Commissioning of the Varian universal interstitial cylinder system for HDR brachytherapy of gynecological cancer

Purpose

This paper outlines the commissioning of the Varian (VMS, Varian Medical Systems, Palo Alto, CA) Universal Interstitial Cylinder (UIC) applicator set for Ir-192 HDR brachytherapy. The UIC was commissioned for use with CT and MRI and a custom phantom was designed to avoid the introduction of water-like materials into the needle guide tracks. Various marker strands were investigated to determine which allowed the most accurate reconstruction of source positions.

Methods

Planar kV and MV imaging, along with physical measurements and autoradiographs, were used to commission the physical dimensions of all components of the UIC applicator set. CT and MR imaging was used to further commission one configuration of the UIC with UCP and eight interstitial needles in a simulated clinical setup using a GYN phantom. Three different methods of channel identification were compared – no radio opaque markers, VMS numbered markers, or nylon coated stainless steel leader wires – to see which best aided in channel identification and image registration. An HDR MRI Lumen marker (C4 Imaging, LLC) was used to verify any applicator rotation on MR scans during image registration. Three types of GYN phantoms were investigated – wet towel, gelatin, and ground beef. Dimensions of all components were compared with vendor provided information, including the solid applicator models, which are based on the computer-aided design model files of the specific applicators.

Results

The dimensions of the applicators could be validated using physical measurements, kV and MV planar imaging, and CT scans. The ground beef based GYN phantom best eliminated the introduction of water into the needle guide tracks that was found when using a water or gel-based phantom. CT scans using no radio opaque markers did not allow the plastic needles to be visualized well enough to digitize source positions. CT scans with VMS markers showed significant artifact. CT scans with the nylon coated stainless steel wires provided the best visibility of the needle locations to aid in digitizing source positions. The use of an MR marker allowed the channel to be identified on the MR scan and confirm rotation for image registration.

Conclusions

The UIC set and applicator configuration was commissioned for CT and MR based treatment planning. The plastic components of the UIC applicator set pose challenges to the commissioning process but the use of radio opaque markers seen on CT combined with MR image registration allow the source positions within the needles, as well as the location of the end of the needles, to be digitized appropriately. A ground beef phantom minimized the fluid introduced into the needle guide track, minimizing any unintended MR and CT signal in the needle guide tracks.