Direct tumor visual feedback during free breathing in 0.35T MRgRT

Abstract To present a tumor motion control system during free breathing using direct tumor visual feedback to patients in 0.35 T magnetic resonance‐guided radiotherapy (MRgRT). We present direct tumor visualization to patients by projecting real‐time cine MR images on an MR‐compatible display system inside a 0.35 T MRgRT bore. The direct tumor visualization included anatomical images with a target contour and an auto‐segmented gating contour. In addition, a beam‐status sign was added for patient guidance. The feasibility was investigated with a six‐patient clinical evaluation of the system in terms of tumor motion range and beam‐on time. Seven patients without visual guidance were used for comparison. Positions of the tumor and the auto‐segmented gating contour from the cine MR images were used in probability analysis to evaluate tumor motion control. In addition, beam‐on time was recorded to assess the efficacy of the visual feedback system. The direct tumor visualization system was developed and implemented in our clinic. The target contour extended 3 mm outside of the gating contour for 33.6 ± 24.9% of the time without visual guidance, and 37.2 ± 26.4% of the time with visual guidance. The average maximum motion outside of the gating contour was 14.4 ± 11.1 mm without and 13.0 ± 7.9 mm with visual guidance. Beam‐on time as a percentage was 43.9 ± 15.3% without visual guidance, and 48.0 ± 21.2% with visual guidance, but was not significantly different (P = 0.34). We demonstrated the clinical feasibility and potential benefits of presenting direct tumor visual feedback to patients in MRgRT. The visual feedback allows patients to visualize and attempt to minimize tumor motion in free breathing. The proposed system and associated clinical workflow can be easily adapted for any type of MRgRT.


| INTRODUCTION
In previous studies, tumors in the thorax were shown to move up to 5 cm 1 and rotate up to 45°2 during respiration. Conventional respiratory motion-compensation techniques such as surrogatebased respiratory gating, breath-hold, and marker-based tumor tracking [3][4][5] are clinically useful for tumor motion management but significant variations in cycle-to-cycle breathing can cause treatment inaccuracies. 6,7 Recently, several respiratory monitoring systems 3,7-10 were introduced for respiratory motion management in radiotherapy providing respiratory guidance during radiotherapy in addition to medical imaging. 11,12 For instance, audio-visual biofeedback 7 uses a noninvasive external marker to measure abdominal motion and uses audio-visual (AV) tools to return that information to the patient for respiratory motion guidance. Audio-visual biofeedback can reduce average cycle-to-cycle variations in breathing displacement and period by up to 50% and 70%, respectively. 7,13 However, the applications of this system may be limited by an insufficient correlation between tumor and surrogate motion. 6,14 Real-time 2D tumor tracking in MR-guided radiotherapy (MRgRT) became clinically available in 2014. 15,16 MRgRT improves local control and spares critical organs by providing superior soft tissue contrast resolution and real-time imaging-based delivery. However, irregular tumor motion still hinders treatment efficiency in gated radiotherapy. Visual guidance systems in MRgRT have been introduced for improving tumor motion control in voluntary breathhold. 17,18 For example, Kim et al. displayed the treatment delivery system (TDS) operator screen inside the bore of the treatment system by using a video signal splitter and an MR-compatible beam projector. In a similar approach, de Koste et al. displayed the TDS on an MR-compatible monitor by using a video signal splitter and an adjustable mirror. The splitters supply the video signal of the TDS computer to an in-room display device. There are two challenges associated with the splitter-based approach: (a) the TDS display depends on video signal of a splitter; (b) displaying the entire TDS screen to patients includes unnecessary information that may confuse the patient, thus requiring further processing for advanced visual guidance. 18 In our study, we developed a visual guidance system which does not impact the TDS display. In addition, a customizable visual guidance display was added without intensive programming that optimizes the information provided to the patient for efficient guidance.
Through the study, we implemented the visual guidance system in a clinical workflow and investigated its impact on tumor motion control during free breathing in 0.35 T MRgRT.   First, we set up dual screen output on the TDS system with vendor's support such that the TDS display at the treatment console is independent from our signal capturing device. An Epiphan displayed the images inside the bore of the treatment system. Additionally, an adjustable stand was developed and used to adjust the display location for patients and a projector keystone correction was used to minimize image distortion due to oblique projection as shown in Fig. 1(a).

2.C | Clinical workflow of direct tumor visualization
Visual biofeedback based on direct tumor visualization was applied in our adaptive radiotherapy workflow. 15,19 The adaptive treatment planning and delivery workflows have been discussed in previous reports. 15,16 The additional workflow specific to providing direct tumor visualization to the patient is described below. Figure

3.A | Implementation of direct tumor visualization
The direct tumor visualization system was successfully implemented on a 0.35 T MRgRT system in our clinic. Implementation included the system installation, training and instruction material preparation, workflow development, and staff training. System adjustment and optimization, updates to materials, and refresher training were conducted in a developing loop. Patient introduction to the system and projector adjustment required approximately seven minutes to perform prior to treatment, including five minutes for education at the first fraction and two minutes for projector adjustment at each fraction.

3.B | Evaluation of tumor motion control and
Beam-on time In this study, we used a frame grabber instead of a video signal splitter. Since the TDS system requires a high display resolution, the TDS control room display resolution must not be compromised by any secondary display device. A secondary monitor or a projector might reduce the TDS display resolution, but the configured frame grabber does not. In addition, when a video signal splitter is powered off, neither output display receives a video signal because the video signal splitter produces both signals. In contrast, by using a frame grabber the TDS display remains independent and its operation is unaffected by screen grabber status. 17,18 This is important in preventing treatment delays due to potential splitter malfunction or troubleshooting.
Video signal capturing by the frame grabber provided more options in signal processing than using a video signal splitter. Once A similar MRI-compatible display was used in our study compared to previous studies. 18 Since we displayed the images inside the bore of the treatment system using a projector like Kim et al., the resulting images were distorted due to the oblique projection, so a keystone function of the projector was used to correct image distortion. This approach is sufficient to remove the projection screen and adjustable mirror required for a conventional setup. In our approach, we developed an adjustable stand which can be used to adjust the projection angle and display location for patients Through this study, we investigated the implementation of the  motion is not updated, the visual guidance appeared frozen to the patient.
Target tracking assessment of patients with and without the visual guidance system in place showed that the duty cycle remained similar. However, the target tracking contour was more likely to extend a greater distance outside of the gating contour without visual guidance. The target tracking contour had a greater range of motion outside the gating contour and was more likely to be outside of the target window, indicating that providing visual guidance has the potential benefit of reducing motion range. However, an additional study with randomized patient cohorts would be required to determine the true benefit. During treatment, patients were observed to be more conscious of their breathing and attempting to keep the displayed target contour within the gating contour, especially when the beam-on indicator was active. Patients were not instructed to alter their breathing during the training session but changed their breathing pattern when presented with the beam-on indicator. The addition of training time to familiarize patients with the system may reduce any total time benefit of the system, however time spent outside of the MRI bore is more desirable than treatment time in the bore where the patient may experience discomfort due to treatment positioning. It is noted that this study did not consider number of treatment beams, beam segments, monitor units, or setup difficulty, which could all have a significant impact on the total time spent in the treatment room by the patient. In the future, we will conduct additional studies to assess the time impact in further detail.

| CONCLUSION
We demonstrated the clinical feasibility of direct tumor visualization to patients in MRgRT. It allows tumor motion control in free breathing, with the potential to reduce on-table treatment time and tumor motion range. Clinical workflow for the proposed system can be easily adapted for any type of MRgRT.