Surrogating tumour cell density in head and neck cancer: [¹⁸F]FDG PET- versus ADC (MRI)-based approaches

In this study we examined the correlation between standardized uptake value (SUV) of [¹⁸F]fluorodeoxyglucose (FDG) and apparent diffusion coefficient (ADC) within the gross tumor volume (GTV) of patients with head and neck squamous cell carcinoma (HNSCC). In addition, we assessed the comparability of cell density (ρ) estimates obtained from FDG PET and MRI data.

Twenty-one HNSCC patients from a prospective FMISO imaging trial underwent pre-treatment PET/CT and MRI. We assessed correlations between FDG SUV (mean, max) and ADC (mean, min) within the GTV using Pearson's correlation coefficient. The tumor cell density within the GTV was calculated from FDG SUV and from ADC maps. For the estimation of ADC-based cell density, we used a published tumor cell volume fraction (v_TC). Agreement between FDG- and ADC-derived cell density estimates was assessed. The best-fitting v_TC* was computed to achieve equal mean ρ_ADC and ρ_FDG for each patient and was compared to the literature.

The SUV and ADC metrics showed up to moderate negative correlations, but none of them were statistically significant at p < 0.05. The correlation of SUV_mean vs. ADC_mean with Pearson's correlation coefficient r = -0.426 and p = 0.054 and SUV_max vs. ADC_min with r = -0.414 and p = 0.062 suggested a weak negative trend. The average and standard deviation of mean ρ_FDG and ρ_ADC across our cohort were (1.8 ± 0.6) × 10⁸ cells/ml and (3.3 ± 0.2) × 10⁸ cells/ml. The difference between the mean ρ_FDG and ρ_ADC was statistically significant (p < 0.001). To achieve equal mean ρ_ADC and ρ_FDG for each patient, the mean optimal v_TC* with standard deviation was 0.29 ± 0.09. Although significantly lower than the published mean v_TC​ (0.54), v_TC* lies within the published range of v_TC for HNSCCs (0.28 to 0.75).

ADC and SUV metrics exhibited moderate but marginally insignificant correlation in this dataset. Although not directly interchangeable, the two methods provide comparable, clinically relevant cell density estimates, offering flexibility to use the most accessible modality for individualized treatment planning.

Registered at German Clinical Trials Register on 20/08/2015 (DRKS00003830).