Organ-contour-driven auto-matching algorithm in image-guided radiotherapy.

PURPOSE: This study aimed to demonstrate the potential clinical applicability of an organ-contour-driven auto-matching algorithm in image-guided radiotherapy. METHODS: This study included eleven consecutive patients with cervical cancer who underwent radiotherapy in 23 or 25 fractions. Daily and reference magnetic resonance images were converted into mesh models. A weight-based algorithm was implemented to optimize the distance between the mesh model vertices and surface of the reference model during the positioning process. Within the cost function, weight parameters were employed to prioritize specific organs for positioning. In this study, three scenarios with different weight parameters were prepared. The optimal translation and rotation values for the cervix and uterus were determined based on the calculated translations alone or in combination with rotations, with a rotation limit of ±3°. Subsequently, the coverage probabilities of the following two planning target volumes (PTV), an isotropic 5 mm and anisotropic margins derived from a previous study, were evaluated. RESULTS: The percentage of translations exceeding 10 mm varied from 9% to 18% depending on the scenario. For small PTV sizes, more than 80% of all fractions had a coverage of 80% or higher. In contrast, for large PTV sizes, more than 90% of all fractions had a coverage of 95% or higher. The difference between the median coverage with translational positioning alone and that with both translational and rotational positioning was 1% or less. CONCLUSION: This algorithm facilitates quantitative positioning by utilizing a cost function that prioritizes organs for positioning. Consequently, consistent displacement values were algorithmically generated. This study also revealed that the impact of rotational corrections, limited to ±3°, on PTV coverage was minimal.

Development and multi-institutional evaluation of a new phantom for verifying beam-positioning errors at off-isocenter positions.

PURPOSE: Single-isocenter stereotactic radiotherapy for multiple brain metastases requires highly accurate treatment delivery at off-isocenter positions (off-iso). This study aimed to verify the beam-positioning errors at off-iso using a newly developed phantom tested at multiple institutions. METHODS: The off-iso phantom comprised five stainless-steel balls with a 3-mm diameter placed at the center and at four peripheral positions on a diagonal line. Each ball was placed 3.5 cm apart along each of the three axes. Two patterns of the phantom setup were defined as 0° and 90° phantom rotations to evaluate the beam-positioning error, which is the distance between the center of the ball and the irradiated field on the electronic portal imaging device. Furthermore, the reproducibility of the beam-positioning errors was verified by evaluating their standard deviation (SD) at a single institution, which included five measurements for two treatment machines. The errors were evaluated at multiple institutions using eight treatment machines. RESULTS: The measurement time from setup to image acquisition was approximately 20 min for two patterns. The SD of the beam-positioning errors in the reproducibility tests was 0.41 mm. In the multi-institutional evaluation, the beam-positioning error at the isocenter position was within 1.00 mm of the AAPM-RSS tolerance, with the exception of two linacs. The largest beam-positioning error (1.36 mm) was observed 7.5 cm away from the isocenter in three directions at a gantry angle of 180°. CONCLUSIONS: The developed phantom can be applied as a new tool for establishing beam-positioning errors in single-isocenter stereotactic radiotherapy at off-isocenter positions.

Three-dimensional assessment of interfractional cervical and uterine motions using daily magnetic resonance images to determine margins and timing of replanning.

PURPOSE: This study was conducted to determine the margins and timing of replanning by assessing the daily interfractional cervical and uterine motions using magnetic resonance (MR) images. METHODS: Eleven patients with cervical cancer, who underwent intensity-modulated radiotherapy (IMRT) in 23-25 fractions, were considered in this study. The daily and reference MR images were converted into three-dimensional (3D) shape models. Patient-specific anisotropic margins were calculated from the proximal 95% of vertices located outside the surface of the reference model. Population-based margins were defined as the 90th percentile values of the patient-specific margins. The expanded volume of interest (expVOI) for the cervix and uterus was generated by expanding the reference model based on the population-based margin to calculate the coverage for daily deformable mesh models. For comparison, expVOIconv was generated using conventional margins: right (R), left (L), anterior (A), posterior (P), superior (S), and inferior (I) were (5, 5, 15, 15, 10, 10) and (10, 10, 20, 20, 15, 15) mm for the cervix and uterus, respectively. Subsequently, a replanning scenario was developed based on the cervical volume change. ExpVOIini and expVOIreplan were generated before and after replanning, respectively. RESULTS: Population-based margins were (R, L, A, P, S, I) of (7, 7, 11, 6, 11, 8) and (14, 13, 27, 19, 15, 21) mm for the cervix and uterus, respectively. The timing of replanning was found to be the 16th fraction, and the volume of expVOIreplan decreased by >30% compared to that of expVOIini . However, margins cannot be reduced to ensure equivalent coverage after replanning. CONCLUSION: We determined the margins and timing of replanning through detailed daily analysis. The margins of the cervix were smaller than conventional margins in some directions, while the margins of the uterus were larger in almost all directions. A margin equivalent to that at the initial planning was required for replanning.