The role of potassium in depth profiling of the tumor border in bone-invasive oral cancer using laser-induced breakdown spectroscopy (LIBS): a pilot study

Abstract

Purpose: Microscopic tumor spread beyond the macroscopically visible tumor mass in bone represents a major risk in surgical oncology, where the spatial complexity of bony resection margins cannot be countered with rapid bone analysis techniques. Laser-induced breakdown spectroscopy (LIBS) has recently been introduced as a promising option for rapid bone analysis. The present study aimed to use LIBS-based depth profiling based on electrolyte disturbance tracking to evaluate the detection of microscopic tumor spread in bone.

Methods: After en bloc resection, the tumor-infiltrated mandible section of a patient's segmental mandibulectomy specimen was natively investigated using LIBS. Spectral and electrolytic depth profiles were analyzed across 30 laser shots per laser spot position in healthy bone and at the tumor border. For the histological validation of the lasered positions, the mandibular section was marked with a thin separating disc.

Results: Solid calcium (Ca) from hydroxyapatite and soluble Ca from dissolved Ca can be reliably differentiated using LIBS and reflect the natural heterogeneity of healthy bone. Increased potassium (K) emission values in otherwise typically healthy bone spectra are the first spectral signs of tumorous bone invasion. LIBS-based depth profiles at the tumor border region can be used to track tumor-associated changes within the bone with shot accuracy based on the distribution of K.

Conclusion: Depth profiling using LIBS might enable the detection of microscopic tumor spread in bone. In the future, direct electrolyte tracking using LIBS should be applied to other intraoperative challenges in surgical oncology to advance rapid bone analysis by spectroscopic-optical techniques.

Keywords: Bone invasion; Depth profiling; Head and neck cancer; Laser-induced breakdown spectroscopy (LIBS); Microscopic tumor spread; Potassium.

Fig. 1

Mandible section with significant tumorous bone invasion. Bone section from a segmental mandibulectomy specimen with vertical and horizontal markings to histologically validate the laser spot positions A, B, C, and D in the H&E stained section. a Macroscopic view. b Microscopic (0.5× magnified) view with the corresponding positions. c Microscopic (5× magnified) view with a focus on spots B and C, which were acquired in the outgrowths of tumor-associated stromal tissue in the bone–tumor interface

Fig. 2

Spectral depth profiling in healthy bone material. Typical LIBS emission spectra of healthy bone at laser spot position A. Selected depth profile spectra at laser shot 2 (a), shot 5 (b), shot 11 (c), shot 20 (d), and shot 25 (e). Note the consistently low K emission lines in the spectra from healthy bone. All spectra have been normalized to base peak intensity (BPI)

Fig. 3

Depth profiling of different elements in a healthy bone substance. Depth profiles of different elements in healthy bone (laser spot position A). The contents of K, Na, solid Ca, and soluble Ca are shown by their emission lines, covering approximately 100 μm. The strong changes in the solid Ca and soluble Ca emission lines reflect the heterogeneity of the tissue itself. Dotted line with squares: Soluble Ca, measured as CaO and CaOH species. Thin solid line with squares: Solid Ca from hydroxyapatite. Bold line with triangles: Na. Bold line with round dots: K. The missing dots at laser shot 30 indicate LIBS spectra that did not fulfill the QC criteria

Fig. 4

Spectral depth profiling on the tumor border. Typical LIBS spectra of tumor border area, measured at laser spot position B. Selected depth profile spectra at laser shot 2 (a), shot 5 (b), shot 7 (c), shot 10 (d), and shot 15 (e). Note the increase in K with the increasing depth of laser shots

Fig. 5

Depth profiling by selected electrolytes near the tumor border. Depth profiles of K and solid Ca at two positions (laser spot position B [a] and laser spot position C [b]) from the tumor border. The figures show a steady increase in K and a decrease in solid Ca with the increasing depth of shots. The tumor-associated stromal tissue starts at laser shot 10 (a) and laser shot 17 (b). The tumor-associated stromal tissue in both spots reached similar peak areas for K. Solid lines used for solid Ca. Dotted lines were used for K. Missing dots: Spectra that did not fulfill the QC criteria and were discarded. The vertical scale on the left represents the peak area of solid Ca, and the vertical scale on the right represents the peak area of K