Dosimetric evaluation on the impact of delineation of brachial plexus in radiotherapy planning of nasopharyngeal carcinoma

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Dosimetric evaluation on the impact of delineation of brachial plexus in radiotherapy planning of nasopharyngeal carcinoma

 

Author: Li, Chi Him
Title: Dosimetric evaluation on the impact of delineation of brachial plexus in radiotherapy planning of nasopharyngeal carcinoma
Degree: M.Sc.
Year: 2015
Subject: Nasopharynx -- Cancer -- Radiotherapy.
Hong Kong Polytechnic University -- Dissertations
Department: Dept. of Health Technology and Informatics
Pages: x, 55 leaves : color illustrations ; 30 cm
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b2801364
URI: http://theses.lib.polyu.edu.hk/handle/200/7909
Abstract: Introduction: The brachial plexus (BP) originates from the cervical to upper thoracic spinal cord. Radiotherapy (RT) for patients with nasopharyngeal carcinoma (NPC) may result in high BP dose resulting in radiation-induced brachial plexopathy (RIBP). Treatments for RIBP are symptomatic and often ineffective affecting the quality of life of the patient. Purpose: This study is to evaluate the feasibility of the Radiation Therapy Oncology Group (RTOG)-endorsed BP contouring atlas with the help of computed tomography (CT) and magnetic resonance images (MRI) registration. The aim of this study was to evaluate the radiation dose to BP of the original plans and to analyze the dosimetric consequences on applying dose constraints to BP in tomotherapy planning optimization of NPC. Materials & Methods: It was a retrospective study on 15 NPC cases treated radically with tomotherapy in Hong Kong Sanatorium & Hospital from year 2012-13. Apart from the original treatment plan (Plan A) in which no dose constraint was applied to BP, two new plans (Plan B & C) were computed using the same set of planning CT images and the same planning parameters. Plan B consisted of BP contours based on RTOG-endorsed atlas while that for Plan C based on MR images registered with the planning CT images. Additional BP dose constraints were added to both Plan B & C aiming to keep the maximum dose (Dmax) below 60Gy. The end-point was that both plans achieved the same dose outcome as Plan A and the Dmax of the non-target BP was kept <60Gy. The new BP contours based on both methods were also added to Plan A to evaluate whether the original BP dose exceeded the RTOG recommended dose limit using the MIM software. For each patient, dose-volume histograms (DVHs) of the BP for both Plan A, B & C were generated for dose comparison. Plan differences were also quantified by comparing the BP volumes, Dmax, mean dose (Dmean) and dose received by 5%, 10% and 15% (D5%, D10% and D15%). BP contours by two methods on both axial CT slices and corresponding beam’s eye views (BEVs) were also compared visually.
Results: The mean BP volume contoured by RTOG-endorsed contouring atlas was 19.04 ± 3.50mL (range: 11.8-24.6mL) versus 10.44 ± 2.00mL (range: 6.3-14.1mL) by MRI. The mean BP overlapping volume was 1.9mL (range: 0.38-4.03mL). There was significant difference between two different BP contouring methods (p=0.000). The average Dmax of BP of Plan A & B were 62.0Gy versus 60.0Gy (Lt) and 61.6Gy versus 59.9Gy (Rt) (p=0.002 & 0.003 respectively). The average Dmax of BP of Plan A & C were 60.8Gy versus 58.6Gy (Lt) and 60.5Gy versus 58.3Gy (Rt) (p=0.005 & 0.016 respectively). There were also significant dose reduction among different Plans on Dmean, D5%, D10% & D15% (p<0.05). Conclusion: Contouring BPs based on two methods showed significant difference (p=0.000). Applying BP dose constraints during tomotherapy plan optimization for NPC patients could significantly reduce the BP dose (Dmax, Dmean, D5%, D10% & D15%) (p<0.05) without affecting the doses to targets and other organ-at-risks.

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