Correlation of Radiobiological Assays of Hypoxic Fraction with Phosphorus-31 Magnetic Resonance Spectroscopy across Multiple Tumor Lines

Hypoxic fractions were measured in multiple tumor models and compared to³¹ P magnetic resonance spectroscopy (MRS) parameters. Hypoxic fractions were measured by excision assays in intramuscular and subcutaneous RIF-1 tumors, large and small SCCVII tumors, and KHT tumors. The hypoxic fractions ranged from 3% (small SCCVII) to 21% (KHT). Measurements of³¹ P MRS parameters (indices of tumor metabolism) were made under the same conditions used in the hypoxic fraction assays. The <tex-math>${\rm PCr}/{\rm P}_{{\rm i}}$</tex-math> ratios in the five tumor models were strongly correlated r²=0.98, τ = 1.00, P < 0.02) with their hypoxic fractions, and the <tex-math>${\rm NTP}/{\rm P}_{{\rm i}}$</tex-math> ratio was weakly correlated (r²=0.67, τ = 0.60, P < 0.17) with hypoxic fraction. Smaller values of the <tex-math>${\rm PCr}/{\rm P}_{{\rm i}}$</tex-math> and <tex-math>${\rm NTP}/{\rm P}_{{\rm i}}$</tex-math> ratios were found in the tumors with larger hypoxic fractions. These experiments indicate that³¹ P MRS may provide a relative measurement of hypoxic fractions across different tumor lines that can be applied non-invasively to individual tumors.