The necessity of 4D-motion monitoring for thoracic tumors treated with pencil beam scanning proton therapy:a comprehensive 4D-imaging study

Purpose/ObjectiveFor pencil beam scanning proton therapy (PBS-PT), moving targets remain challenging due to the interplay effect. Even when using motion mitigation strategies, one needs to be aware of motion variations. We investigated weekly and daily motion variations to define the most optimal motion monitoring protocol for PBS-PT treatments of lung cancer patients.Material/MethodsFor 20 stage II-IV (N)SCLC patients 4DCT imaging was performed during treatment simulation (week 0) and weekly during the treatment course. GTVs were delineated on the maximum inspiration and expiration 4DCT phases and the centroid 3D-vector translations were evaluated. For one patient, daily 3D-vector centroid 4DCBCT motion was evaluated additionally. ResultsA median initial tumor motion (Figure 1) of 1.3 mm (range: 0.0 – 7.4 mm) was observed. GTV motions varied each week; 7 out of 20 patients showed motion variation >3 mm compared to the motion measured in week 0. Figure 2 shows that motion amplitudes extracted from weekly 4DCTs were not predictive for motion amplitudes extracted from daily 4DCBCTs.ConclusionFor a considerable part of the patients, the motion measured in week 0 based on weekly repeat 4DCT imaging was not predictive for motion in the following weeks. Daily motion measured by 4DCBCT imaging for one patient suggests that weekly measured 4DCT motion is not predictive for the daily motion in between the weekly 4DCTs. This indicates that breathing motion differs from day to day and daily 4D-imaging is therefore needed to assure safe PBS-PT treatments for lung cancer patients.