Planning strategies for inter-fractional robustness in pancreatic patients treated with scanned carbon therapy

Abstract

BackgroundManaging inter-fractional anatomy changes is a challenging task in radiotherapy of pancreatic tumors, especially in scanned carbon-ion delivery. This treatment planning study aims to focus on clinically feasible solutions, such as the beam angle selection and margin design to increase the robustness against inter-fractional uncertainties.MethodsThis study included 10 patients with weekly 3D-CT imaging and physician-approved Clinical Target Volume (CTV). The study was directed to keep the CTV-coverage using six beam angle configurations in combination with different Internal Target Volume (ITV) concepts. These were: geometric-margin (symmetric 3 and 5 mm margin); range-equivalent margins with an isotropic HU replacement; and to evaluate the need of asymmetric margins the water-equivalent range path (WEPL) was determined per patient from the set of CTs.Plan optimization and forward dose calculation in each week-CT were performed with the research treatment planning system TRiP98 and the plan quality evaluated in terms of CTV coverage (V95CTV) and homogeneity dose (HCTV = D5-D95).ResultsThe beam geometry had a substantial impact on the target irradiation over the treatment course, with the single posterior or two beams showing the best average coverage of the CTV. The use of geometric margins for the more robust beam geometries showed acceptable results, with a V95CTV of (99.2 ± 1.2)% for the 5 mm-margin. For the non-robust configurations, due to substantial changes in the radiological depth, the use of this margin results in a V95CTV that might be below 80%, only showing improvement when the range changes are included.ConclusionsSelection of adequate beam configurations and treatment margins in ion-beam therapy of pancreatic tumors is of great importance. For a single posterior beam or two beam configurations, application of geometrical margins compensate for dose degradation induced by inter-fractional anatomy changes for the majority of the analyzed treatment fractions.Electronic supplementary materialThe online version of this article (doi:10.1186/s13014-017-0832-x) contains supplementary material, which is available to authorized users.

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“…This, on the other hand, allows for flexibility in the design and hence a positioning of corset opening and buckles, which would avoid beam edge effects. The design of the 3DPE corset considered that beam angles between 40° and 100° were a priori not intended for pencil beam scanned PT planning since the presence of the stomach and the large intestine prevents the usage of left lateral (oblique) beam angles for pancreatic cancer patients . Hence, in our study, the left lateral part was chosen as an opening.…”

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“…Furthermore, comparably small uncertainties in patient positioning, stopping power calculations and anatomical changes might have significant consequences like hot or cold spots in the resulting dose distribution in the patient. A major topic in treatment planning therefore is the plan robustness against these uncertainties [104][105] [106] [107]. This also includes the impact of a variable RBE in proton treatment planning, which could be…”

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“…1 ) [ 39 ]. The one-field setup with a single posterior field was also promising, although the maximum doses in the myelon were thoroughly high [ 40 ]. This can be due to many reasons, however, one explanation can be overdosage in the Bragg Peak region potentially due to higher biological effects in the distal edge of the Bragg Peak of particle beams.…”

“…Nevertheless, Batista et al have presented data about pancreatic cancer, that supported this hypothesis. A single posterior field and two oblique posterior fields are superior in case of robustness [ 40 ]. But, dose deposition by a single field leads to high integral dose in its trajectory, resulting in high dose deposition in the spinal cord itself, probably violating general QUANTEC (Quantitative Analyses of Normal Tissue Effects in the Clinic) constraints [ 69 ].…”