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KMID : 0390320150250010007
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Chungbuk Medical Journal
2015 Volume.25 No. 1 p.7 ~ p.14
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Setup Error Measurement Using EPID for Rectal Cancer Patients
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Kim Won-Dong
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Abstract
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Purpose : The purpose of this study was to measure the three-dimensional setup error with electronic portal imaging devices (EPID) in the rectal cancer patients whose treatment position is unstable.
Materials and Methods : Ten rectal cancer patients treated from Aug. 2014 to Feb. 2015 with 3D conformal radiotherapy(3D CRT) were enrolled. Average 5 EPID images a patient were taken during radiotherapy. By image analysis software(Coherence Oncologist WorkspaceR), EPID images were compared with reference images digitally recontructed from planning CT. The three-dimensional translational (anterior-posterior;△AP, left-right;△LR, superior-inferior;△SI) setup errors were mesaured and the undirected total displacement(△iso) of the treatment isocenter from the simulation isocenter were determined by the equation.
Results : The analysis of all patients group revealed that average unidirectional translational setup errors were 0.16 cm, -0.15 cm and 0.07 cm in △AP, △LR and △SI direction, respectively with a standard deviation of around 0.5 cm. The average undirected △iso was 0.84 cm with a standard deviation of 0.25 cm. Individual patient analysis revealed 7 of 10 patients had statistically significant nonzero mean translational setup error implying the systemic error.
Conclusion : Average of translational setup errors measured in this study approached the zero with a standard deviation of around 0.5 cm. It is reasonable to add a standard deviation of around 0.5 cm margin to the clinical target volume (CTV) to account for setup errors. Strict and frequent position verification and repositioning with EPID just befor radiotherapy can eliminate individually specific systemic error and lead to adding a smaller margin to CTV and less radiotherapy induced complication.
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KEYWORD
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Setup error, EPID, Rectal cancer
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