A Simulation Study of a Radiofrequency Localization System for Tracking Patient Motion in Radiotherapy.

One of the most widely used tools in cancer treatment is external beam radiotherapy. However, the major risk involved in radiotherapy is excess radiation dose to healthy tissue, exacerbated by patient motion. Here, we present a simulation study of a potential radiofrequency (RF) localization system designed to track intrafraction motion (target motion during the radiation treatment). This system includes skin-wearable RF beacons and an external tracking system. We develop an analytical model for direction of arrival measurement with radio frequencies (GHz range) for use in a localization estimate. We use a Monte Carlo simulation to investigate the relationship between a localization estimate and angular resolution of sensors (signal receivers) in a simulated room. The results indicate that the external sensor needs an angular resolution of about 0.03 degrees to achieve millimeter-level localization accuracy in a treatment room. This fundamental study of a novel RF localization system offers the groundwork to design a radiotherapy-compatible patient positioning system for active motion compensation.

Inter-fractional variation of markers and applicators in single-implant high-dose-rate interstitial brachytherapy for gynecologic malignancies.

PURPOSE: Implanted fiducial markers are a commonly used tool in delineating the CTV in high-dose-rate interstitial brachytherapy (HDR-ISBT) for gynecologic malignancy, but their reliability in gynacological sites is not well understood. These markers and interstitial applicators can experience interfractional motion due to organ swelling or other anatomical changes. The purpose of this study was to evaluate the spatial variation of these features. METHODS AND MATERIALS: The spatial positions of 50 implanted markers and 202 needles were tracked in 15 patients treated over 70 fractions of HDR brachytherapy. Marker and/or needle coordinates were extracted from CT images with contours and dose distributions. Automated analysis determined marker self-consistency and displacements between various elements of the implant. RESULTS: From start to end fraction, the relative positions of the markers experienced an average magnitude displacement of 4.5 ± 3.0 mm while the average displacement of the applicator tips was 11 ± 8 mm, relative to their respective centers of mass (CM). CONCLUSIONS: Markers implanted lateral and superior to the CTV experience greater drift than other implant locations.

Respiratory cycle characterization and optimization of amplitude-based gating parameters for prone and supine lung cancer patients.

The selection of posture between supine and prone induces changes in the characteristics of respiratory patterns in lung cancer patients. We characterize these differences, as well as introduce two new metrics to describe the quality of amplitude-based gating. The stability of the following metrics were measured for 134 supine-and-prone-paired individual breathing sessions from 22 patients: amplitude, period, inhale-to-exhale period ratio, and location of end-of-exhale and end-of-inhale peaks. A new normalization characteristic of typical amplitude was introduced for comparing patients based on external surrogates. This metric was used to characterize the baseline drift and to compare the overall gating efficiency between different amplitude-based gating parameters in a new proposed metric called the gating efficiency index. While the choice of supine or prone posture had negligible impact on the overall duty cycle or gating efficiency, some metrics showed greater difference, especially with prone showing reduced variability in period, inhale-to-exhale period ratio, amplitude, and relative amplitude of end-of-exhale. Therefore, the breathing pattern resulting from prone positioning was found to be more favorable due to less intrafraction variation. The gating efficiency index was used to quantitatively show that narrow amplitude gating windows near end-of-exhale have the best balance of decreased motion variability within gating while maintaining the longest duty cycle.

Technical Note: The design, construction, and evaluation of a liquid-based single phantom solution for TG128 brachytherapy ultrasound QA.

PURPOSE: Since the publication of the AAPM TG128 report for the quality assurance (QA) of prostate brachytherapy ultrasound systems, no commercially available phantoms have been developed which satisfy all of the task group recommendations. Current solid phantoms require a separate user-implemented setup using a container with liquid medium to evaluate the alignment between the needle template and the electronic grid, a test of geometric accuracy with critical implications in dosimetric quality. Utilizing a 3D printer, we constructed a cost-effective, liquid-based phantom that provides a complete TG128 solution which improves the efficiency of brachytherapy ultrasound QA. METHODS: The TG128 report was used to guide the design process of the liquid-based phantom. The needle template and electronic grid alignment setup served as the foundation with specific components developed to integrate all remaining tests. Water was chosen as the liquid medium, with speed of sound adjusted to 1,540 m/s via salinity per the task group recommendations. The proof of concept was evaluated by comparing the time stamps labeled on QA images between the liquid-based phantom and a commercially available one for both a new and experienced user. RESULTS: A TG128 QA trial run demonstrated that all recommended tests can be completed with the single phantom setup. Evaluation of the time data revealed a total QA duration of 45 min (average of two trials) with the liquid-based phantom, compared to 70 and 90 min with the commercial phantom for a new and experienced user. CONCLUSIONS: The liquid-based phantom is specifically designed to satisfy the recommendations of the TG128 report. The incorporation of 3D printing allows simple design modifications to adapt the phantom on-the-fly if needed. The resulting product improves the efficiency of brachytherapy ultrasound QA by eliminating the need for multiple phantom setups.

Radiotherapy-Compatible Robotic System for Multi-Landmark Positioning in Head and Neck Cancer Treatments.

The spine flexibility creates one of the most significant challenges to proper positioning in radiation therapy of head and neck cancers. Even though existing immobilization techniques can reduce the positioning uncertainty, residual errors (2-3 mm along the cervical spine) cannot be mitigated by single translation-based approaches. Here, we introduce a fully radiotherapy-compatible electro-mechanical robotic system, capable of positioning a patient's head with submillimeter accuracy in clinically acceptable spatial constraints. Key mechanical components, designed by finite element analysis, are fabricated with 3D printing and a cyclic loading test of the printed materials captures a great mechanical robustness. Measured attenuation of most printed components is lower than analytic estimations and radiographic imaging shows no visible artifacts, implying full radio-compatibility. The new system evaluates the positioning accuracy with an anthropomorphic skeletal phantom and optical tracking system, which shows a minimal residual error (0.7 ± 0.3 mm). This device also offers an accurate assessment of the post correction error of aligning individual regions when the head and body are individually positioned. Collectively, the radiotherapy-compatible robotic system enables multi-landmark setup to align the head and body independently and accurately for radiation treatment, which will significantly reduce the need for large margins in the lower neck.

An electromechanical, patient positioning system for head and neck radiotherapy.

In cancer treatment with radiation, accurate patient setup is critical for proper dose delivery. Improper arrangement can lead to disease recurrence, permanent organ damage, or lack of disease control. While current immobilization equipment often helps for patient positioning, manual adjustment is required, involving iterative, time-consuming steps. Here, we present an electromechanical robotic system for improving patient setup in radiotherapy, specifically targeting head and neck cancer. This positioning system offers six degrees of freedom for a variety of applications in radiation oncology. An analytical calculation of inverse kinematics serves as fundamental criteria to design the system. Computational mechanical modeling and experimental study of radiotherapy compatibility and x-ray-based imaging demonstrates the device feasibility and reliability to be used in radiotherapy. An absolute positioning accuracy test in a clinical treatment room supports the clinical feasibility of the system.