Evaluation of indexes for tumor growth and effect of radiation therapy using MRS

Changes in the phosphate metabolism of the transplanted SCCVII (squamous cell carcinoma VII) tumor were studied before and after irradiation using in vivo 31P-magnetic resonance spectroscopy (MRS). Phosphocreatine (PCr), γ-, α-, β-ATP, inorganic phosphate (Pi), phosphomonoester (PME), and phosphodiester (PDE) were detected in 31P-MR spectra of tumor tissues. The curve fitting method was employed for calculating each peak of 31P-MR spectra (PCr, β-ATP, Pi, PME, PDE). The values of PCr, PCr/Pi, β-ATP/Pi and pH decreased in proportion to the growth of tumor, whereas that of Pi, PME and PDE increased. But the changes of β-ATP were minimal in degree. Based on the fact that the ratio of PCr/Pi highly correlated with tumor volume and showed the most remarkable change among various parameters, it might be the useful index for the tumor growth. Because the phospholipid metabolite PME was detected in the earlier stage of tumor growth, PME might be suitable for detecting the existence of viable tumor cells. 31P-MRS parameters, such as PCr/Pi, β-ATP, β-ATP/Pi and pH once increased after irradiation, and then they have decreased once again with tumor regrowth. The values of Pi, PME and PDE have decreased once transiently after irradiation, and thereafter they increased gradually. The ratio of β-ATP/Pi showed the most remarkable increase by irradiation as compared with the other parameters. Therefore, it may be employed as a therapeutic index in radiation therapy. Because β-ATP/Pi decreased again after irradiation prior to regrowth of tumor, it may be used to predict the regrowth of tumor. From these results, it was suggested that 31P-MRS was useful monitoring viable tumor growth, effects of radiation therapy, and regrowth after irradiation. (author)