Summary

Cervical lymph node metastases in oral cancer patients are a frequent occurrence and important prognostic factor. Anatomical and molecular imaging modalities can identify neck metastases with varying sensitivity and specificity but perform poorly in clinically negative neck nodes with microscopic disease. In this article the authors investigated the use of porphyrin-lipid nanotheranostics (PS) for multimodal detection of neck disease in preclinical models of oral cancer. PS nanoparticles were radiolabelled with positron-emitting Copper-64 and uptake in the tumour and cervical lymph nodes was measured with serial PET/MR imaging.

Results from nanoScan^® PET/MRI

PET/MR imaging was performed using a 1T PET/MR small animal scanner (nanoScan^® Mediso, Budapest, Hungary). For intratumoural lymphatic mapping experiments, ⁶⁴Cu-PS (80–120 MBq ⁶⁴Cu, 0.5 mg pyro-lipid, IT) were administered into the tongue tumour. PET/MR imaging was performed at timepoints: 1 h, 3 h, 6 h, 12 h, 24 h, 48 h, and 72 h post-IT injection. PET acquisition times ranged from 10–25 min and T1 3D material maps were collected for MRI-based attenuation correction using parameters: 256 x 182 matrix, 0.35 x 0.35 x 0.60 mm³ voxel size, 18.98 ms repetition time, 2.9 ms echo time, 15° flip angle.

For comparison experiments between systemically injected PET radiotracers, ¹⁸F-FDG was injected and imaged 24 h prior to experiments involving ⁶⁴Cu-PS. 18F-FDG (46 MBq 18F/kg, IV) was administered via the tail vein and PET/MR imaging performed after ~45 min of uptake. PET acquisition time was 10 min using the same imaging parameters described above. The following day, ⁶⁴Cu-PS (250–500 MBq ⁶⁴Cu/kg, 0.5–1.0 mg/kg pyro-lipid, IV) were administered via tail vein and PET/MR imaging repeated as described above at 3 h and 24 h post-IV injection using a 15–20 min PET acquisition time. All PET images were reconstructed using parameters: 400-600 keV energy window, 105 x 105 x 237 matrix, 0.4 mm³ voxel size.

• Using the Mediso nanoScan^® PET/MR scanner, serial PET/MR imaging was performed from 1 to 72 hours after intratumoral administration of ⁶⁴Cu-PS. The PET images clearly visualized sentinel and downstream cervical lymph nodes as early as 1 hour after injection, with the highest contrast observed between 12 and 24 hours (see Figure 1.). Metastatic nodes tended to accumulate and retain more nanoparticle than benign nodes. However, despite excellent visualization of lymphatic drainage pathways, the PET signal differences between metastatic and benign nodes were not statistically significant. Consequently, intratumoral ⁶⁴Cu-PS PET/MR was valuable for lymphatic mapping and surgical planning but was insufficient as a stand-alone diagnostic method for nodal staging.

Fig 1. Representative MIPs from serial 64Cu-PS PET/MR imaging post-IT injection (80–120 MBq ⁶⁴Cu, 0.5 mg pyro-lipid). Pathological staging of nodes indicated on 3 h image. ⁶⁴Cu PET signal intensity arbitrarily scaled to the 1 h timepoint. (B) Healthy rats (i.e., without tongue tumours) administered 64Cu-PS into the tongue and serially imaged. ⁶⁴Cu PET signal intensity scaling as in (A).

• The authors then evaluated systemic administration of ⁶⁴Cu-PS. PET/MR imaging performed on the scanner showed that at 3 hours post-injection most nanoparticles remained in the circulation, resulting in relatively high background activity. By 24 hours, background activity had decreased and the tongue tumors and cervical lymph nodes became clearly visible (Figure 2.).
  The PET/MR images demonstrated strong passive accumulation of ⁶⁴Cu-PS within the tumors, with tumor SUVmean increasing significantly between 3 and 24 hours. Comparison with ¹⁸F-FDG PET showed that the 24-hour ⁶⁴Cu-PS PET images achieved tumor and nodal contrast comparable to conventional FDG imaging. Histological validation further confirmed that PET signal corresponded closely with nanoparticle fluorescence and autoradiographic localization within tumor tissue.

Fig 2. (A) Representative MIPs of ¹⁸F-FDG PET/MR images 45 min post-IV injection (46 MBq 18F/kg), and ⁶⁴Cu-PS PET/MR images from 3 h and 24 h post-IV injection (250–500 MBq ⁶⁴Cu/kg, 0.5–1.0 mg/kg pyro-lipid) in tumour-bearing rats. Magnified view of outlined neck nodes in insets. Arrows denote tumour. ⁶⁴Cu PET signal intensity arbitrarily scaled to the 3 h timepoint. (B) Representative MIPs of ⁶⁴Cu-PS PET/MR images from healthy rats (i.e., without tongue tumours).

• Figure 3. presents the main PET imaging results. At 24 hours after intravenous injection, the scans visualized 83% of harvested cervical lymph nodes, including most metastatic nodes. Quantitative PET analysis showed significantly higher ⁶⁴Cu-PS uptake in metastatic lymph nodes compared with benign nodes. Importantly, ⁶⁴Cu-PS PET outperformed conventional ¹⁸F-FDG PET. Whereas FDG uptake did not significantly differ between metastatic and benign nodes, ⁶⁴Cu-PS produced a substantially better ROC performance (AUC ≈ 0.78), demonstrating superior ability to identify microscopic nodal disease.

Fig 3. Representative MIPs of ⁶⁴Cu-PS PET/MR images from 24 h post-IV injection (250–500 MBq 64Cu/kg, 0.5–1.0 mg/kg pyro-lipid). ⁶⁴Cu-PS uptake in pN0 vs pN+ staged nodes.

Overall, the study demonstrated that ⁶⁴Cu-PS PET/MR imaging on the Mediso nanoScan platform provides a sensitive multimodal approach for detecting occult cervical lymph node metastases, outperforming conventional MRI morphology assessment and showing better diagnostic accuracy than ¹⁸F-FDG PET, while also enabling complementary fluorescence-guided surgical navigation.

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