Purpose

To develop and compare tumor-control probability (TCP) models for single-fraction stereotactic radiosurgery (SRS) for brain metastasis (BMs) with and without retreatment.

Methods and materials

We developed three different schemas to model TCP of BMs treated with linear accelerator-based SRS. Dose to 99% of each planning target volume (PTV D₉₉) and 6-month local control were fit using linear-quadratic-linear (LQ-L) models based on equivalent-dose conversions in 2 Gy (EQD2). The M1 schema had separate LQ-L TCP models for initial dose (M1-initial) and retreatment dose (M1-retreat), and the M2 schema had an LQ-L model using the sum of 50% of the initial SRS dose plus the retreatment SRS dose. The M1-initial and M1-retreat schema modeled local control after first SRS to 48 lesions (patients = 22) and second SRS to 46 lesions (patients = 21). The M0 schema included a whole data set of 349 lesions (patients = 136) receiving first SRS (no retreatment and M1-initial).

Results

LQ-L models fitted the data well (χ² = 0.059-0.525 and P = 0.999-1.000). For M0 and M1-retreat, the fitted models EQD2₅₀ and γ₅₀ parameters were similar. The LQ-L fitted EQD2₅₀ was ∼8.0 Gy for M0 and M1-retreat, ∼24 Gy for M1-initial, and ∼19 Gy for M2. The model fitted γ₅₀ was 0.1 Gy for M0, M1-retreat, and M2 and 0.5 for M1-initial. For the PTV D₉₉ of 10 and 20 Gy, the steepest to shallowest dose-response or largest change in TCP, that is, TCP_20Gy - TCP_10Gy, was observed in M1-initial (0.49) and M2 (0.17). M0 and M1-retreat showed a similar change in TCP of 0.21.

Conclusions

The model-fitted parameters predicted the recurrent BMs required a higher threshold dose and had a steeper dose-response for first SRS versus second SRS and M0. Alternatively, the recurrent BMs required ∼2 Gy higher predicted PTV D₉₉ dose for first SRS to achieve the same TCP of 0.75 compared with second SRS and M0. Further investigations on larger patient cohorts are needed for validating our findings in predictive modeling of recurrent BMs.