Nivolumab: Patients will receive 240 mg nivolumab (dissolved in 250 mL NaCl 0.9%) administered intravenously over 30 minutes. The first dose will take place one or two days before the IRE treatment. The second and third doses will be given 2 and 4 weeks post-IRE. Bloodwork will be done just prior to each treatment.

IRE is performed in the CT scanner. Patients will receive a general anesthetic, and an ultrasound will be performed to locate the designated metastasis. Ultrasound-guided electrode placement (3 or 4 depending on cancer size, shape, and location) will be performed by interventional radiologist Dr. Chris Wall and the IRE device will be activated and used as per the technique of Martin et al.

Patients will receive 240 mg nivolumab as above and also receive 8 mg of CpG-ODN dissolved in 1 mL normal saline administered peritumorally just before the IRE treatment. Three or four electrodes will be placed using a combination of ultrasound and CT guidance in preparation for treatment. Then using the electrodes as landmarks, injection of ¼ or 1/3 cc of the CpG-ODN solution will be performed near each of the 3 or 4 electrodes to achieve a peritumoral administration of the drug.

IRE is performed in the CT scanner. Patients will receive a general anesthetic and an ultrasound will be performed to locate the designated metastasis. Ultrasound-guided electrode placement (3 or 4 depending on cancer size, shape, and location) will be performed by interventional radiologist Dr. Chris Wall and the IRE device will be activated and used as per the technique of Martin et al.

Nivolumab: Patients will receive 240 mg nivolumab (dissolved in 250 mL NaCl 0.9%) administered intravenously over 30 minutes. The first dose will take place one or two days before the IRE treatment. The second and third doses will be given 2 and 4 weeks post-IRE. Bloodwork will be done just prior to each treatment.

Complications (Clavien-Dindo classification of complications) at 30 days. The Clavien-Dindo Classification of Complications involves assigning a numbered classification to each complication, from 1-5, with 1 being a complication without need for any intervention required to 5 which is death of the patient. (a higher number indicates a higher severity). This is a validated measure in use for over 25 years.

Complications (Clavien-Dindo classification of complications) at 90 days. The Clavien-Dindo Classification of Complications involves assigning a numbered classification to each complication, from 1-5, with 1 being a complication without need for any intervention required to 5 which is death of the patient. (a higher number indicates a higher severity). This is a validated measure in use for over 25 years.

Two radiologists (one diagnostic and the other interventional) will examine the scans of all patients. They will select a colorectal metastasis from the scan just before the intervention that is between 2-3.2 cm in maximal diameter, accessible to percutaneous ablation, and treatable according to the interventional radiologist's opinion. Another metastasis, with a maximal diameter of less than 4 cm, will be selected to monitor for the abscopal effect (reduction in size of a colorectal liver metastasis that was not treated with IRE).

After 3 months, the radiologists will review the imaging again. They will assess the treatment mass area for any contrast uptake indicating incomplete ablation. The second metastasis will be measured in the same dimensions as before treatment. The difference in maximal diameter of the second metastasis will be calculated as (diameter before IRE - diameter after IRE) / diameter before IRE and expressed as a percentage.

Distribution of Carcinoembryonic (CEA)-specific CD8+ T cells, CD4+ T cells, type-1 and 2 macrophages (M1 and M2), regulatory T (Treg) cells on the flow-cytometry plot.