2369 - Temporary Organ Displacement for Sacral Chordoma Stereotactic Radiosurgery

Purpose/Objective(s): Sacral chordomas abut the rectum, limiting ablative stereotactic radiosurgery (SRS). We hypothesized that temporary organ displacement (TOD) would reduce rectal dose while improving target coverage.

Materials/Methods: We retrospectively reviewed all (N=15) patients with sacral chordoma treated with single-fraction SRS (24 Gy) using TOD at one institution. TOD was performed by interventional radiology via percutaneous placement of an 8-French drainage catheter into the presacral space. Immediately prior to simulation and treatment, saline mixed with iodinated contrast was instilled and cone-beam CT was obtained to confirm displacement. For each patient, a paired dosimetric comparison was performed between a plan generated on the pre-TOD CT (pre-TOD plan) and the delivered clinical plan generated after TOD (post-TOD plan). The primary endpoint was paired change in rectum Dmax (Gy). Secondary endpoints included PTV V100 (%) and the pass rate for PTV V100 >95%. Paired comparisons used Wilcoxon signed-rank tests; pass rates used McNemar’s exact test. Grade =3 adverse events (G3+ AEs) were recorded. Progression-free survival (PFS) and overall survival (OS) from the SRS date were evaluated via Kaplan–Meier analysis.

Results: In this cohort of 15 patients, 11 were treated with definitive SRS alone. Partial sacrectomy was performed in 4 patients, with 3 receiving neoadjuvant SRS and 1 receiving adjuvant SRS. Dosimetric analysis showed median rectum Dmax decreased from 15.91 Gy (IQR 15.65–15.95) pre-TOD to 12.53 Gy (IQR 5.94–15.34) post-TOD (median ? -2.96 Gy; IQR -10.02 to -0.57; p=0.0076). Median PTV V100 increased from 87.3% (IQR 84.2–92.5) to 98.0% (IQR 95.7–98.9) (median ? +6.8 percentage points; IQR +4.65 to +13.05; p=0.00061). The proportion meeting PTV V100 >95% increased from 13.3% to 73.3% (p=0.0117). At a median follow-up of 67.5 months (range 30.8–131.9), there were no GI G3+ AEs and one GU G3+ AE (urinary incontinence; bladder Dmax 7.2 Gy). 5-year PFS was 86.7% (95% CI 56.4–96.5), and 5-year OS was 93.3% (95% CI 61.3–99.0).

Conclusion: This is the largest reported series of sacral chordoma SRS using TOD with pre-TOD / post-TOD dosimetric comparison and long-term follow-up. TOD was associated with clinically meaningful rectal dose reduction and improved PTV coverage, increasing the proportion of plans meeting a prespecified coverage threshold, with low rates of severe toxicity. These data support further evaluation of TOD to facilitate safe dose escalation in SRS for sacral chordomas.