Stereotactic Ablative Radiotherapy (SABR) for Recurrent and/or Previously Irradiated Skull Base Malignancies: An Analysis of Setup and Positioning Accuracy

Purpose/Objectives: Recurrent and/or previously irradiated skull base (SB) malignancies present a therapeutic challenge due to the proximity of critical structures, limiting conventional treatment options. Stereotactic ablative radiotherapy (SABR) delivers high dose radiation over a few sessions, and requires reliable daily setup to minimize unintended translational and rotational movements. Here, we examine setup and positioning accuracy of SABR in SB tumors to further optimize image guidance and PTV margins.

Materials/Methods: We analyzed patients with recurrent and/or previously irradiated SB tumors treated with SABR using our custom cushion-mask-bite block (CMB) immobilization system. Initial setup was performed with ExacTrac infrared tracking, and X-ray images of bony anatomy were compared with the treatment plan to determine setup error (e.g., interfractional error). Patients were then shifted per ExacTrac and repeat verification X-ray was taken to determine residual error. Cone-beam CT (CBCT) was then performed to determine 3D-3D agreement and further shifts were applied as necessary based on soft-tissue anatomy. Finally, ExacTrac X-rays were taken prior to each beam to determine intrafractional error. Translational, rotational, systematic, and random errors were determined and used to calculate 3D vectors and PTV margins. Results were stratified into four groups by SB subsite: anterior (1), central (2), or posterolateral (3) SB, and nasopharynx/retropharynx (4).

Results: A total of 63 patients and 304 individual treatment sessions were analyzed. Median dose and treatment volume were 45 Gy in 5 fractions (range: 21–45 Gy, 3–5 fractions) and 17.2 cm³ (range: 1.5–178.1 cm³), respectively. [Table 1] provides a comprehensive summary of setup and positioning errors. For initial setup, 43/65 patients required shifts of >2 mm or 2 degrees, necessitating PTV margins 34.1 mm to account for interfractional alignment errors in the absence of image guidance. All setup errors were corrected to within 1.3 mm and 1.8 degrees, and only 1.9% of treatment sessions had residual errors >1 mm. CBCT-ExacTrac 3D agreement was ≤1 mm in 271/304 sessions (89.1%) and ≤1.5 mm in nearly all sessions (303/304). Translational differences amenable to CBCT shift (>1 mm) were detected in only 16/304 treatment sessions (5.3%). The largest shifts occurred in group 1 and 4 tumors, indicating physician-based adjustments for differences in bony and soft tissue anatomy. Although rotational setup errors were <1 degrees across subsites, anterior and lateral SB tumors were most susceptible to yaw uncertainties (0.5 vs. 0.08 degrees; p < 0.01), whereas central SB tumors were more susceptible to roll uncertainties (0.3 vs. 0.06 degrees; p < 0.11). Tumors of the retropharynx and nasopharynx appear least susceptible to pitch uncertainties compared with the other subsites (0.3 vs. 0.7 degrees; p < 0.08). Overall, anterior/posterior and pitch errors were most common. PTV margins of 1.3 mm were needed to encompass 95% of the target for a confidence level of 90% of patients.

Conclusion: This is one of the largest setup and positioning analyses for recurrent and/or previously irradiated skull base tumors treated with SABR, demonstrating PTV margins of 1.5 to 2 mm will ensure adequate target coverage independent of SB location. For 0 to 1 mm margins, such as used in radiosurgery, consideration should be given to tumor subsite location and rotation error susceptibility. Further investigation into the clinical implications of these findings is warranted.