Estimation of the risk of secondary malignancies following intraoral electron radiotherapy for tongue cancer patients

Purpose: To measure dosimetric characteristics for linear accelerator-based electron beams, which are applied through locally manufactured acrylic tubes for intraoral radiotherapy and to calculate the secondary cancer risk for organs at risk. Materials and methods: Six different acrylic tubes were exposed to a 6-MeV electron beam; they had tips with three angles (0°, 15° and 30°) and two diameters (2·5 and 3·0 cm). Gafchromic EBT2 film was horizontally and vertically inserted in a solid water phantom to measure the dose profiles and percentage depth doses (PDDs). The measured data from radio-photoluminescence glass dosimeters placed on the neck and both eyes were used to estimate the lifetime attributable risk of secondary cancer resulting from intraoral radiotherapy for tongue cancer. Results: A total of 12 dose profiles were obtained from six different acrylic applicators at 0·5 and 1·28 cm depths. Circular shapes were obtained from 0° applicators, and oval shapes were obtained from 15° and 30° applicators. Absorbed doses at a 0·5 cm depth were higher than those at a 1·28 cm depth. PDD shapes for the six acrylic applicators were similar to those of a normal 6 MeV electron beam. Larger-diameter applicators showed higher PDD than smaller-diameter applicators with the same tip angle. According to our secondary cancer risk estimation, if 100,000 patients received intraoral radiotherapy at 30 years and lived until 80 years, 122 female and 22 male patients would develop secondary thyroid cancer, while 13 female and 18 male patients would develop secondary ocular melanoma or retinoblastoma. Conclusions: Dosimetric characteristics for linear accelerator-based intraoperative radiotherapy treatment beam were confirmed. In addition, we found that 0·1% of tongue cancer patients would get secondary malignancies for both eyes and thyroid from this treatment.