Post-mastectomy radiotherapy involving supraclavical nodes – matchline dose, tumour dose and organs at risk dose

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Date
2009
Authors
Salem Alzoubi, Ahmad Saleem
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Abstract
The purposes of this study were investigated the effect of the overlap resulting from the beam divergence of the tangential fields onto a supraclavicular field in breast irradiation of patients undergoing breast radiotherapy, to examine the absorbed dose to the skin of patients in such treatments and to determine the dose in contralateral breast (CB) from 2-field and 3-field techniques used in postmastectomy chest wall irradiations in a fabricated phantom as well as in patients. A heterogeneous fabricated phantom of an adult medium female torso, a set of thermoluminescent dosimeters (TLDs), megavoltage therapy films, and an n-type VeriDose diode were used in the dose measurements. Four different techniques were used in this study, overlap technique, couch rotating technique which is routinely applied in Mount Miriam Cancer Centre, Penang, a gap of 5 mm technique using asymmetric jaws to block the supraclavicular field which is routinely applied in Pantai Mutiara Hospital, Penang, and a 5 mm gap technique using symmetrical field. All treatments have been done with three-dimensional conformal radiation therapy (3D-CRT) using 6 MV photon beams. The treatment planning system (TPS) (Plato RPS V2.6.3, Nucleotron) uses the pencil beam algorithm for dose calculations. The TPS dose calculations were normalized at the isocentre of the tangential fields and at dmax for the supraclavicular field. Three sagittal planes from the supraclavicular field down to the tangential fields and one transversal plane along the match line of the three fields were selected to compare the measured and calculated dose in the match region. Dose measurements were performed on thirty-four patients undergoing external beam radiotherapy at the Mount Miriam Cancer Centre, during the period between June 2008 and September 2008. The mean reproducibility of TLD and film readings was within 3% for one standard deviation. Doses from films and TLDs were compared to the calculated doses from the TPS. In the matching selected regions, the measured and calculated doses agreed within 11 % for films and 12 % for TLDs. In phantom, where care was taken to achieve the necessary match, gap and overlap, the total uncertainty for the diode from both the entrance and the exit dose calibration was within 5 % for reference and non-reference conditions. The measurements and calculations doses for the patients are approximately linearly related to each other for entrance and exit dose. The average ratio of the measured to the calculated CB dose using the pencil beam algorithm was approximately 53% while at dmax the ratio was < 11%. The mean and median measured internal CB dose at different depths in a phantom at surface was 5.47 ± 0.22 cGy and 5.44 cGy respectively.
Description
PhD
Keywords
Science Physic , Post-mastectomy radiotherapy , Supraclavical nodes
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