The absorbed dose evaluation of radiation therapy of early glottic cancer by the difference in X-ray energy. Investigation with a film type dosimeter

Radiation therapy is an established method for treatment of early glottic cancers. Since the larynx is a thin wedge-shaped structure in the anterior neck adjacent to the airway, the absorbed doses to the lesion may be diminished because of build-up and build-down. The dose has been measured with conventional measuring systems such as thermoluminescent dosimetry (TLD). In this study, we employed Gafchromic MD-55-2 film (Nuclear Associates, Inc) as a dosimeter, for it can be set on the area of interest and with a measurability of a dose range of 3 to 100 Gy, and this radiometer material is similar in soft tissue of the human body. The dose distributions to the larynx were investigated with this film using a neck phantom under radiation beam energies of 4, 6 and 10 MV X-rays. The neck phantom was irradiated using 4, 6 and 10 MV X-rays, each with right and left lateral parallel-opposed fields, total radiation dose of 20 Gy, fields size of 6 cm x 6 cm and 15 degrees wedge filter. As a result, we observed secondary build-up and build-down curves in tissue in the vicinity of air cavities, especially at 10 MV X-rays. In the anterior commissure·center of glottic region, the absorbed doses decreasing rates of absorbed dose to 20 Gy with 4, 6, and 10 MV X-rays were 6.15%·7.35%, 8.90%·9.45% and 15.6%·12.7% respectively. These findings suggest that patients with early glottic cancer with anterior commissure invasion may receive more effective treatment with 4 MV X-rays rather than with 6 MV or 10 MV X-rays. (author)