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).