Quantitative second harmonic generation microscopy for characterizing collagen remodeling in papillary thyroid...

Thyroid cancer is the most common endocrine malignancy. Although fine needle aspiration (FNA) is the gold standard for diagnosis, 10% to 20% of FNAs yield indeterminate cytology, of which 20% to 30% prove malignant. Such diagnostic uncertainty drives repeat FNAs, costly molecular tests, and unnecessary surgeries, leading to increased healthcare costs, reduced quality of life, and emotional distress to patients. Current methods provide limited structural information about the extra-cellular matrix, despite collagen remodeling being a hallmark of malignancy. A rapid, label-free, and architecture-based diagnostic approach has the potential to significantly improve real-time or near-real-time decision-making and reduce unnecessary biopsies and interventions.

We use a systematic, quantitative framework based on second harmonic generation (SHG) microscopy to differentiate between normal and cancerous (papillary thyroid carcinoma [PTC]) human thyroid tissues through collagen architecture signatures.

Formalin-fixed paraffin-embedded unstained tissue sections ( $5\mu m$ thick) of confirmed PTC and normal thyroid tissue were obtained under an approved IRB and imaged under a multiphoton microscope using an excitation wavelength of 803 nm. An emission filter of $400\pm 10\mathrm{nm}$ was used to filter the SHG emission from the sample. Thirty-five metrics capturing intensity, geometric, textural, and frequency-domain collagen features were quantified. A linear mixed-effects model was used for quantitative comparison between cancer and normal groups, accounting for the hierarchical structure of the data and rigorously assessing metric significance despite limited sample size.

Of the 35 metrics evaluated, 19 remained statistically significant after false discovery rate (FDR) adjustment for multiple comparisons. The corresponding effect sizes ranged from moderate to large, indicating substantial separation between normal and cancerous samples. Five of the remaining 16 metrics, although not statistically significant, were consistently either higher or lower in one group compared with the other, suggesting directional consistency. This revealed robust and biologically coherent alterations in collagen organization despite small cohort size. These included consistent increases in spectral power across multiple spatial frequency bands and greater collagen heterogeneity in cancer tissue.

The convergence of significant and directionally consistent metrics underscores the strong diagnostic potential of SHG-derived collagen architecture. Integrating these quantitative features into an automated analysis platform could enable rapid, objective, and reproducible optical biopsies, with the potential to reduce indeterminate FNAs, guide intraoperative decision-making, and improve diagnostic accuracy in routine thyroid cancer care.