A noninvasive in vivo method has been developed and optimized for measuring platinum concentrations in the kidneys of patients receiving chemotherapy. The method is based on polarizing the X-ray beam from an orthovoltage radiotherapy treatment unit, the Pantak DXT-300, and using the beam to produce emission of the characteristic platinum X-rays from the kidney. The platinum is derived from platinum-based chemotherapy drugs, such as cisplatin and its analogues (carboplatin and iproplatin), used to treat cancer patients. The clinical motivation for measuring the platinum concentration in both the kidneys and the tumor is to optimize the treatment by establishing the relationships between the accumulation of the drug at those sites. Such clinical information could be valuable in maximizing the therapeutic ratio toward the tumor tissue and limiting the hazards to the kidney. The performance of the system was experimentally optimized with respect to the applied X-ray tube voltage, filter material, and polarizer. Additionally, the MCNP-4B Monte-Carlo computer code was used to estimate the optimum shielding materials, the thickness surrounding the X-ray tube, and the arrangement of collimators, to protect patients from the hazards of the scattering radiation. Clinical measurements can be made with a combination of a bilayer of copper and silicon as polarizer, a 0.25-mm tin filter introduced in the path between the X-ray beam and the polarizer, and an operating voltage of 220 kV. The minimum detection limit achieved with this arrangement is 16 ppm, for a kidney depth of 3 x 3 cm, with a skin dose of 1.6 mGy and measurement time of 2,000 seconds.