Feasibility of magnetic resonance-guided high-intensity focused ultrasound treatment targeting distinct nodular lesions in neurofibromatosis type 1

Tissue specific considerations in implementing high intensity focussed ultrasound under magnetic resonance imaging guidance

High-intensity focused ultrasound can ablate a target permanently, leaving tissues through which it passes thermally unaffected. When delivered under magnetic resonance (MR) imaging guidance, the change in tissue relaxivity on heating is used to monitor the temperatures achieved. Different tissue types in the pre-focal beam path result in energy loss defined by their individual attenuation coefficients. Furthermore, at interfaces with different acoustic impedances the beam will be both reflected and refracted, changing the position of the focus. For complex interfaces this effect is exacerbated. Moreover, blood vessels proximal to the focal region can dissipate heat, altering the expected region of damage. In the target volume, the temperature distribution depends on the thermal conductivity (or diffusivity) of the tissue and its heat capacity. These are different for vascular tissues, water and fat containing tissues and bone. Therefore, documenting the characteristics of the pre-focal and target tissues is critical for effective delivery of HIFU. MR imaging provides excellent anatomic detail and characterization of soft tissue components. It is an ideal modality for real-time planning and monitoring of HIFU ablation, and provides non-invasive temperature maps. Clinical applications involve soft-tissue (abdomino-pelvic applications) or bone (brain applications) pre-focally and at the target (soft-tissue tumors and bone metastases respectively). This article addresses the technical difficulties of delivering HIFU effectively when vascular tissues, densely cellular tissues, fat or bone are traversed pre-focally, and the clinical applications that target these tissues. The strengths and limitations of MR techniques used for monitoring ablation in these tissues are also discussed.

Surgical Treatment and Complications of Deep-Seated Nodular Plexiform Neurofibromas Associated with Neurofibromatosis Type 1

Background: Nodular plexiform neurofibromas in individuals with neurofibromatosis type 1 often cause significant symptoms and are treated with surgical excision despite the potential risk of complications. This study aimed to clarify the surgical outcomes of deep-seated nodular plexiform neurofibromas and identify the factors associated with postoperative complications. Methods: We retrospectively reviewed patients with neurofibromatosis type 1 who underwent surgical excision for deep-seated nodular plexiform neurofibromas in our hospital from 2015 to 2021. Enucleation while preserving the nerve fascicles was attempted first, and en bloc resection, ligating the nerve origin in cases in which the parent nerve was entrapped by the tumor, making the tumor difficult to dissect, was performed. Results: In 15 patients, 24 nodular plexiform neurofibromas received surgical excision. Sixteen tumors were enucleated, and eight were en bloc resected. The symptoms of all 10 patients with preoperative symptoms resolved after surgery. Four patients developed new neurological deficits immediately after surgery, two of whom had retained neurological symptoms at the last visit, but these symptoms were mild. Conclusions: The present study demonstrates that surgical treatment of nodular plexiform neurofibromas, even deep-seated neurofibromas, is safe with a low risk of severe complications and improvement in preoperative symptoms.