Evaluation of radiation dose effect by high atomic number materials for superficial radiation therapy
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Although great achievements have been made in improving radiation therapy, the issue of non-specific irradiation of normal tissue still remains. One of the promising treatment methods being explored involves using high atomic number (Z) materials to enhance the radiation dose in tumor. Such treatment aims to create a variation in the attenuation coefficients of the cancer cell compared to those of the surrounding normal tissue. The goal of creating such a variation is to dramatically increase the amount of effective dose received by the tumor without increasing the amount received by the surrounding healthy tissue. Several high Z materials have been investigated for this application. Physical considerations such as the incident photon energy, geometric considerations involving the depth of the tumor, and concentration of the high Z materials have been investigated. This study is to experimentally quantify the effect using gold nanoparticles (AuNPs) and iodine solutions. Iodine solution (Omnipaque, GE Healthcare, Inc. NJ) and AuNPs (AuroVist, Nanoprobes, Yaphank NY) are uniformly distributed in each cylinder phantom (1.6 cm diameter and 2.0 cm depth) separately. Concentrations of Iodine and AuNPs were varied from 40 to 225 mg/ml and 16.0 mg/ml to 42.7 mg/ml, respectively. The Iodine solutions were irradiated with 75 to 150 kVp x-rays from a superficial x-ray therapy machine at doses of 250 to 400 cGy. The AuNPs solutions in the cylinder were irradiated with 40 to 150 kVp x-rays. The phantom was placed at the center of the cone to ensure a uniform radiation field. Radiation doses were measured using GafChromic EBT2 films (International Specialty Products, Wayne NJ). Dose enhancement factors (DEF), i.e., the ratio of dose to high-Z material versus dose to water, were calculated and plot as functions of concentration and kVp. Experimental DEFs varied between 1.01 and 1.33. The DEFs increased with increasing concentration and varied with changing kVp. The maximum DEF measured for Iodine solution was 1.33 at 300 mg/ml and 150 kVp. The maximum DEF measured for AuNPs was 1.32 at 62.25 mg/ml and 40 kVp. The target volume covered by dose enhancement is larger with relatively lower concentrations. The magnitude of dose enhancement due to AuNPs and Iodine presence has a dependency with the concentration and kVp. In clinical applications, the concentration of the high-Z material and kVp should also be selected to suit the depth and volume of the target tumor.