A comparison of stereotactic radiotherapy (SRT) and three-dimensional conformal radiotherapy (3DCRT) in boosting recurrent nasopharyngeal carcinoma

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A comparison of stereotactic radiotherapy (SRT) and three-dimensional conformal radiotherapy (3DCRT) in boosting recurrent nasopharyngeal carcinoma

 

Author: Choi, Tak-shun
Title: A comparison of stereotactic radiotherapy (SRT) and three-dimensional conformal radiotherapy (3DCRT) in boosting recurrent nasopharyngeal carcinoma
Degree: M.Sc.
Year: 2001
Subject: Nasopharynx -- Cancer -- Radiotherapy
Hong Kong Polytechnic University -- Dissertations
Department: Multi-disciplinary Studies
Dept. of Optometry and Radiography
Pages: xix, 141 leaves : ill. (some col.) ; 30 cm
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b1569101
URI: http://theses.lib.polyu.edu.hk/handle/200/406
Abstract: Background: Linac based stereotactic radiotherapy (SRT) is being used with increasing frequency to boost recurrent nasopharyngeal carcinoma (NPC). It is carried out with a circular collimator with rotation of arcs, which is optimal for small and spherical target volumes. However, most of the patients with recurrent NPC have large and/or irregularly shaped tumours, often requiring the use of multiple isocentres, which will increase the dose inhomogeneity inside the target volume. In this study, the maximum diameter of planned target volume (PTV) to be treated by SRT was in the range of 3.75-7.75 cm (median of 4.75 cm). An alternate method to use external radiotherapy for NPC recurrence is three-dimensional conformal radiotherapy (3DCRT), which uses non-coplanar beam direction and customised shielding blocks to spare the critical structures. Purpose: The author undertook a study to find out which external beam radiotherapy technique (SRT or 3DCRT) is better for boosting recurrent NPC by comparing dose conformity and homogeneity of target volume, and dose sparing effects on organs-at-risk. At the same time, some selection criteria, such as size, dimension, shape and position of target volume relative to normal structures, can be established to determine the better technique. Methods and Materials: 23 patients with residual or recurrent NPC that have been planned with XKnife 4.0 (treatment planning system for SRT) were replanned using Helax TMS 5.0 (treatment planning system for 3DCRT). There were 3 treatment arms in this study: (1) SRT - planned target volume (PTV) was the gross target volume (GTV) plus a 2mm margin (2) 3DCRT - PTV was the same as arm (1) (3) 3DCRT - target volume was the PTV of arm (1) plus a 5 mm margin A conformity index, homogeneity index were used to compare dose conformity and homogeneity respectively. Dose volume histograms (DVHs) and normal tissue complication probability (NTCP) were also analyzed for the dose sparing effects on OARs. The relationships between the treatment parameters of SRT (number of isocentres used and total arc angles)/physical characteristics of the target volume. (size, shape, maximum dimensions in X, Y and Z-axis and relative position of target volume) and dose conformity/homogeneity were studied. Results: Dose conformity and homogeneity of PTV in 3DCRT are much better than in SRT (p< 0.001). Dose sparing of the brainstem, optic chiasma, left/right optic nerves and left/right eyes by SRT are significantly better than by 3DCRT (p< 0.001). However, there is no significant difference in dose sparing to spinal cord (p> 0.05). In SRT, dose conformity is correlated with the number of isacentres, dimensions and size of target volume (p< 0.05). Dose homogeneity is correlated with the number of isocentres, total arc angles and size of target volume (p< 0.05). For 3DCRT, dose conformity and homogeneity are correlated with size and dimensions of target volume (p< 0.05). Conclusions: SRT and 3DCRT can be used to deliver a boost dose of radiation to recurrent nasopharyngeal carcinoma, but 3DCRT is found to be more appropriate in boosting recurrent NPC with respect to SRT.

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