Difficulties with recruiting participants for the study due to the COVID-19 pandemic.

In a non-anatomical resection of a liver tumor, only the part of the liver with the tumor and a safety margin of 5 - 10 mm are resected. This is done to ensure a negative resection margin, which means that no tumor cells are at the boundary of the resection. These non-anatomical resections can be performed repeatedly in case of recurrence. However, compared to anatomical resections, it is more challenging to keep a negative resection margin as anatomical landmarks cannot be used for intra-operative guidance.

In this study, the investigators aim to clinically evaluate a 3d navigation system, where navigated intra-operative ultrasound data is used to create a virtual model and a surgical plan.

Surgical resection is the current gold standard for curative care of primary and metastatic hepatic tumors. This procedure involves the removal of the part of the liver where the tumor is located. This is typically achieved by removing the segments containing the tumor, so called anatomical resections. The downside of this technique is that it also removes a large part of healthy liver tissue. Recently, non-anatomical resections are becoming more popular, as they spare more healthy liver tissue than anatomical resections with similar oncological outcomes. In a non-anatomical resection, only the part of the liver with the tumor and a safety margin of 5 - 10 mm are resected. This is done to ensure a negative resection margin, which means that no tumor cells are at the boundary of the resection. These non-anatomical resections can be performed repeatedly in case of recurrence. However, compared to anatomical resections, it is more challenging to keep a negative resection margin as anatomical landmarks cannot be used for intra-operative guidance.

In the beginning of a non-anatomical resection, a resection line is drawn onto the liver surface to visualize where the resection shall be started. During the resection process, intra-operative ultrasound is used to confirm a safe distance to the tumor. Finally, once the depth is reached, the distance to the tumor is again confirmed on ultrasound and the tumor is removed. This is a challenging process which depends on the operator's ability and experience with mentally reconstructing the spatial relationships of the ultrasound image and the intra-operative scene. Additionally, the resection margin introduces artifacts and makes it harder to visualize the safety distance to the tumor on ultrasound.

To overcome these challenges, image-guidance systems have been introduced into the surgical workflow. These systems measure the pose of the surgical instruments and display their position on a virtual model of the anatomy. They mainly rely on a registration process to align a preoperative model with the patient's anatomy intraoperatively. This process is time-consuming, complex and error prone which is the main reason why such systems are rarely used.

In this study, the investigators aim to clinically evaluate a different approach, where navigated intra-operative ultrasound data is used to create a virtual model and a surgical plan on the spot. This does not require a separate registration process. With this approach a virtual draft of the surgical plan is created, which serves as a rough guidance map through the procedure. The investigators hypothesize that using such an intra-operative surgical draft allows the surgeon to acquire a negative resection margin.

Inclusion Criteria:

• Patients which are regularly scheduled for an open surgical liver resection
• At least one tumor considered for non-anatomical resection
• Lesion is visible on ultrasound imaging
• Informed Consent as documented by signature (Appendix Informed Consent Form)
• Age >= 18 years

Exclusion Criteria:

• Other clinically condition or disease that would (as deemed by the operating surgeon) significantly increase the risk of surgery
• Lesion is close to major vessel (< 10 mm)
• Lesion is too large to be visualized on ultrasound imaging
• Emergency
• Subjects not able to give informed consent (dementia)
• Women of childbearing potential (less than 1 year post-menopausal)