A versatile phantom-based framework for end-to-end testing of (online) adaptive proton therapy in the head-and-neck...

Clinical implementation of online adaptive proton therapy requires rigorous testing of all facility-specific components. This work aimed to develop a multi-purpose phantom-based framework capable of testing the efficacy of diverse adaptive proton therapy workflows.

A custom-built, 3D-printed phantom mimicking different cavity fillings and swelling in the head-and-neck region was designed. The baseline scenario was compared with the anatomically altered scenario employing an initial and an adapted treatment plan. Across six institutes, dose deterioration and recovery were measured using an ionisation chamber and radiochromic films. Tools supporting adaptive proton therapy, i.e. log-file-based dose reconstruction, cone-beam computed tomography (CBCT)-based synthetic CTs, prompt- gamma imaging and proton radiography were included depending on their availability to demonstrate the framework's versatility.

Median dose deviation between ionisation chamber measurements and calculation was -0.7%[IQR=1.2%] averaged over all institutes and scenarios without significant differences. Not adapting to the anatomical change led to a 13.9%[IQR=1.5%] median dose decrease inside the target and 169.2%[IQR=11.7%] in the buildup region. Adapting to the changed anatomy, a median 0.2%[IQR=0.8 %] difference in the target and maximum 3.3% in the buildup could be achieved. Gamma-pass-rate (2%, 2mm) analysis of the radiochromic film measurements revealed a comparable trend. Institute-specific tests showed good compatibility with the various adaptive workflows tools.

The combination of the developed multi-purpose phantom and the end-to-end testing framework could reliably detect dosimetric effects of anatomical changes and provided comparable results across six institutes.