Author: Cheng, Chi-yuen Harry
Title: Dose analysis of two radiotherapy techniques using different beam directional moulds in the treatment of nasopharyngeal carcinoma
Degree: M.Sc.
Year: 2000
Subject: Nasopharynx -- Cancer -- Radiotherapy
Hong Kong Polytechnic University -- Dissertations
Department: Multi-disciplinary Studies
Department of Optometry and Radiography
Pages: ix, 75 leaves : ill. (chiefly col.) ; 30 cm
Language: English
Abstract: Purpose: Traditionally, two beam directional moulds have been used on each NPC patient treated by radiotherapy in Queen Elizabeth Hospital (QEH). This is called the two moulds technique in which a flexed neck mould is used in phase I facio-cervical technique and an extended neck mould in phase II split-field technique and for the booster treatment. However, due to the increased tightness of resources, the treatment technique has been modified. The new technique is called the single mould technique which uses one beam directional mould throughout the whole treatment course. This new technique treats the patient's neck at a semi-extended position. Since the treatment position is different from the two moulds technique, the anatomical structures covered by the fields would be different which may lead to a change of dose distribution to the nasopharynx (NP), parapharyngeal space (PPS) and organs at risk (OARs) may be changed. This study is used to evaluate these two techniques in terms of dose distribution to the NP, PPS and OARs, and to identify the dose characteristics of individual treatment technique. Method: Computed tomographic images of 19 patients were loaded into a radiotherapy planning computer equipped with 3D planning facilities. Treatment plans simulating the radiotherapy techniques were computed. A 3D computer planning system was used to study the differences in the dose distribution of NP, PPS and OARs between the single mould and two moulds techniques. The dose distribution of the NP, PPS and different OARs were analysed by DVH. Results: The mean NP doses for the single mould technique and the two moulds technique were 71.09 and 71.57Gy respectively (p=0.421). The dose range of the NP for the single mould technique was 49.76 to 75.66Gy whereas the two moulds technique was 49.21 to 77.29Gy. The mean doses to the ipsilateral PPS for the single mould technique and two moulds technique were 74.31 and 75.38Gy respectively (p=0.062). The dose ranges of the ipsilateral PPS for the single mould technique and the two moulds technique were 56.10 to 80.30Gy and 61.33 to 81.37Gy respectively. Doses to the optic nerve, spinal cord, brain stem, temporal lobe and lens were similar and did not show significant difference (p>0.05). Conclusion: The dose distribution and dose homogeneity of the nasopharynx and OARs for the single mould technique and the two moulds technique were comparable. For the PPS, the dose distribution of the two moulds technique was slightly better than that of the single mould technique due to the anatomical position. A fully extended neck mould used for the single mould technique is suggested to improve the dose distribution to the PPS.
Rights: All rights reserved
Access: restricted access

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