Background and purpose: Adaptive field size reduction based on gross tumor volume (GTV) shrinkage imposes risk on coverage. Fiducial markers were used as surrogate for behavior of tissue... Show moreBackground and purpose: Adaptive field size reduction based on gross tumor volume (GTV) shrinkage imposes risk on coverage. Fiducial markers were used as surrogate for behavior of tissue surrounding the GTV edge to assess this risk by evaluating if GTVs during treatment are dissolving or actually shrinking.Materials and methods: Eight patients with oropharyngeal tumors treated with chemo-radiation were included. Before treatment, fiducial markers (0.035 x 0.2 cm(2), n = 40) were implanted at the edge of the primary tumor. All patients underwent planning-CT, daily cone beam CT (CBCT) and MRIs (pretreatment, weeks 3 and 6). Marker displacement on CBCT was compared to local GTV surface displacement on MRIs. Additionally, marker displacement relative to the GTV surfaces during treatment was measured.Results: GTV surface displacement derived from MRI was larger than derived from fiducial markers (average difference: 0.1 cm in week 3). During treatment, the distance between markers and GTV surface on MRI in week 3 increased in 33% > 0.3 cm and in 10%> 0.5 cm. The MRI-GTV shrank faster than the surrounding tissue represented by the markers, i.e. adapting to GTV shrinkage may cause under-dosage of microscopic disease.Conclusions: We showed that adapting to primary tumor GTV shrinkage on MRI mid-treatment is potentially not safe since at least part of the GTV is likely to be dissolving. Adjustment to clear anatomical boundaries, however, may be done safely. (C) 2016 Elsevier Ireland Ltd. Show less
Houweling, A.C.; Fukata, K.; Kubota, Y.; Shimada, H.; Rasch, C.R.N.; Ohno, T.; ... ; Horst, A. van der 2016
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... Show moreBackground 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 (D-98%) 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. (C) 2016 Elsevier Ireland Ltd. All rights reserved. Show less
