Photoacoustic Molecular Imaging for the Identification of Lymph Node Metastasis in Head and Neck Cancer using an anti-EGFR Antibody-Dye Conjugate

Abstract

The presence of lymph node (LN) metastases is an essential prognostic indicator in patients with head and neck squamous cell carcinoma (HNSCC). This study assessed photoacoustic molecular imaging (PAMI) of the anti-epidermal growth factor receptor (EGFR) antibody (panitumumab) conjugated to a near-infrared fluorescent dye, IRDye800CW (panitumumab-IRDye800CW; pan800) for the identification of occult metastatic LNs in patients with HNSCC (n=7). After in vitro photoacoustic imaging characterization of pan800, PAMI was performed on excised neck specimens of patients infused with pan800 prior to surgery. Freshly obtained neck specimens were imaged with three-dimensional, multiwavelength spectroscopic PAMI (680, 686, 740, 800, 860, 924, and 958 nm wavelengths). Harvested LNs were then imaged with a closed-field near-infrared fluorescence imager and histologically examined by the pathologist to determine their metastatic status. A total of 53 LNs with a maximum diameter of 10 mm were analyzed with photoacoustic and fluorescence imaging, of which four were determined to be metastatic on final histopathology. Photoacoustic signal in the LNs corresponding to accumulated pan800 were spectrally unmixed using a linear least square error classification algorithm. Metastatic LNs had a five-fold higher average thresholded photoacoustic signal intensity corresponding to pan800 compared to benign LNs (2.50 ± 1.09 a.u. vs. 0.53 ± 0.32 a.u., p<0.001). Fluorescence imaging showed that metastatic LNs had a two-fold increase in fluorescence signal compared to benign LNs ex vivo (p<0.01, 0.068 ± 0.027 a.u. vs. 0.035 ± 0.018 a.u.) Moreover, the ratio of the average of the highest 10% photoacoustic signal intensity over total average, representative of degree of heterogeneity of pan800 signal in LNs, showed a significant difference between metastatic LNs vs. benign LNs (11.6 ± 13.4 vs. 1.8 ± 0.7, p<0.01) and an area under the receiver operating characteristic (ROC) curve of 0.96 (95% CI; 0.91-1.00). The data indicate that PAMI of IRDye800-labeled tumor-specific antibody may have the potential to identify occult LN metastasis perioperatively in HNSCC patients.