Shallow kinetics induced by a metronome (SKIM): A novel contactless respiratory motion management

Abstract

Objectives: As an alternative to conventional compression amidst the COVID-19 pandemic, we developed a contactless motion management strategy. By increasing the patient's breathing rate to induce shallow breathing with the aid of a metronome, our hypothesis is that the motion magnitude of the target may be minimized without physical contact or compression.

Methods: Fourteen lung stereotactic body radiation therapy (SBRT) patients treated under fast shallow-breathing (FSB) were selected for inclusion in this retrospective study. Our proposed method is called shallow kinetics induced by a metronome (SKIM). We induce FSB by setting the beats-per-minute (BPM) high (typically in the range of 50-60). This corresponded to a patient breathing rate of 25-30 (breathing) cycles per minute. The magnitude of target motion in 3D under SKIM was evaluated using 4DCT datasets. Comparison with free breathing (FB) 4DCT was also made for a subset for which FB data available.

Results: The overall effectiveness of SKIM was evaluated with 18 targets (14 patients). Direct comparison with FB was performed with 12 targets (10 patients). The vector norm mean ± SD value of motion magnitude under SKIM for 18 targets was 8.2 ± 4.1 mm. The mean ± SD metronome BPM was 54.9 ± 4.0 in this group. The vector norm means ± SD values of target motion for FB and SKIM in the 12 target sub-group were 14.6 ± 8.5 mm and 9.3 ± 3.7 mm, respectively. The mean ± SD metronome BPM for this sub-group was 56.3 ± 2.5.

Conclusion: Compared with FB, SKIM can significantly reduce respiratory motion magnitude of thoracic targets. The difference in maximum motion reduction in the overall vector norm, S-I, and A-P directions was significant (p = 0.033, 0.042, 0.011). Our proposed method can be an excellent practical alternative to conventional compression due to its flexibility and ease of implementation.

Keywords: fast shallow-breathing (FSB); free breathing (FB); metronome; motion management; stereotactic body radiation therapy (SBRT).

FIGURE 1

Illustration of breathing patterns, free breathing (FB), shallow breathing (SB) and shallow kinetics induced by metronome (SKIM). Blue arrows indicate the period of breathing cycle while yellow ones do the amount of air inhaled per breathing cycle. Note: The total amount of air inhaled in SKIM is same as in FB but breathing magnitude in each breathing cycle in SKIM is half of that in FB in this simple illustration.

FIGURE 2

A brief comparison between free‐breathing (FB) and shallow kinetics induced by metronome (SKIM), presents different features, including breathing patterns and internal target volume (iGTV). Our proposed SKIM works with a metronome within a range between 50 and 60 BPM, enabling not only to reduce the breathing motion but also regularize the breathing pattern. Note: The breathing patterns and iGTV above are for illustrative purpose only and do not reflect a clinical scenario.

FIGURE 3

The comparison of the vector of motion magnitude of the target between free‐breathing (FB) and shallow kinetics induced by metronome (SKIM) for the 12 targets. The vector mean ± SD values of motion magnitude of the target for FB and SKIM were 14.6 ± 8.5 mm and 9.3 ± 3.7 mm. Note: Both FB and SKIM data sets were retrievable for 12 targets only (out of 18 targets) thus, comparison was limited to them.

FIGURE 4

Example of difference in the external body contour with hard compression between CT simulation and treatment CBCT.

FIGURE 5

SKIM CBCT alignment with the iGTV and PTV displayed based on the CBCT alignment for patient 8 (LLL target), patient 1, and patient 6. These cases represent a moderate, high, and low degree of tumor motion, respectively.