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SU‐FF‐J‐118: Lung Target Volume Inaccuracies Due to Motion in Non‐Gated and Gated CT Images
Author(s) -
Wells D,
Lim J
Publication year - 2005
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.1997664
Subject(s) - nuclear medicine , physics , rotation (mathematics) , duty cycle , volume (thermodynamics) , optics , mathematics , medicine , geometry , voltage , quantum mechanics
Purpose: To quantify lung planning target volume (PTV) inaccuracies due to motion in non‐gated and gated CT images. Method and Materials: A motorized test object: sphere diameter (2.5, 5.0 cm), orbit diameter (1, 2 cm) and period (3–5 sec.) was constructed. For non‐gated helical CT, pitch, slice width, interval and acquisition time was varied. For gated CT, phase and slice acquisition time was varied. Structures were automatically contoured and a 1 cm margin added using a 3D growing tool. A performance metric, normalized volume (NV), was defined as measured PTV divided by theoretical volume (NV=1, static conditions). A uniform margin, based on a spherical model, was calculated over a range of orbital speeds. External marker motion for ten lung cancer patients was measured and minimum residual motion was calculated for a duty cycle spanning CT slice acquisition time. Results: Optimal non‐gated helical protocols (time: 1.0 sec./rotation, pitch: 0.4–0.7, width: 2–3 mm, interval: 1 mm) achieved NV>0.85 over a range of clinically relevant orbital speeds; a standard protocol for lung (time: 1.5 sec./rotation, pitch: 1.5, width/interval: 3 mm) achieved NV>0.70. Gated protocols (time: 1.0 sec./rotation, width/interval 2–3 mm) achieved NV>0.65 over the same range of orbital speeds. For the lung patients, average orbital speed of the external marker ranged from 0.5 to 1.5 cm/sec. and residual motion ranged from 1.5 to 4.6 mm. Assuming a 1:1 ratio of marker to tumor motion, margins for non‐gated CT of 0.7 to 1.5 mm (optimal) and 1.5 to 3 mm (standard) were required to achieve NV=1. For gated CT, margins of 0.3 to 1 mm were required. Conclusion: Inaccuracies in PTV delineation due to tumor motion occur in both non‐gated and gated CT images. Average orbital speed of an external marker for lung patients may be used to estimate tumor motion margin.