The Risk of Radiation-Induced Carcinogenesis after External Beam Radiotherapy of Graves’ Orbitopathy

Abstract

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.

Comparative analysis of dosimetric parameters of three different radiation techniques for patients with Graves’ ophthalmopathy treated with retro-orbital irradiation

“…It is usually reserved for cases with moderate and severe degree of exophthalmos or compressive optic neuropathy when decompressive surgery is not immediately available or if patients are medically contraindicated for surgery. Though producing conflicting and controversial results as demonstrated in several double-blind randomized-controlled trials and numerous non-randomized trials with a small potential risk of long-term adverse events, orbital irradiation is still an acceptable treatment option for such disease [24-29]. There are also disputes on the optimal dose of orbital irradiation [30,31].…”

Ophthalmic and adnexal complications of radiotherapy

“…1999) have published theoretical data suggesting that the risk of any malignancy following EBRT for thyroid ophthalmopathy ranges from 0.3% to 1.4%. The theoretical risk of developing a fatal malignancy following this form of therapy has been estimated as ranging from 0.3% to 0.7% (Akmansu et al. 2003).…”

Orbital carcinomas treated with adjuvant intensity‐modulated radiation therapy

“…[5][6][7] Conventional 2D external beam radiotherapy with a single en face photon or electron beam, or via wedged anterior and lateral fields, have been historically utilized after orbital exenteration; albeit with the risk of contralateral eye toxicity, hypothalamic and pituitary dysfunction, and radiation-associated secondary cancers. [8][9][10] Electrons have attractive depth dose characteristics with rapid dose fall off and can be delivered safely to orbital targets with excellent coverage, but are limited by their depth of coverage, particularly for PNI requiring coverage of nerve tracks. Threedimensional conformal external beam radiotherapy allows for some improved tumor dose conformality but produces unacceptable plans for treatment of irregular targets close to critical structures.…”