A novel dose‐based positioning method for CT image‐guided proton therapy

Abstract

Purpose: Proton dose distributions can potentially be altered by anatomical changes in the beam path despite perfect target alignment using traditional image guidance methods. In this simulation study, the authors explored the use of dosimetric factors instead of only anatomy to set up patients for proton therapy using in-room volumetric computed tomographic (CT) images. Methods: To simulate patient anatomy in a free-breathing treatment condition, weekly time-averaged four-dimensional CT data near the end of treatment for 15 lung cancer patients were used in this study for a dose-based isocenter shift method to correct dosimetric deviations without replanning. The isocenter shift was obtained using the traditional anatomy-based image guidance method as the starting position. Subsequent isocenter shifts were established based on dosimetric criteria using a fast dose approximation method. For each isocenter shift, doses were calculated every 2 mm up to ±8 mm in each direction. The optimal dose alignment was obtained by imposing a target coverage constraint that at least 99% of the target would receive at least 95% of the prescribed dose and by minimizing the mean dose to the ipsilateral lung. Results: The authors found that 7 of 15 plans did not meet the target coverage constraint when using only the anatomy-based alignment. After the authors applied dose-based alignment, all met the target coverage constraint. For all but one case in which the target dose was met using both anatomy-based and dose-based alignment, the latter method was able to improve normal tissue sparing. Conclusions:The authors demonstrated that a dose-based adjustment to the isocenter can improve target coverage and/or reduce dose to nearby normal tissue.

Head and neck IMPT probabilistic dose accumulation: Feasibility of a 2 mm setup uncertainty setting

“…Moreover, as more experience is gained, the interfractional motion is expected to decrease due to improved immobilization and treatments are expected to become more robust to motion due to improvements in treatment planning and plan adaptations. Lastly, optimizing patient positioning to minimize the dose perturbation can improve target coverage without increasing the setup uncertainty setting [34]. Future work on estimation of stopping power ratios using dual-energy CT can help reduce the range errors found in proton radiography and reduce the required range uncertainty setting [23,35].…”

Potential proton and photon dose degradation in advanced head and neck cancer patients by intratherapy changes

“…The standard image guidance for positioning in proton therapy is orthogonal X‐ray42 which has limitations for 3D target positioning. More advanced 3D imaging techniques like in‐room and cone‐beam CT become more and more available and might be beneficial,43, 44, 45 also for direct dose recalculations, dose accumulation and treatment adaptations. Estimations for accumulated doses during the treatment course were not feasible here due to the limitation of having only one control CT available.…”

Current state and future applications of radiological image guidance for particle therapy

“…An interesting alternative approach to re‐planning is dose‐guided positioning where treatment plan adaptation is emulated by optimizing the patient positioning to yield an optimal dose distribution in terms of target coverage and organ at risk sparing, potentially reducing the quality assurance workload associated with the creation of a new treatment plan. The necessary data for this procedure are the SPR distribution corresponding to the anatomy of the day as well an updated structure set.…”