Feasibility study of patient-specific four-dimensional in vivo tracking system for high-dose-rate brachytherapy: Experimental evaluation

Background

High-dose-rate (HDR) brachytherapy using Iridium-192 as a radiation source is widely employed in cancer treatment to deliver concentrated radiation doses while minimizing normal tissue exposure. In this treatment, the precision with which the sealed radioisotope source is delivered significantly impacts clinical outcomes.

Purpose

This study aims to evaluate the feasibility of a new four-dimensional (4D) in vivo source tracking and treatment verification system for HDR brachytherapy using a patient-specific approach.

Methods

A hardware system was developed for the experiments, featuring a high-resolution compact gamma camera with a redesigned diverging collimator, enhanced detector, and precision control system. The collimator was redesigned to improve spatial resolution by reducing the hole size and increasing the hole array, while reducing the pixel size of the detector and increasing the number of pixels. The performance was evaluated using Monte Carlo simulations, which demonstrated significant improvements in spatial resolution. Experiments were conducted in a controlled setup using a phantom to simulate clinical conditions. The phantom was positioned at various distances from the gamma camera (327.30, 377.30, and 427.30 mm) and imaged at multiple angles. The accuracy of the system was tested in four different cases: three with fixed distances and one employing a multi-focusing method. The multi-focusing method allows the gamma camera to adjust its focus based on the anatomical characteristics of individual patients, thereby enhancing source-tracking accuracy. The performance of the system was evaluated under these four different scenarios. The Euclidean distance and three-dimensional gamma analysis were used to evaluate tracking accuracy and dose distribution.

Results

The redesigned collimator demonstrated significant improvements (compared to the previous design) in the spatial resolution of the gamma camera, showing 34.21% and 23.46% enhancements in the horizontal and vertical profiles, respectively. These improvements in gamma camera resolution are crucial for enhancing the tracking system's accuracy. The experimental results demonstrated varying degrees of accuracy across different cases, reflecting the performance of the system under different conditions. The average Euclidean distance errors were Case 1 (327.30 mm): 1.358 mm; Case 2 (377.30 mm): 1.731 mm; Case 3 (427.30 mm): 1.973 mm; and Case 4 (multi-focusing): 1.527 mm. The gamma pass rates for the four cases were:- Case 1: 86.39%; Case 2: 75.28%; Case 3: 72.22%; and Case 4: 81.67% (1 mm/1%). For the 2 mm/2% criterion, the gamma pass rates were 97.11, 94.72, 92.38, and 96.78% for Cases 1, 2, 3, and 4, respectively. Case 4 (multi-focusing) showed an improvement over Case 3, with a 22.6% reduction in the average Euclidean distance error and a 13.1% increase in the gamma pass rate (1 mm/1%).

Conclusion

These results demonstrate that the new 4D in vivo source tracking and treatment verification system for HDR brachytherapy is feasible and has potential clinical benefits.