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Dosimetric superiority of low melting-point lead in the radiotherapy of Graves’ ophthalmopathy

Keywords: Graves' ophthalmopathy , radiotherapy , multileaf collimator , low-melting-point lead , radiotherapy planning , computer-assisted

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Abstract:

Objective  To compare the dosimetric difference between the two beams formed by low melting-point lead (LML) and multileaf collimator (MLC) in orbit radiotherapy, so as to select the one with a lower lens dose for treatment of Graves' ophthalmopathy. Methods  Patients with unilateral and bilateral Graves' ophthalmopathy (10 cases each) suitable for radiotherapy were selected for dosimetric comparison. The identical sketching principle of target volume was employed, and the prescribed dose of planning target volume (PTV) was 2000 cGy/10 times. The distribution of radiation field in unilateral group was 3 fields (2 X-ray fields + 1 electron field), and in bilateral group was 4 fields (2 X-ray fields + 2 electron fields), and LML and MLC were employed to form the radiation field. The conformity index (CI) and dose volume histogram (DVH) were compared between the two formation methods of radiation field; the effective penumbra area of the half radiation field formed by the two methods and its influence on the lens dose were analyzed with flushing-free film and dose analysis software. Results  The average dose of the affected side lens in MLC unilateral group was 582±34cGy, and of the lens of uninjured side was 160±22cGy, the CI of target volume was 0.69; the average dose of the left and right lens in MLC bilateral group was 591±47cGy and 585±52cGy, respectively, and the CI was 0.67. The average dose of the affected side lens in LML unilateral group was 252±45cGy, and that of the uninjured side lens was 148±19cGy, and the CI was 0.71; the average dose of the left and right lens in LML bilateral group was 247±44cGy and 256±42cGy, respectively, and the CI was 0.68. When the X-ray energy was setup at 4MV and 8MV, the half radiation field was 5cm×5cm with a depth of 4cm, and the effective penumbra area of LML was 3mm smaller than that of MLC. Conclusion  A small radiation area formed by LML may be more appropriate, and it may not only diminish the penumbra of radiation field, also it significantly reduce the irradiation dose imparted to patients' affected side lens.

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