Radiation-induced changes in phosphorus T1values in human melanoma xenografts studied by 31P-MRS

31P-magnetic resonance spectroscopy (MRS) has been shown to be a promising method for monitoring tumor response to radiation therapy. The purpose of the work reported here was to investigate whether the usefulness of 31P-MRS might be enhanced by measurement of spin-lattice relaxation times (T1s) in addition to resonance ratios. The work was based on the hypothesis that tumors having a high probability of being controlled locally would show shortened T1s during the treatment course due to reoxygenation and development of necrosis. BEX-t human melanoma xenografts, which show efficient reoxygenation and development of necrosis following single dose irradiation, were used as tumor models. Tumors were treated with single doses of 5.0 or 15.0 Gy and the T1s of the inorganic phosphate and nucleoside triphosphate β resonances were measured as a function of time after irradiation by using the superfast inversion recovery method. Fractional tumor water content was determined by drying excised tumors at 50°C until a constant weight was reached. The T1s in irradiated tumors were either longer than or not significantly different from those in unirradiated control tumors. The increase in the T1s following irradiation coincided in time with a radiation-induced increase in tumor water content, suggesting a causal relationship. The effects of reoxygenation and development of necrosis on T1s were probably overshadowed by the effects of tumor water content. Consequently, the usefulness of 31P-MRS in monitoring tumor response to radiation therapy might not be significantly enhanced by measurement of T1s.