The loading of tissue with iodine can result in the enhancement of the radiation dose absorbed from low-energy x-ray or gamma ray sources. We have explored the potential of this phenomenon for radiation therapy. We have demonstrated the effect of iodine concentration and radiation quality on this dose enhancement in lymphocytes, we have calculated the effect of such enhancement on depth dose distributions in the brain, we have estimated the iodine content in two human brain tumors during computerized tomography (CT) scans, we have studied the dispersion of the iodine contrast media after direct injection into rabbit tumors, and we have demonstrated that the combination of x-ray and contrast media injection is far more effective than either agent alone in causing the regression of mouse tumors. These results suggest that there may be a therapeutic advantage from loading tumors with iodine and treating them with low-energy photons.

The Zenkoku-shijo-sugaku-danwakai, 3 (1), 212-214 (1948).14 Schouten, J. A., and Haantjes, J., "Ueber die konforminvariante Gestalt der Maxwellschen Gleichungen und der elektromagnetischen Impulsenergie gleichungen," Physica, 1, 869-872 (1934).* Schouten, J. A., and Haantjes, J., "Ueber die konforminvariante Gestalt der relativistischen Bewegungsgleichungen, Proc.In the article "On the Interpretation of Multi-Hit Survival Curves," these PROCEEDINGS, pages 696-712, December, 1949, equation (21) on p. 702 should read m S= II [1k(1-ekiD)] (21) and equation (22), same page, should read S = [1 -(1 e kD)]m (22)

In x-ray phototherapy of brain tumours, the tumour is loaded with iodine and exposed to kilovoltage x-rays. Due to the high photoelectric cross sections of iodine, substantial photoelectric interactions occur. The flux of photoelectrons, characteristic x-rays and Auger electrons produce a localized dose enhancement. A modified computed tomography scanner, CTRx, can be used both for tumour localization and delivery of the dose enhancement therapy. Monte Carlo methods were employed to simulate the treatment of iodinated brain tumours with a CTRx. The calculated results reveal the effect of tumour iodine concentration on dose distribution, the degree of skull bone sparing with the application of multiple arcs, and the homogeneity of tumour dose distribution versus iodine concentration. A comparison with 10 MV stereotactic radiosurgery treatment shows the potential of CTRx treatment relative to conventional treatment modalities.

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