Evaluating stereotactic accuracy with patient-specific MRI distortion corrections for frame-based radiosurgery.

Purpose: This study examines how MRI distortions affect frame-based SRS treatments and assesses the need for clinical distortion corrections.

Methods: The study included 18 patients with 80 total brain targets treated using frame-based radiosurgery. Distortion within patients' MRIs were corrected using Cranial Distortion Correction (CDC) software, which utilizes the patient's CT to alter planning MRIs to reduce inherent intra-cranial distortion. Distortion was evaluated by comparing the original planning target volumes (PTV) to targets contoured on corrected MRIs (PTV). To provide an internal control, targets were also re-contoured on uncorrected (PTV) MRIs. Additional analysis was done to assess if 1 mm expansions to PTV targets would compensate for patient-specific distortions. Changes in target volumes, DICE and JACCARD similarity coefficients, minimum PTV dose (D), dose to 95% of the PTV (D95%), and normal tissue receiving 12 Gy (V), 10 Gy (V), and 5 Gy (V) were calculated and evaluated. Student's t-tests were used to determine if changes in PTV were significantly different than intra-contouring variability quantified by PTV.

Results: PTV and PTV relative changes in volume were 6.19% ± 10.95% and 1.48% ± 32.92%. PTV and PTV similarity coefficients were 0.90 ± 0.08 and 0.73 ± 0.16 for DICE and 0.82 ± 0.12 and 0.60 ± 0.18 for JACCARD. PTV and PTV changes in D were -0.88% ± 8.77% and -12.9 ± 17.3%. PTV and PTV changes in D95% were -0.34% ± 5.89 and -8.68% ± 13.21%. The 1 mm expanded PTV targets did not entirely cover 14 of the 80 PTV targets. Normal tissue changes (V, V, V) calculated with PTV were (-0.09% ± 7.39%, -0.38% ± 5.67%, -0.08% ± 2.04%) and PTV were (-2.14% ± 7.34%, -1.42% ± 5.45%, -0.61% ± 1.93%). Except for V, all PTV changes were significantly different (p < 0.05) than PTV.

Conclusion: MRIs used for SRS target delineation exhibit notable geometric distortions that may compromise optimal dosimetric accuracy. A uniform 1 mm expansion may result in geometric misses; however, the CDC algorithm provides a feasible solution for rectifying distortions, thereby enhancing treatment precision.