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

Abstract

Hypoxic fractions were measured in multiple tumor models and compared to 31P 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 of 31P MRS parameters (indices of tumor metabolism) were made under the same conditions used in the hypoxic fraction assays. The PCR/Pi ratios in the five tumor models were strongly correlated (r2 = 0.98, tau = 1.00, P < 0.02) with their hypoxic fractions, and the NTP/Pi ratio was weakly correlated (r2 = 0.67, tau = 0.60, P < 0.17) with hypoxic fraction. Smaller values of the PCr/Pi and NTP/Pi ratios were found in the tumors with larger hypoxic fractions. These experiments indicate that 31P MRS may provide a relative measurement of hypoxic fractions across different tumor lines that can be applied non-invasively to individual tumors.