Motion Management for Lung Stereotactic Ablative Radiotherapy

Abstract

Intra-fraction organ motion poses significant challenges to thorax irradiation. With respiratory- induced motions having the potential to degrade image quality and accuracy of the spatial dose distribution, leading to tumour overdosage and/or underdosage. This is a particular issue for stereotactic ablative radiotherapy (SABR).

The first question in this study aimed to determine the amount of lung tumour motion in a large cohort of SABR patients through the retrospective analysis of n = 363 4DCT images. Additionally, to assess the correlation between internal tumour motion and external surrogate motion, the breathing signals of n = 260 patients were analysed using in-house software. The second question in this study sought to estimate the magnitude of motion-induced dosimetric errors associated with SABR plans using in-house software. The effects of motion amplitudes, the number of breaths (NBs), plan complexity, and target size were examined using sinusoidal motion in the superior-inferior direction. A worst-case interplay scenario of a patient breathing with high motion amplitude and low NBs was estimated. Finally, the geometric consequences of organ motion were quantified for n = 257 4DCT datasets by ranking the images based on the severity of the artifacts and the number of couch positions. The effect of internal breathing characteristics and the external surrogate on the number of artifacts was studied.

The amount of tumour motion was found to be low for tumours close to the apex of the lung, while tumours located near the diaphragm exhibited the largest motion, with a maximum amplitude of 2.67 cm. Tumour motion proved predictable based on anatomical lobes and anterior-posterior location. However, there was no evidence of tumour motion predictability based on its volume. The internal/external correlation was weak for tumours in the upper lobe and moderate for tumours near the diaphragm. Additionally, lower interplay dose discrepancies were associated with a high number of breaths (NBs). There was no evidence of the effect of plan complexity or target size in the considered sample. Interplay dose errors would be a concern in patients with NBs ≤ 20. Finally, all considered 4DCT images suffered from motion artifacts, indicating a lack of improvements in 4DCT since its inception. Breathing irregularity and higher mean motion amplitudes were associated with high motion artifacts.

Given that lung cancer patients often exhibit irregular breathing due to the underlying disease, the implementation of breathing regulators (i.e., ABC system), along with audio-visual coaching, becomes essential for the majority of lung SABR patients to limit the adverse impacts of organ motion. Future implementation of adaptive radiotherapy for lung SABR would enhance precision, account for anatomical changes, and optimize dose delivery, ultimately improving therapeutic outcomes.