KEY POINTS
- This retrospective pattern-of-failure analysis included 209 sacral segments in 109 patients treated with SBRT between 2010 and 2021. MRI-confirmed local failure developed in 48 segments from 25 patients, which formed the principal failure-analysis cohort.
- Most failed segments involved S1 (42%) or S2 (31%). Radioresistant histology was present in 54%, and 71% had baseline extraosseous extension.
- Among all treated segments, nonadherent contours produced more failures inside the initially involved sector than guideline-adherent contours: 37.1% versus 17.8% (p = 0.021).
- Failure in guideline-recommended adjacent sectors occurred in 22.9% of nonadherent segments versus 2.3% of adherent segments (p < 0.001). Failure in sectors not recommended for coverage remained uncommon.
- Among the 15 nonadherent segments that failed, 11 failures (73%) involved sectors that guidelines had recommended but that were incompletely contoured or omitted. Three failures occurred exclusively within these missed sectors.
- In contrast, 30 of 33 failures (91%) after guideline-adherent contouring occurred exclusively within sectors already included in the treated clinical target volume, suggesting biological or dose-related failure rather than geographic miss.
- Missed sectors that later failed received a median EQD2 of 38.7 Gy, compared with 48.2 Gy in the corresponding treated clinical target volume (p < 0.001). Volume coverage, equivalent uniform dose, D50%, D80%, and D90% were also significantly lower.
- Extraosseous progression reflected baseline anatomy: every extraosseous failure involved the epidural space, 89% involved a neural foramen, and 79% extended into the paraspinal compartment. The study was retrospective, single-centre, and not designed to determine the optimal elective-sector dose.
CLINICAL TAKEAWAY
Sacral metastases should be contoured according to sector-based spine SBRT guidance rather than treated as isolated non-spine bone lesions. Limiting the target to visible tumour or incompletely covering an involved sector creates a measurable risk of preventable geographic failure.