Case report of the first lung cancer patient treated with passive scatter proton therapy at the University Proton Therapy Dresden

Purpose: We present the data of the first lung cancer patient treated at the University Proton Therapy Dresden with double scattering with respect to dosimetric stability throughout the treatment course.
Material and Methods: In August 2016, the first lung cancer patient was enrolled in the proton therapy arm of the clinical trial PRONTOX (NCT02731001). The patient with a stage III disease underwent multiple CT imaging: a pre-treatment 4DCT which was used for treatment planning and sequential 4DCT imaging in treatment position once a week (5 in total) for evaluation of motion and anatomical changes. The tumour GTV was contoured on all 4DCT phases and its centre of mass was used for motion assessment. A double scattering proton treatment plan with 3 beams was generated on the average CT using the iCTV (iGTV+involved lymph node stations+8mm) as target. The iGTV was overwritten with an average density. Margins and smearing were applied following Moyers et al. 2001. A robustness assessment was undertaken before treatment by evaluating the dose on all 4DCT phases, and recalculating the doses with uncertainties of ±3.5% HU and 8 set-up shifts of x,y,z=±3 mm. A re-evaluation of the dose distribution was performed weekly on the sequential average CT. For this purpose, the delineated contours were transformed with deformable image registration to match the new CT data.
Results: Pre-treatment motion of the GTV was below 1.5 mm in all directions and did not change during treatment. The pre-treatment robustness evaluation showed median changes below 5% for all evaluated OAR parameters, not exceeding the dose constraints, and a median dose coverage drop of iCTV from V95=99.8 % to V95=97.7%, which was deemed acceptable. OAR parameters evaluated on the sequential CT scans increased throughout the treatment by maximum 8% in the worst case, but not exceeding any constraints, while iCTV coverage was only slightly decreased (worst case drop of 0.1% in V95). An anatomical and dosimetric comparison of the planning CT and the sequential CT which had the worst iCTV coverage is shown in Figure 1.
Conclusion: The first lung cancer patient double scattering proton treatment at the University Proton Therapy Dresden was safely implemented. Within the framework of the trial, follow-up data regarding side effects and outcome will be collected and analysed. The collected data will also be used for evaluations of interplay and motion mitigation for pencil beam scanning treatment, including daily recorded breathing patterns and irradiation log files to enable the implementation of this technique in future.