Thoracoscopic ultrasonography for localization of subcentimetre lung nodules

Localization of small, non-visible and non-palpable subcentimetre nodules can be challenging during video-assisted thoracoscopic surgery (VATS). Intraoperative ultrasonography is an option for localization of such lesions, yet this technology has not been fully adapted to thoracic surgery. The objective of this study was to assess a newly developed thoracoscopic ultrasound for localization and biopsy of subcentimetre pulmonary nodules in animal models.

A prototype convex probe ultrasound thoracoscope (XLTF-UC180, Olympus Medical Systems Corp.) was used in this study. Multiple 5% agar pseudo-tumours were created in porcine lungs (n = 10) and assessed for localization with different frequencies (5.0-12.0 MHz) in deflated lungs. The evaluated pseudo-tumours were divided into two groups based on the distinctness of the tumour margin on the ultrasound images and compared in terms of the size and depth of the tumours. The visualization of real tumours and the biopsy capability were assessed using rabbit VX2 lung tumour models (n = 7).

The thoracoscopic ultrasound clearly visualized normal lung structures within a 1.5-cm depth including small vessels and bronchioles less than 5 mm in diameter in the completely deflated lung. Twenty-eight of 30 agar pseudo-tumours (93.3%) were successfully detected in deflated lungs (average size: 8.5 ± 2.1 mm; average depth: 7.4 ± 7.5 mm and depth range: 0-24.8 mm). Two tumours were not detected due to residual air surrounding the tumour. Higher frequency (12 MHz) tended to show more distinct margins of the targets. Indistinct tumours were located significantly deeper in the lung than the distinct tumours (14.11vs 2.42 mm), regardless of them being in a similar size range. VX2 tumours were identified as heterogeneous isoechoic lesions and adequate tissue sampling for diagnosis was achieved using a dedicated needle.

The newly developed convex probe ultrasound thoracoscope was capable of localizing subcentimetre nodules in the porcine deflated lung as well as of obtaining sufficient sampling from lung tumours in the rabbit model, which may enable single-port VATS lung nodule biopsy in a human clinical setting. However, the depth of the tumours significantly influenced the quality of ultrasound images. Complete collapse of the lung and use of high frequency may facilitate achieving distinct visualization of the targets.