The risk of radiation-induced carcinogenesis after external beam radiotherapy of Graves' orbitopathy

Purpose: In order to estimate the risk of radiation-induced carcinogenesis after external beam radiotherapy of Graves' orbitopathy, we made dosimetric measurements with thermoluminescent dosimeters in a male Rando phantom using our treatment technique. Methods: We produced dose-volume histograms from 5-mm CT slices using our treatment planning system. In this way we estimated the irradiation dose received by the main surrounding organs such as the brain, the bone and the eyeball. We use 6-MV X-ray for the treatment of Graves' orbitopathy and our total dose is 2,000 cGy, 200 cGy per fraction. Clinical target volume was limited to the retrobulbar orbital content and medial rectus muscle, sparing the lenses, lacrimal glands and the sella turcica. All dosimetric measurements with thermoluminescent dosimeters were made 3 times. Results: We found that the dose to the right lens was 101.6 mGy and to left lens 103.4 mGy. Average absorbed doses to red bone marow, thyroid, lung, esophagus, bone surface, skin and brain, respectively, were estimated to be 523, 21, 4, 5, 81, 29, 47 mGy. Then, using tissue weighting factors specified by the International Commission of Radiation Protection, the resulting total effective dose was calculated to be 66.8 mSv. Conclusion: The risk factor for fatal cancer development derived from human epidemiological data is presently assumed to be 10% per Gy. In patients treated by our method of retro-orbital external beam radiotherapy for Graves' orbitopathy, there is a 0.0067 (or 7 per 1,000 persons or 0.7%) risk of developing fatal radiation-induced cancer. Copyright (C) 2003 S. Karger AG, Basel.