Ying Wang1, Fu Jin1,2, Juan Zhou3, Huanli Luo2. 1. 1 Department of Radiation Oncology, Chongqing Cancer Institute, Chongqing, China. 2. 2 Division of Physics, Chongqing Cancer Institute, Chongqing, China. 3. 3 Forensic Identification Center, College of Criminal Investigation, Southwest University of Political Science and Law, Chongqing, China.
Abstract
OBJECTIVE: In order to not add extra uncertainty to radiotherapy planning process, accurate margin algorithm is necessary, therefore we propose a centre-shift method to estimate its accuracy. METHODS: A series of spherical phantoms are used with different CT slice separations (SSs) and pixel sizes (PSs). They are grown by different margins, and displaced geometric centres provide a window on accuracy. Volume difference between pre- and post-expansion is also calculated to double-check the accuracy. RESULTS: The measured margin nearly varies as multiples of PS in the transaxial plane; in the superior-inferior direction, it is approximately equal to SS when the ideal margin is smaller than SS. A sphere's volume of <1 cm(3) is underestimated by 3-70% for all PSs, and 2-100% for SS of ≤5 mm but overestimated up to 112% for >5 mm. For volumes of >1 cm(3), relative volume error decreases, and it is nearly zero for >100 cm(3). The dependence of margin accuracy on SS and PS is largely eliminated by volume difference method. CONCLUSION: We have proposed a simple method to estimate margin-growing accuracy and suggested corrective action to minimize the variation. ADVANCES IN KNOWLEDGE: One big difference from the previous results is that SS and PS both influence the accuracy of margin growth and volume calculation in Eclipse(™) treatment planning system (Varian Medical System, Palo Alto, CA).
OBJECTIVE: In order to not add extra uncertainty to radiotherapy planning process, accurate margin algorithm is necessary, therefore we propose a centre-shift method to estimate its accuracy. METHODS: A series of spherical phantoms are used with different CT slice separations (SSs) and pixel sizes (PSs). They are grown by different margins, and displaced geometric centres provide a window on accuracy. Volume difference between pre- and post-expansion is also calculated to double-check the accuracy. RESULTS: The measured margin nearly varies as multiples of PS in the transaxial plane; in the superior-inferior direction, it is approximately equal to SS when the ideal margin is smaller than SS. A sphere's volume of <1 cm(3) is underestimated by 3-70% for all PSs, and 2-100% for SS of ≤5 mm but overestimated up to 112% for >5 mm. For volumes of >1 cm(3), relative volume error decreases, and it is nearly zero for >100 cm(3). The dependence of margin accuracy on SS and PS is largely eliminated by volume difference method. CONCLUSION: We have proposed a simple method to estimate margin-growing accuracy and suggested corrective action to minimize the variation. ADVANCES IN KNOWLEDGE: One big difference from the previous results is that SS and PS both influence the accuracy of margin growth and volume calculation in Eclipse(™) treatment planning system (Varian Medical System, Palo Alto, CA).
Authors: Charles Mayo; Mary K Martel; Lawrence B Marks; John Flickinger; Jiho Nam; John Kirkpatrick Journal: Int J Radiat Oncol Biol Phys Date: 2010-03-01 Impact factor: 7.038
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