Changes in ³¹P nuclear magnetic resonance with tumor growth in radioresistant and radiosensitive tumors

In vivo ³¹P nuclear magnetic resonance (³¹P NMR) spectroscopy has been used to compare metabolic profile with tumor radiosensitivity. A radioresistant mammary carcinoma (MCa) and a radiosensitive methylcholanthrene-induced fibrosarcoma (Meth-A) were studied by ³¹P NMR spectroscopy in the tumor volume range of approximately 100-1200 mm³. The MCa showed a constant pH in this volume range; the ratio of phosphocreatine to inorganic phosphate (PCr/P(i)) for 160-300 mm³ tumors was 0.33 ± 0.11 (mean ± standard deviation) and did not change (0.29 ± .09) for tumors in the volume range of 600-1200 mm³. In comparison, the Meth-A showed a decrease in tumor pH as volume increased from 160-300 mm³ (pH 7.16 ± .04) to 600-1200 mm³ (pH 6.94 ± .07). Tumor PCr/P(i) decreased from 0.70 ± .16 (160-300 mm³) to 0.33 ± .16 (600-1200 mm³). The radiation doses for control of MCa-induced tumors in 50% of the treated tumors ranged from 65 (150-250 mm³) to 71 Gy (1000-1300 mm³) and for the Meth-A-induced tumors ranged from 35 (150-250 mm³) to 38 Gy (1000-1300 mm³). These results sugest that ³¹P NMR spectra may be a qualitative predictor of tumor hypoxia, although further studies of human and rodent tumors are necessary to support this hypothesis.