Objective : With the growing use of medical imaging the incidence of thyroid cancer is increasing, now ranking as the 9th most common cancer worldwide. However, the death rate from thyroid cancer remains low, with the increased incidence mostly driven by small nodules that had previously gone undiagnosed. Given this growing incidence of small thyroid cancers, there is a need for the development of new treatment options, especially non-invasive techniques. Robotically Assisted Sonic Therapy (RAST) is an ablation modality based on histotripsy, a non-invasive, non-thermal method of focused ultrasound tissue destruction. The purpose of this study was to evaluate the feasibility of creating a precise ablation zone within a porcine thyroid model using RAST.
Methods : Four female swine underwent eight RAST thyroid model ablations. With the animals in the supine position, an ablation was targeted in the thymus and thyroid tissue within the superficial neck of the animal. The choice to include thymus in addition to thyroid tissue was done due to the small size of the porcine thyroid. In addition, the porcine thymus is located in an anatomically similar position in comparison to the human thyroid. The combination of thymus and thyroid tissue allowed for a target of comparable size and depth in relation to the human thyroid. RAST was performed with a custom therapy transducer (VortxRx Gen. 1, Histosonics, Inc.). Two ablations were performed in each animal, one with a prescribed 1.0 x 1.0 x 1.0 cm spherical ablation zone (prescribed volume=0.5 cm3) and the other a 1.0 x 1.0 x 2.0 cm ovoid ablation zone (prescribed volume=1.1 cm3), targeted within the thymus/thyroid tissue. Post procedural magnetic resonance imaging (MRI) was obtained immediately following the procedures and ablations reviewed for size and appearance. The animals were sacrificed immediately following imaging, and the ablation zones were removed at necropsy for histologic analysis.
Results : All procedures were well-tolerated, with the creation of circumscribed ablation zones similar to the prescribed volume. The 1.0 x 1.0 x 1.0 cm ablation zones had a mean volume of 0.68 cm3 (SD +/-0.11), and measured 0.95 (0.10), 0.95 (0.06) and 1.1 (0.00) in AP, transverse, and CC dimensions. While the 1.0 x 1.0 x 2.0 cm ablation zones had a mean volume of 1.20 cm3 (SD +/-0.40), and measured 1.13 (0.13), 1.23 (0.10) and 2.0 (0.16) in AP, transverse, and CC dimensions. Mean depth of the superficial aspect of the ablation zones was 1.9 cm (1.3-2.2cm). On MRI, there was central non-enhancement consistent with necrosis. There were areas of T1 hyperintensity within the ablation due to blood products. Ablation zones were fairly well demarcated from adjacent tissue, with expected surrounding perfusional changes. Within two of the eight ablations there was mild edema and enhancement within the overlying musculature consistent with muscle injury. There was no injury to overlying subcutaneous fat or skin.
Conclusions : RAST is capable of creating precise ablations within a porcine thyroid model, without damage to the overlying skin or subcutaneous fat, and mild overlying muscle injury in two of the eight ablations.