Background and purpose We evaluated the robustness of carbon ion therapy for pancreatic cancer patients by investigating the impact of interfractional anatomical changes on the accumulated dose when using bony anatomy- and fiducial marker-based position verification. Material and methods Carbon ion treatment plans were created for 9 patients in this retrospective planning study. The planning CT was deformably registered to each daily cone-beam CT (CBCT). The gastrointestinal gas volume visible on each CBCT was copied to these deformed CT images. Subsequently, the fraction doses were calculated by aligning the treatment plan according to a bony anatomy- and a fiducial marker-based registration. We compared the accumulated fraction doses with the planned dose using dose-volume histograms (DVHs) of the internal gross tumour volume (iGTV), internal clinical target volume (iCTV), duodenum, stomach, liver, spinal cord and kidneys. Results iCTV coverage (D98%) was on average reduced from 98.6% as planned to 81.9% and 88.6% for the bony anatomy- and marker-based registrations, respectively. DVHs of the duodenum showed large differences between the planned and accumulated dose. Conclusions Severe reductions in dose coverage of the target due to interfractional anatomical changes were observed in both position verification methods.
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