  Background and purpose: To investigate the possible contribution of indirect
damage and damage saturation to tumour control obtained with SBRT/SRS
treatments for early-stage NSCLC and brain metastases. Methods and Materials:
We have constructed a dataset of early-stage NSCLC and brain metastases
dose-response. These data were fitted to models based on the linear-quadratic
(LQ), the linear-quadratic-linear (LQL), and phenomenological modifications of
the LQ-model to account for indirect cell damage. We use the
Akaike-Information-Criterion formalism to compare performance, and studied the
stability of the results with changes in fitting parameters and perturbations
on dose/TCP values.
  Results: In NSCLC, a modified LQ-model with a beta-term increasing with dose
yields the best-fits for $\alpha$/$\beta$=10 Gy. Only the inclusion of very
fast accelerated proliferation or low $\alpha$/$\beta$ values can eliminate
such superiority. In brain, the LQL model yields the best-fits, and the ranking
is not affected by variations of fitting parameters or dose/TCP perturbations.
  Conclusions: For $\alpha$/$\beta$=10 Gy, a modified LQ-model with a beta-term
increasing with dose provides better fits to NSCLC dose-response curves. For
brain metastases, the LQL provides the best fit. This might be interpreted as a
hint of indirect damage in NSCLC, and damage saturation in brain metastases.
The results for NSCLC are strongly dependent on the value of $\alpha$/$\beta$
and may require further investigation, while those for brain seem to be clearly
significant. Our results can assist in the design of improved radiotherapy for
NSCLC and brain metastases, aiming at avoiding over/under-treatment.
