Patient Set-up in Radiotherapy with Video-based Positioning System

A comparison of patient position displacements from body surface laser scanning and cone beam CT bone registrations for radiotherapy of pelvic targets

BACKGROUND AND PURPOSE

Optical surface detection has attractive features as a mean in radiotherapy for patient positioning tasks such as set-up, monitoring and gating. To aid in hitting radiotherapy targets the correlation between detected surface displacements and internal structure displacements is crucial. In this study, we compare set-up displacements derived from a body surface laser scanning (BSLS) system to displacements derived from bone registrations with a cone beam computed tomography (CBCT) system in order to quantify the accuracy and applicability of BSLS for fractionated treatments in the pelvic region.

MATERIAL AND METHODS

Displacements from concurrent BSLS and CBCT registrations were compared for 40 patients treated in the pelvic region for a total of 170 set-ups. Surface data captured by BSLS at the first treatment fraction (BSLSref) was used as main reference for the BSLS system, while bony structures from the planning CT were used as a reference for the CBCT method. As comparison, the patient outline extracted from the planning CT was used as BSLS reference (CTref). The displacements detected by the CBCT system (skin-marks-only) was also used for comparison.

RESULTS

The mean differences (± 1 SD) between the BSLS and CBCT displacements were -0.01 (± 0.17) cm, 0.00 (± 0.21) cm and 0.01 (± 0.17) cm in the lateral, longitudinal and vertical directions, respectively. The median length of the difference was 0.26 cm (0.24-0.29 cm, 95% CI). The median of the difference between CBCT and BSLS displacements based on CTref was 0.37 cm (0.30-0.39 cm) and the median for skin-marks-only was 0.38 cm (0.34-0.42 cm).

CONCLUSIONS

The BSLS system is a good supplement to the CBCT system for accurate set-up for fractions when no CBCT is deemed necessary for pelvic targets. Inter-fractional skin movement in relation to bone was estimated to be 0.2 cm in the lateral (X), longitudinal (Y) and vertical direction (Z), respectively.

3-D model-based multiple-object video tracking for treatment room supervision

We present a method to monitor a patient and the equipment in a radiotherapy treatment room, by exploiting the information in the treatment plan, enriched with other elements such as visual, geometric, and "semantic" information. Using all these information items, and a generic model, a virtual environment of the scene is created, with maximum precision. The images resulting from video sequences with several cameras are also used to confront the filmed information on the scene and its numerical representation. The method is based on the features of the scene elements, and on a fuzzy formalism. The feasibility of the method is being quantitatively evaluated in the absence of treatment, to be further exploited in a module for external control by video in real conditions.

Benefits of Six Degrees of Freedom for Optically Driven Patient Set-up Correction in SBRT

To quantify the advantages of a 6 degrees of freedom (dof) versus the conventional 3- or 4-dof correction modality for stereotactic body radiation therapy (SBRT) treatments. Eighty-five patients were fitted with 5–7 infra-red passive markers for optical localization. Data, acquired during the treatment, were analyzed retrospectively to simulate and evaluate the best approach for correcting patient misalignments. After the implementation of each correction, the new position of the target (tumor's center of mass) was estimated by means of a dedicated stereotactic algorithm. The Euclidean distance between the corrected and the planned location of target point was calculated and compared to the initial mismatching. Initial and after correction median±quartile displacements affecting external control points were 3.74±2.55 mm (initial), 2.45±0.91 mm (3-dof), 2.37±0.95 mm (4-dof), and 2.03±1.47 mm (6-dof). The benefit of a six-parameter adjustment was particularly evident when evaluating the results relative to the target position before and after the re-alignment. In this context, the Euclidean distance between the planned and the current target point turned to 0.82±1.12mm (median±quartile values) after the roto-translation versus the initial displacement of 2.98±2.32mm. No statistical improvements were found after 3- and 4-dof correction (2.73±1.22 mm and 2.60±1.31 mm, respectively). Angular errors were 0.09±0.93° (mean±std). Pitch rotation in abdomen site showed the most relevant deviation, being − 0.46±1.27° with a peak value of 5.46°. Translational misalignments were −0.68±2.60 mm (mean±std) with the maximum value of 12 mm along the cranio-caudal direction. We conclude that positioning system platforms featuring 6-dof are preferred for high precision radiation therapy. Data are in line with previous results relative to other sites and represent a relevant record in the framework of SBRT.