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Evaluating the Need for CBCT Image Guidance for Brain SRS with ExacTrac

R Martin*, T Briere , E Han , MD Anderson Cancer Ctr., Houston, TX


(Sunday, 7/14/2019) 4:00 PM - 4:30 PM

Room: Exhibit Hall | Forum 6

Purpose: Image guidance is an important part of hypo-fractionated brain SRS with multiple options available. This work focuses on the potential added benefit of CBCT image guidance in a linac setting that utilizes ExacTrac. A patient population with suspected alignment challenges (post-surgery where the bone flap rests against the headrest) was studied specifically.

Methods: 45 patients who had received brain SRS on a linac were divided into 3 groups of 15 patients each: post-surgery with bone flap touching headrest, post-surgery with bone flap away from headrest, and no surgery. VMAT treatment plans included a 2mm GTV to PTV expansion margin. Image guidance included ExacTrac (0.7mm/0.8� tolerance) for patient positioning, CBCT, and orthogonal kV for verification purposes followed by ExacTrac before each arc. Offline rigid registration of the treatment to reference CBCT was performed using MosiaQ’s maximization mutual information (MMI) algorithm and 10cm bounding box around isocenter.

Results: Mean shifts for each translational and rotational direction were within 0.5mm and 0.5â?° for all patients. Maximum shifts were 1mm and 1.4â?°, with only 3 patients with shifts over 1â?° (1.1â?°, 1.1â?°, and 1.4â?°). No significant difference in shifts in any direction between the three groups was observed except an increase in yaw rotation for no surgery patients (p=0.01). However, the average in this case was still less than 1â?° (0.7â?°).

Conclusion: Residual shifts after ExacTrac alignment as found by CBCT were determined to be small enough to be able to safely omit CBCT image guidance for brain SRS with a 2mm PTV margin. Clinical factors including ExacTrac tolerances, PTV expansion margin, proximity to critical structures, and number of targets per isocenter should be considered when deciding whether to use CBCT for a particular patient or institution.


Cone-beam CT, Stereotactic Radiosurgery, Brain


Not Applicable / None Entered.

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