Abstract
BackgroundUncertainties associated with the delivery of treatment to moving organs might compromise the accuracy of treatment. This study explores the impact of intra-fractional anatomical changes in pancreatic patients treated with charged particles delivered using a scanning beam. The aim of this paper is to define the potential source of uncertainties, quantify their effect, and to define clinically feasible strategies to reduce them.MethodsThe study included 14 patients treated at our facility with charged particles (protons or 12C) using intensity modulated particle therapy (IMPT). Treatment plans were optimized using the Treatment Planning System (TPS) Syngo® RT Planning. The pre-treatment dose distribution under motion (4D) was simulated using the TPS TRiP4D and the dose delivered for some of the treatment fractions was reconstructed. The volume receiving at least 95% of the prescribed dose (V95CTV) and the target dose homogeneity were evaluated. The results from the 4D dose calculations were compared with dose distributions in the static case and its variation correlated with the internal motion amplitude and plan modulation, through the Pearson correlation coefficient, as well the significant p-value. The concept of the modulation index (MI) was introduced to assess the degree of modulation of IMPT plans, through the quantification of intensity gradients between neighboring pencil beams.ResultsThe induced breathing motion together with dynamic beam delivery results in an interplay effect, which affects the homogeneity and target coverage of the dose distribution. This effect is stronger (∆V95CTV > 10%) for patients with tumor motion amplitude above 5 mm and a highly modulated dose distribution between and within fields. The MI combined with the internal motion amplitude is shown to correlate with the target dose degradation and a lack of plan robustness against range and positioning uncertainties.ConclusionsUnder internal motion the use of inhomogeneous plans results in a decrease in the dose homogeneity and target coverage of dose distributions in comparison to the static case. Plan robustness can be improved by using multiple beams and avoiding beam entrance directions susceptible to density changes. 4D dose calculations support the selection of the most suitable plan for the specific patient’s anatomy.
Highlights
Uncertainties associated with the delivery of treatment to moving organs might compromise the accuracy of treatment
The dose distributions, namely the static, the 4D Dose Simulation (4DDSim), and 4DDReco, were evaluated by taking as metric the volume receiving at least 95% of the prescribed dose (V95CTV) and the target dose homogeneity (HCTV = D5-D95)
The main component of the motion was detected in the cranio-caudal direction, followed by the anterior-posterior direction
Summary
Uncertainties associated with the delivery of treatment to moving organs might compromise the accuracy of treatment. This study explores the impact of intra-fractional anatomical changes in pancreatic patients treated with charged particles delivered using a scanning beam. Anatomical changes during the course of the treatment, as well as tumor volume changes, intestine and stomach filling and loss of adipose tissue, have been discussed in a recent publication [5].This study, will address the impact of intra-fractional changes. Intra-fractional anatomical variations, i.e. the induced breathing motion, together with dynamic beam delivery, has been shown to affect the dose distribution in terms of homogeneity and target coverage [6]. This so-called interplay effect must be quantified for each pathology and facility-specific configuration of the beam delivery system
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