Computed Surgical Preoperative Planning for Tumors in the Internal Auditory Canal

Surgical access to the internal auditory canal (IAC) for vestibular schwannoma (VS) resection with the aim of hearing preservation can be achieved through either a middle cranial fossa (MCF) or retrosigmoid (RS) approach. Indications for a specific surgical approach are based on patient anatomy, review of preoperative imaging, results of audiometric testing, as well as surgeon and patient preference. Objective methods for evaluating surgical approach are not well described. We aim to build a computer model of the patient specific anatomy and tumor to objectively score and compare surgical approaches based on traversed structures within the surgical corridor accessing IAC VS for hearing preservation surgery.

Patients who underwent hearing preservation surgery for VS primarily located in the IAC with preoperative MRI were queried from a surgical database from January 1, 2018 to March 1, 2019. Data collected included surgical approach, preoperative imaging, and surgical outcome. The preoperative MRI was segmented semiautomatically using 3D Slicer to create a 3D segmentation mask, including tumor, vasculature, bone, nervous system, and inner ear structures. Each voxel was assigned a morbidity score based on clinical experience by a group of skull base surgeons, and surgical paths were scored based on the voxels contained within the path. For each patient, standard MCF and RS craniotomy boundaries were drawn onto the segmentation mask. A morbidity value was computed for both the MCF and RS approach using a weighted sum of the lowest cost surgical path between each extrema point on the tumor and a defined set of entry points within the boundaries of the craniotomy. The final output of the algorithm was a scalar total morbidity value for each approach. A lower morbidity value corresponded to more favorable surgical access.

Seven patients met inclusion criteria, two underwent RS approach and five underwent MCF approach. All patients had serviceable hearing preoperatively (pure tone average ≤50 dB and word recognition score ≥50%). The median tumor volume was 1,936.5 mm³ (8.8 SD) for the RS patients; and 171.2 mm³ (120.7 SD) for the MCF patients.

For patients who underwent MCF craniotomy, the computed morbidity value of the surgical approach for the MCF approach was lower than the RS approach in all five patients. For patients who underwent RS craniotomy, the computed morbidity value for surgical approach was lower for the RS approach than the MCF approach in one out of two patients.

In a retrospective case series of seven patients who underwent hearing preservation surgery for IAC VS, the patient specific surgical approach derived by this objective analysis corresponded to the actual surgery performed in six of seven patients. Further research is needed to assess the utility of quantitative, objective methods for surgical planning as part of clinical decision-making for selecting the optimal surgical approach for hearing preservation surgery in vestibular schwannomas of the internal auditory canal. Computational surgical planning may also be expanded to more complex tumors of the lateral skull base.

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Publication History

Article published online:
12 February 2021