Stereotactic radiosurgery (SRS) demands precise dose shaping around critical structures, yet black-box AI systems have limited clinical adoption due to opacity concerns. We tested whether chain-of-thought reasoning improves agentic planning in a retrospective cohort of 41 patients with brain metastases treated with 18 Gy single-fraction SRS. We developed SAGE (Secure Agent for Generative Dose Expertise), an LLM-based planning agent for automated SRS treatment planning. Two variants generated plans for each case: one using a non-reasoning model, one using a reasoning model. The reasoning variant showed comparable plan dosimetry relative to human planners on primary endpoints (PTV coverage, maximum dose, conformity index, gradient index; all p > 0.21) while reducing cochlear dose below human baselines (p = 0.022). When prompted to improve conformity, the reasoning model demonstrated systematic planning behaviors including prospective constraint verification (457 instances) and trade-off deliberation (609 instances), while the standard model exhibited none of these deliberative processes (0 and 7 instances, respectively). Content analysis revealed that constraint verification and causal explanation concentrated in the reasoning agent. The optimization traces serve as auditable logs, offering a path toward transparent automated planning.
@article{arxiv.2512.20586,
title = {Automated stereotactic radiosurgery planning using a human-in-the-loop reasoning large language model agent},
author = {Humza Nusrat and Luke Francisco and Bing Luo and Hassan Bagher-Ebadian and Joshua Kim and Karen Chin-Snyder and Salim Siddiqui and Mira Shah and Eric Mellon and Mohammad Ghassemi and Anthony Doemer and Benjamin Movsas and Kundan Thind},
journal= {arXiv preprint arXiv:2512.20586},
year = {2025}
}