A Simulation-Free Adaptive Radiotherapy Workflow for Helical Tomotherapy: Concept Study of Platform Requirements on the Accuray Radixact System

Abstract Background and purpose. Simulation-free adaptive radiotherapy, in which the patient's existing diagnostic CT replaces the planning CT and the treatment plan is created or adapted with the patient on the couch, is no longer a proposal. It has been implemented clinically on cone-beam CT-guided platforms in cohorts of up to 43 patients, and its constituent steps are all published: pre-planning on the diagnostic CT with planning templates, deformable propagation of contours onto the daily image, physician review, on-couch re-optimization, and same-session delivery. None of that work has been done on helical tomotherapy, which has a same-session lineage of its own in STAT RT, a feature that planned on a megavoltage CT acquired at the console but contoured on the day and carried no plan forward from a diagnostic CT. This paper asks what would have to be true for the same workflow to run on an Accuray Radixact system, and answers it in the specific terms of that platform. What is established, and what is not. We take the workflow itself as prior art and do not claim it. What is specific to helical tomotherapy, and what this paper contributes, is an analysis of four platform constraints that the cone-beam CT literature does not encounter in the same form. The first is the image-value-to-density calibration required to compute dose on megavoltage CT, and its sensitivity to acquisition mode and reconstruction algorithm. The second is the 40 cm megavoltage CT reconstruction field of view, which truncates the external contour of larger patients and forces a merged-image construction whose donor is a deformed diagnostic CT. The third is the optimization and dose-calculation time of helical delivery, heavier than the static-angle equivalent, which raises a question we identify rather than answer: which step binds the on-couch session. The fourth is a discontinuity in the planning interface, where the deformable re-contouring session ends at the structure set rather than continuing into plan creation. Method of the analysis. Each constraint is quantified from the published tomotherapy literature where a measurement exists, and identified as unmeasured where it does not. The result is a specification with defined commissioning tasks, a staged validation program with explicit gates, and a set of recommendations to the manufacturer. Conclusions. This is an analysis, not a measurement: no dose has been delivered and no dosimetric quantity reported here was measured by us. The registration, contouring, and plan-creation path the workflow depends on has, however, been exercised on the author's Precision 2.0.1.1 installation, so the open questions are dosimetric and organizational rather than whether the software will begin the sequence at all. Its value is that it converts a general clinical workflow into the particular set of physics and software questions a tomotherapy department would have to answer first, and states which of those questions the literature already answers and which it does not. Keywords: simulation-free radiotherapy; same-session radiotherapy; diagnostic CT-based planning; helical tomotherapy; Radixact; megavoltage computed tomography; ClearRT kilovoltage computed tomography; image-value-to-density table; treatment planning template; palliative radiotherapy