PO17: Assessment of a Prototype Gynecologic US/MR/CT Phantom for Utilization in Interstitial Brachytherapy Training

Abstract

<h3>Purpose</h3> Simulation-based education has been previously demonstrated to be efficacious for teaching intracavitary (IC) brachytherapy. In principle, the success of this "hands-on" approach could be extended to interstitial (IS) and/or hybrid IC/IS brachytherapy. Yet, this education technique has not been adopted and one reason may be the shortcomings of current simulation models. Highly-desirable features which were not available in current simulation models included: non-stylized realistic anatomical representation, compatibility with image-guidance (ultrasound, CT, and/or MRI), practical material for needle placement, and commercial availability. Our work presents results from our evaluation of the physical, radiographic, and imaging characteristics of a new commercial prototype phantom designed for simulation-based education. <h3>Materials and Methods</h3> The SIM (Simulated Inanimate Models, Pittsford, NY) gynecological trainer prototype phantom was constructed based on a representative female pelvis CT imageset to provide physically realistic anatomy within a block of tissue-like material. The original CT imageset was truncated to a region of interest with dimensions of 160 × 160 × 220 mm. To provide a planning imageset, the phantom was imaged using CT with 1 mm slice reconstruction and using an axial T2 MRI sequence with 1.5 mm slice reconstruction. We assessed the anatomic dimensions of the model, physical compatibility with different hybrid applicators, and the quality of the radiographic model on imaging. After the implant was approved, a planning CT was taken, an HR-CTV was drawn, and the plan quality was evaluated. After clinical assessment, feedback was provided to the manufacturer for consideration in future designs. <h3>Results</h3> The phantom consisted of a 15 mm introitus which preceded a cylindrical air cavity with length of 75 mm and average diameter of 20 mm with irregular ridges intended to represent a vaginal canal which was capped by a pliable opening of 3 mm intended to represent an intact cervical os. Near the cervical region, the cylindrical air cavity widened from 20 to 28 mm. Proximal to the cervical os, there was a conical air cavity 80 mm in length which represented an intrauterine canal. In addition to the air cavities and tissue, the phantom was composed of three other distinct materials designed to represent bones, bladder, and rectum - each of which was distinguishable on US-, CT- and MR-based imaging. Within the phantom, an experienced physician was able to place a tandem-and-ovoid (TO) applicator with a 30 degree curved tandem, but was restricted to ovoid sizes ≤ 20 mm. To supplement the TO, the physician was able to place three IS needles to produce a realistic hybrid IC/IS implant intended to cover a hypothetical 35 × 35 × 40 mm³ HR-CTV. Coronal and sagittal views of the phantom post-implant planning CT imageset with <i>in situ</i> hybrid IC/IS equipment are shown in Figure 1. Figure 1 also contains the final plan with HR-CTV D_90% = 102% with the purple HR-CTV contour and red 100% isodose line. The primary clinical feedback provided to the manufacturer on this prototype phantom included: enlarging the vaginal canal dimensions to physically accommodate larger IC applicators; adding a visual/palpable cervical tumor to provide a target and recreate the needle-placement decision-making process. <h3>Conclusions</h3> We report the basic characteristics of a novel gynecologic phantom prototype which intended for simulation-based training. This phantom provided an anatomically-realistic "hand-on" tool composed of various materials which were each distinguishable on three modalities: US, CT, MR. Clinical feedback was provided to the manufacturer for consideration in the next generation of this phantom.