Computer-assisted surgical planning and navigation in maxillo-mandibular tumor resection

Introduction

Surgical treatment of maxillo-mandibular tumors aims for in-sano resection. Besides information about dignity (benign/malignant) and entity of the tumor provided via specimen taking and histopathological examination, the extent of the tumor is crucial for surgical treatment planning. Tumor extent is typically based on the clinical picture and radiological imaging. While localized and sharply circumscribed tumors can be resected relying on the clinical picture alone, more advanced tumor disease challenges the surgeon: the urge for a more extensive resection can be in conflict with the maintenance of important anatomical structures.

Computer-assisted surgical planning (CaSP) can help extract all the relevant information from radiological data sets by displaying the complex three-dimensional (3D) anatomy of the head and neck region and visualizing the tumor extent. In localizations where the radiological imaging is not certain, further pathohistological examination might be needed to elucidate the true tumor extent via tumor mapping. A trajectory-based surgical specimen taking using intraoperative navigation tools allows a minimally invasive approach combined with a precise localization of the uncertain anatomical areas marked in the CaSP. The pathological examination results, which are linked to the coordinates of the navigated specimen taking, can be transferred to the CaSP, thus enhancing the information content even further.

Segmentation of the tumor extent can be performed on the basis of these enhanced data sets. Adding a sufficient 3D resection margin depending on the tumor entity, while respecting critical anatomical structures, results in the display of the needed resection “wall” for both the soft and hard tissue. Regarding the bony part of the resection, cutting guides can be virtually designed and fabricated via rapid prototyping to transport the information of the virtually defined resection border into the theater. In cases of primary reconstruction, vector-based drilling guides for indexing the fixation holes for the patient-specific reconstruction plates can be added to these guides. Also, bony flap raising can be facilitated by designing and manufacturing harvesting guides, which help with the contouring and fixation of the transplant.

Compared to conventional “free-hand” resection, CaSP provides some advantages in facilitating the surgery (see Table 22.1 ). As long as high-quality 3D radiological data sets can be obtained, there is no medical contraindication for the use of CaSP in tumor resection. However, these techniques do not reflect the gold standard of pathohistology, which limits their use regarding the aim of curative oncological surgery: pathohistological R0-resection.

Clinical considerations

In the treatment of head and neck cancer a comprehensive staging is mandatory. This involves a complete history of other diseases, especially other entities of cancer, previous cancer treatments, or side diseases that limit the cancer treatment options, such as advanced cardiovascular or neurological disease. Clinical examination involves the inspection and palpation of the primary tumor formation, exclusion of second tumor formation, and palpation and ultrasound of the neck looking for lymph node metastasis. For completion of the staging with regard to primary tumor extent and cervical lymph node metastasis, further 3D radiological imaging of the head and neck region, from the skull base to the upper thoracic aperture, is needed. In cases of advanced disease a scan of the thorax and abdomen is recommended to rule out distant metastasis.

When it comes to CaSP, radiological scanning is key, as the whole procedure is based on these examinations. Therefore, a high-quality computed tomography (CT) scan following a standardized protocol is recommended, fulfilling certain criteria for tumor resection planning:

• Complete coverage of the relevant regions.

• Removal of all foreign bodies (dental prostheses, jewelry, face masks) to reduce artifacts.

• In case navigation is required, ensure correct prescan insertion of navigation markers.

• Reduction of movement artifacts in patients with neurological disorders, anxiety diseases, mental disabilities, dementia and children by use of sedation/narcosis.

• Biphasic contrast medium administration protocol (split bolus).

• Reconstruction of thin slices with a thickness of 1 mm or less.

• Means of reduction of metal artifacts (e.g., using single-energy-based metal artifact reduction).

A high-quality CT scan is the prerequisite for a high-resolution 3D surface model of the facial skull, which serves as the basis for the design of resection guides. Enhanced by contrast medium, this offers a good enough soft tissue contrast for differentiating tumor tissue. In cases of severe artifacts due to metallic dental restorations overshadowing the region of the tumor formation, information content is limited and has to be completed by the clinical picture. In case of reduced soft tissue contrast, a magnetic resonance imaging (MRI) scan should be obtained, which can be merged with the CT scan to combine the information content of both modalities.

In cases of planned navigated procedures, reference markers are needed either as transcutaneous bone-fixed screws (e.g., mandibulo-maxillary fixation [MMF] screws made of steel or titanium) or removable dental splints with osteosynthesis screws attached to the splint. If these reference markers have not been in place during the initial scan, another cone-beam CT scan with inserted reference markers is sufficient and can be merged with the initial scan. If navigation is needed for the region of the mandible, a fixed reproducible position of the lower jaw in relation to the upper jaw and skull base is obligatory. This can be achieved by fixation in occlusion (habitual if well defined or with a bite splint) or, if an open mouth is needed during navigation, inserting a patient-specific bite wedge during the CT scan and during surgery.

In cases of primary bony reconstruction, a CT scan of the donor site is needed, which has to match similar criteria mentioned above. In particular, a reconstruction of thin slices with a thickness of 1 mm or less is needed for well-fitting harvesting guides. Moreover, contrasting in the arterial phase gives insight into potential peripheral artery disease which might expel transplant harvesting due to insufficient vascular supply of the donor region.

In summary, close cooperation with the radiologist is recommended to prevent unnecessary repetition of examinations and thus minimize radiation exposure, avoid unnecessary delay of treatments, save costs and time, and avoid staff overload and patient frustration.

Workflow of computer-assisted design/computer-assisted manufacturing

Please see Fig. 22.1 and ppt 22.1.