starts recording the patient's respiratory curve, it commands the patient to perform DIBH. When the amplitude value of the respiratory curve reaches the set threshold, CT scanning begins.

Figure 2: Respiratory curve of a patient recorded by the RGSC system

Figure 3 shows the comparison of CT images between Free Breathing (FB) and DIBH scans of the same patient, where image a is an FB scan with a distance of 1.36cm between the heart and chest wall, and image b is a DIBH scan with a distance of 2.57cm between the heart and chest wall. It can be seen that DIBH scanning increases the distance between the chest wall target area and the heart compared to FB scanning.

Figure 3: DIBH CT and FB CT images

Design DIBH treatment plan

In order to improve treatment efficiency, DIBH plans to use VMAT technology, using 6 MV-FFF energy X-rays, with a maximum dose rate set at 1400 MU/min. Using lead gate following technology, four sub arcs are set, and the frame starting angle and small machine head angle of the four sub arcs are set. The lead gate setting is shown in Figure 4.

Figure 4: Field setup for DIBH plan

Figure 5 summarizes the dosimetric data comparison of DIBH plan for 20 patients with left breast cancer after radical surgery including chest wall area and clavicle area and FB plan for 20 patients with left breast cancer after radical surgery including chest wall area and clavicle area.

Figure 5: Comparison of Dosimetric Parameters between DIBH Plan and FB Plan

Perform DIBH treatment

As shown in Figure 6, the patient is lying on the treatment bed and can see their breathing amplitude through the Visual Coaching Device (VCD) above their head. When the patient DIBH, the white bar in the VCD screen stays in the dark blue area, and the machine can exit.
Patients can perform DIBH on their own or follow the instructions of the technician. VCD effectively solves two situations that existed in previous DIBH: (1) the patient's DIBH inhalation is insufficient and cannot reach the beam exit threshold. The technician prompts the patient to inhale more, but the patient cannot understand how much more is inhaled, and it is easy to inhale too much or too little. (2) The patient's DIBH inhales too much, exceeding the beam discharge threshold. The technician prompts the patient to exhale more, but the patient cannot understand how much more exhalation is and is prone to vomiting more or less. Both of these situations greatly affect the treatment efficiency of DIBH.

Figure 6: VCD

By using VCD in conjunction with breathing, a complete gated CBCT image can be obtained through one or more DIBHs. After obtaining the CBCT image of DIBH, use TrueBeam's 6-dimensional bed to match with PTV as the target and bone markers as auxiliary. From Figure 7, it can be seen that the gated CBCT image of DIBH matches well with the positioning CT image, with the target area, sternum, ribs, heart and other active parts perfectly matched. After the target area matching is accurate, a 6-dimensional bed is used to correct the positioning error.

Figure 7: Image registration of gated CBCT

DIBH treatment video

DIBH treatment time

As shown in Figure 8, read the treatment time record in the RT Summary of TPS. Figure 9 summarizes the comparison of DIBH planned treatment time (excluding gated CBCT scan time) and FB planned treatment time (excluding CBCT scan time) for 20 patients. It can be seen that DIBH treatment time is longer than free breathing treatment time, but this 3-minute exit time is also acceptable.

Figure 8: Record of treatment time

Figure 9: Comparison of treatment time between DIBH plan and FB plan

Summary

To sum up, the use of RGSC system and VCD to achieve DIBH radiotherapy after left breast cancer surgery can reduce the dose of left lung and heart, while improving the accuracy of target area treatment. The use of VCD enables patients to breathe DIBH "at their heart" or better listen to technicians' commands to carry out DIBH, greatly improving the treatment efficiency of DIBH.

Hospital Introduction
As a large comprehensive hospital, Jinshazhou Hospital of Guangzhou University of Traditional Chinese Medicine has linear accelerators such as Varian TrueBeam 2.7, Halcyon 3.0, and Ethos before the radiotherapy program. It treats over 200 patients every day, including over 70 treated on TrueBeam, 5-10 HyperArc, and more than 20 respiratory gated.