FROM ANATOMY TO DELIVERY: PHYSICS-ALIGNED CREDIT ASSIGNMENT FOR VMAT RADIOTHERAPY PLANNING

Radiation therapy planning translates a precise spatial balance between curative target coverage and healthy organ sparing into executable machine instructions. Volumetric modulated arc therapy (VMAT) realizes this delivery through a rotational trajectory of tightly coupled multileaf collimator apertures and beam intensities. Existing automated approaches struggle to connect clinical objectives directly to these machine parameters. A common approach to addressing this challenge is to use two-stage pipelines that optimize intermediate dose representations without physical constraints, causing the clinical trade-offs to degrade when eventually converted into deliverable actions. Conversely, methods that act directly on machine variables typically rely on a single plan-level clinical reward. This global scalar creates a severe credit assignment bottleneck, failing to isolate which specific leaf or intensity adjustment improved the target or harmed an organ. Differentiating the clinical objective provides local dose sensitivities for every machine variable. We find that translating these independent local gradients into a continuous, deliverable trajectory requires separating physical credit construction from policy learning. We introduce Physics-Aligned Credit for VMAT (PAC-VMAT) to realize this separation. The framework evaluates the evolving plan through a dose engine to construct explicit, fixed credit targets. Parameterized Leaf and MU policy agents then learn to coordinate these local clinical sensitivities into mechanically feasible aperture and intensity updates. Evaluated on lung and head-and-neck benchmarks, PAC-VMAT substantially improves the clinical target-organ balance over scalar-reward reinforcement learning and direct machine optimization, achieving plan quality comparable to closed-source commercial engines. This direct translation from patient anatomy to executable trajectories reduces optimization time to minutes, safely lowering collateral organ exposure while strictly satisfying mechanical delivery constraints.