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Performance Test Comparing Three Pre-Treatment Isocenter Localization Techniques for Single Fraction Cranial Stereotactic Radiosurgery

B Mathews

B Mathews*, J Saini, D Campbell, CARTI, Little Rock, AR

SU-E-T-416 Sunday 3:00:00 PM - 6:00:00 PM Room: Exhibit Hall

Purpose: To develop a performance test comparing three pre-treatment isocenter localization techniques when using head-frame vs. immobilization mask for cranial Stereotactic Radiosurgery (SRS). This study will compare pre-treatment positioning techniques using laser alignment vs x-ray verification using ExacTrac or On Board Imaging.

Methods: A RANDO anthropomorphic head phantom was fitted with an in-house polystyrene insert to allow EDR2 film measurements in two orthogonal planes. A pin hole was pricked on each film to serve as a target during treatment planning. For each trial (three total), a CT scan was acquired of the phantom equipped with either an immobilization mask or invasive head frame. Treatment planning employed iPlan Image v4.1 and iPlan Dose v4.1.1. Positioning of the phantom equipped with the head-frame was performed by aligning vault lasers to coincide with cross-hair labels on a target positioner box. Setups utilizing an immobilization mask were verified by x-ray verification using ExacTrac and On Board Imaging, and if alignment were not within tolerance, then shifts were made using a 6D robotic couch. Gantry star-shot irradiation was performed using a 5mm cone to evaluate the differences between radiation isocenter and the target. The mean and standard deviations were calculated for differences in the x-, y-, and z-coordinate axes.

Results: Positional accuracy using ExacTrac for mask based SRS resulted in 1.10±0.86, 0.67±0.83, and 0.59±0.48mm for cross plane, inline, and vertical measurements, respectively. Differences for frame based SRS were 0.93±0.43, 0.76±0.18, and 0.34±0.12mm for cross plane, inline, and vertical measurements, respectively. Results for mask based SRS using OBI will soon follow.

Conclusions: Although the frame-based SRS technique generated smaller standard deviations, the mean difference from target to radiation isocenter for both techniques fall within the statistical uncertainty of one another. Planning margins must take into account target size when treating small lesions for both techniques.

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