In vivo31 P nuclear magnetic resonance (<tex-math>${}^{31}{\rm P}\ {\rm NMR}$</tex-math>) spectroscopy has been used to compare metabolic profiles with tumor radiosensitivity. A radioresistant mammary carcinoma (MCa) and a radiosensitive methylcholanthrene-induced fibrosarcoma (Meth-A) were studied by31 P NMR spectroscopy in the tumor volume range of approximately <tex-math>$100-1200\ {\rm mm}^{3}$</tex-math>. The MCa showed a constant pH in this volume range; the ratio of phosphocreatine to inorganic phosphate (<tex-math>${\rm PCr}/P_{{\rm i}}$</tex-math>) for <tex-math>$160-300\ {\rm mm}^{3}$</tex-math> tumors was 0.33 ± 0.11 (mean ± standard deviation) and did not change (0.29 ± .09) for tumors in the volume range of <tex-math>$600-1200\ {\rm mm}^{3}$</tex-math>. In comparison, the Meth-A showed a decrease in tumor pH as volume increased from <tex-math>$160-300\ {\rm mm}^{3}$</tex-math> (pH 7.16 ± .04) to <tex-math>$600-1200\ {\rm mm}^{3}$</tex-math> (pH 6.94 ± .07). Tumor <tex-math>${\rm PCr}/P_{{\rm i}}$</tex-math> decreased from 0.70 ± .16 (<tex-math>$160-300\ {\rm mm}^{3}$</tex-math>) to 0.33 ± .16 (<tex-math>$600-1200\ {\rm mm}^{3}$</tex-math>). The radiation doses for control of MCa-induced tumors in 50% of the treated tumors ranged from 65 (<tex-math>$150-250\ {\rm mm}^{3}$</tex-math>) to 71 Gy (<tex-math>$1000-1300\ {\rm mm}^{3}$</tex-math>) and for the Meth-A-induced tumors ranged from 35 (<tex-math>$150-250\ {\rm mm}^{3}$</tex-math>) to 38 Gy (<tex-math>$1000-1300\ {\rm mm}^{3}$</tex-math>). These results suggest that31 P NMR spectra may be a qualitative predictor of tumor hypoxia, although further studies of human and rodent tumors are necessary to support this hypothesis.

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