Session Item

Despite potential improvement in survival after stereotactic ablative radiotherapy (SABR) for oligometastasis, concerns over its safety remain. To mitigate the risk of toxic effects, target coverage is sacrificed to prioritize organs-at-risk (OAR) during SABR planning. This study evaluated the effect of this practice on dosimetry, local recurrence (LR), and progression-free survival (PFS) in patients treated from a population-based phase II trial.

The single-arm phase II SABR-5 trial treated patients with up to 5 oligometastases between November 2016 and July 2020. The protocol-specific planning objective was to cover 95% of the planning treatment volume (PTV) with 100% of the prescribed dose, however PTV coverage must be reduced as needed to meet OAR constraints. This trade-off was measured using the coverage compromise index (CCI), computed as minimum dose received by the hottest 99% of the PTV (D99) divided by the prescription dose. Under-coverage was defined as CCI < 0.90. The potential association between CCI and outcomes (LR and PFS) was evaluated.

A total of 549 lesions from 381 patients were assessed. Mean CCI was 0.88 (95% confidence interval [CI], 0.86-0.89), and 196 (36%) lesions were under-covered. The highest mean CCI (0.95; 95% CI, 0.93-0.97) was seen in non-spine bone lesions (n=116), while the lowest mean CCI (0.71; 95% CI, 0.69-0.73) was seen in spine lesions (n=104). On multivariable analysis, CCI < 0.90 did not predict for worse local recurrence or progression-free survival.

Under-coverage of the PTV is not associated with worse local recurrence and progression-free survival rates in this large, population-based phase II trial. Combined with low rates of toxic effects, this study supports the practice of prioritizing OAR constraints during SABR treatment planning for oligometastases.