Feasibility of Remotely Controlled Capsule to Enable Directionality and Higher Dose Rates for Conventional Low Dose Rate Brachytherapy Using Topas
Authors: Nguyen Junia , Lee Hoseon , Kollar Zsolt , Hesketh Peter J. , Yu David 
Published: August 2024

With low dose radiation (LDR) brachytherapy, thus far, only low dose rates could be used with low energy level radiation sources at 30 keV or less due to the risk of radiation to healthy cells, tissue, and organs. The radiation seeds are placed in the location of the tumor and is left in the body for the total dose to be continuously administered throughout the decay period of the radiation seed. However, the amount of time to deliver the total dose can take up to several months depending on the type of radiation source, tumor size and location, due to the low dose rate of <2 Gy/hr. In contrast, high dose radiation (HDR) is defined as a dose delivered at a rate >12 Gy/hr, although usually much higher, often in excess of 1 Gy/min, with a higher energy radiation source close to 0.5 MeV.
For the first time, a remotely controllable, implanted radiation capsule is introduced, to enable a very high dose rate of 10 Gy/min for 30 keV I-125 radiation source. Until now, this combination of low energy and high dose rate for implanted radiation seeds for LDR was not possible due to the risk of radiation. The implantable capsule is designed to control this risk of radiation by remotely controlling the opening of the capsule hole. In the open case, the high dose rate of I-125 is delivered for a duration of 1 minute and then closed to stop emission.
For feasibility, a simulation study was conducted using TOPAS for Monte Carlo simulation of radiotherapy. In the top right figures, a cylindrical capsule of height 6 mm, diameter 3 mm, and wall thickness 0.9 mm is designed using gold to contain the radiation and prevent leakage radiation. An opening of 30 degrees was made in the middle of the capsule wall where the radiation was to be emitted. For the radiation source, a 30 keV I-125 was used with a specific activity of 6.42 x 1014 Bq/gram. With a wall thickness of 0.9 mm, calculations showed that 4.4 micrograms of I-125 was needed to increase the source activity for 1.454 x 1020 Bq per second for a high dose rate of 10 Gy/min.
The yellow and red colors in the capsule are both gold with wall thickness of 0.9 mm. The red portion of the cylinder is where the 30 degree opening was defined. The green lines show the paths of the radiation particles.
TOPAS simulation results successfully show that the capsule contains the radiation and the only direction of radiation is through the defined opening in the capsule. The bottom three graphs show the x, y, and z plane cuts showing a directional radiation beam that drops in intensity, from a range of 1.0 to 0, over distance and slightly widens over distance.
To control the opening and closing of the hole in the radiation capsule, an inner cylinder and outer cylinder is used, where the inner cylinder can move up and down. A permanent magnet is attached to the bottom of the inner cylinder and an electromagnet is attached to the base at the bottom of the capsule. An implantable inductive coil receives an AC current from the external inductive coil, which is rectified using a diode and capacitor circuit. This DC current powers the electromagnet, which repels the inner cylinder, moving it up to align the windows of the inner and outer cylinders. This alignment opens the hole and radiation is emitted towards the target tumor. After the dose is delivered, the current is turned off, which turns off the electromagnet. With no repelling force from the electromagnet, the permanent magnet reattaches itself to the base, and the inner cylinder slides down and the windows are no longer aligned. This blocks the radiation from being emitted. The dimensions of the capsule and the windows depend on the radiation source and tumor size. Micromachining and micro 3D printing techniques can be applied for fabrication of the device.
In conclusion, a feasibility study using TOPAS demonstrated that a 0.9 mm gold wall with a hole can contain a very high source activity of I-125 radiation for a very high dose rate of 10 Gy/min and emit a directional beam through the hole. A remotely controllable capsule can enable a high dose rate with LDR brachytherapy that combines the advantages of LDR and HDR, which is minimum radiation risk and minimum treatment time, simultaneously. Potential applications for this is intracavitary cancer treatment such as esophageal or cervical cancer.

Figure 1