Abstract
This study evaluated the accuracy of measuring the motion of an internal target using four‐dimensional computed tomography (4DCT) scanning and the BrainLAB ExacTrac X‐ray imaging system. Displacements of a metal coil implanted in a commercial respiratory phantom were measured in each system and compared to the known motion. A commercial respiratory motion phantom containing a metal coil as a surrogate target was used. Phantom longitudinal motions were sinusoidal with a 4.0 second period and amplitudes ranging from 5–25 mm. We acquired 4DCT and ExacTrac images of the coil at specified respiratory phases and recorded the coordinates of the coil ends. Coil displacement relative to the 0% phase (full‐inhale) position were computed for the ExacTrac and 4DCT imaging systems. Coil displacements were compared to known displacements based on the phantom's sinusoidal motion. Coil length distortion due to 4DCT phase binning was compared to the known physical length of the coil (31 mm). The maximum localization error for both coil endpoints for all motion settings was 3.5 mm for the 4DCT and 0.8 mm for the ExacTrac gating system. Coil length errors measured on the 4DCT were less than 0.8 mm at end inhale/exhale phases, but up to 8.3 mm at mid‐inhalation phases at the largest motion amplitude (25 mm). Due to the fast image acquisition time (100 ms), no coil distortion was observable in the ExacTrac system. 4DCT showed problems imaging the coil during mid‐respiratory phases of higher velocity (phases 20%–30% and 70%–80%) due to distortion caused by residual motion within the 4DCT phase bin. The ExacTrac imaging system was able to accurately localize the coil in the respiratory phantom over all phases of respiration. For our clinic, where end‐respiration phases from 4DCT may be used for treatment planning calculations, the ExacTrac system is used to measure internal target motion. With the ExacTrac system, planning target size and motion uncertainties are minimized, potentially reducing internal target volume margins in gated radiotherapy.PACS number: 87.56.‐v
Highlights
302 Matney et al.: ExacTrac localization in a respiratory phantom volumes
AAPM Report 91 recommends that respiratory gating should be considered when tumor respiratory motion exceeds 5 mm.[12] literature reports that, for most patients, a superior–inferior tumor movement over 2 cm is relatively uncommon.[15] we evaluated phantom target motion amplitudes of 5, 10, 15, 20 and 25 mm motion in the longitudinal or superior–inferior direction of the phantom and 1 cm chest wall amplitude in the anterior– posterior direction
A similar table does not exist for the ExacTrac system because the X-ray images are near instantaneous (100 ms) exposures of the coil, which have minimal motion blurring
Summary
302 Matney et al.: ExacTrac localization in a respiratory phantom volumes. This dose conformation reduces complications by limiting the dose delivered to normal tissues.[1,2] With a relative reduction of dose to normal tissues comes the ability to escalate the dose delivered to the target to increase tumor control probability while maintaining acceptable levels of normal tissue complications.[3,4]One challenge for radiation therapy is compensating for respiratory organ motion. Nongated standard radiation therapy techniques treat a volume that encompasses the tumor at all potential positions within the respiratory cycle.[5,6] While this technique can adequately irradiate the entire target volume, it increases the dose to surrounding normal tissues. Accurate adjustment of the radiation delivery to compensate for respiratory motion will permit a reduction in the planning target volume and, reduce doses to surrounding normal tissues. Potential techniques for incorporating intrafractional respiratory motion into treatment planning/delivery include breath-hold,(7) respiration-gating,(8-10) and target-tracking.[11] Breath-hold techniques actively or passively suspend the patient’s respiration for short intervals and deliver treatment during these intervals. Target-tracking four-dimensional techniques greatly increase the complexity of the treatment delivery All of these techniques require some form of observation of the respiratory motion. Current measurement techniques[12] for tracking respiration include infrared external marker tracking, spirometry, strain gauges, video visual tracking, and fluoroscopic tracking of implanted fiducial markers
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