Author: Estelle Batin, Michael A. Carlson, Nilendu Gupta, Zachary X. Richards, Diana Shvydka 👨🔬
Affiliation: Department of Radiation Oncology, The Ohio State University Wexner Medical Center 🌍
Purpose: With an increase in world-wide proton treatment availability, patients sometimes are transferred between proton and photon modalities. Among the main reasons for modality shifts are proton beam failures, anatomical changes, hospital resource allocation, or mixed-modality planning. Patient support structures must be accurately accounted for in these circumstances.
Methods: A single CT simulation is typically used to generate both proton and photon plans. kVue One™ couch top (CQ Medical), offers nearly identical couch base for proton and photon treatments with interchangeable inserts that can be fit on the CT sim couch. The base, however, requires a model for the treatment planning system (TPS) and is not provided by the manufacturer. Two models were independently created and commissioned to provide close dosimetric agreements between measurements and calculations in TPS, Varian Eclipse and RaySearch Laboratories RayStation for photon and proton beams, correspondingly.
Results: Carbon fiber is used in both couch tops, offering mechanical strength and low beam attenuation. Despite similarity in construction and dimensions, the associated supporting structure models cannot be easily interchanged due to different approaches to beam arrangements: fixed-angle beams for proton plans, vs. VMAT arcs for photons. Since proton beams are never directed through the couch sides, those are disregarded in the proton couch model, focusing on correct representation of the proton beam range pullback. On the contrary, these areas are important for photon VMAT arcs, contributing ~9% segments. We found that for a typical 2 full arc photon plan use of the proton couch model results in ~2% overestimate of the target coverage and critical structure doses, the effect is larger for one-sided partial arcs.
Conclusion: While it may be tempting to use the same couch model switching from proton to photon plans, they are not interchangeable, resulting in dose overestimate of 2% or higher.