## 1 Introduction  
As of 2018, lung cancer is the most commonly diagnosed cancer with circa 12% of tumor cases worldwide [1, 2]. Despite advances in therapy, the five-year survival rate is approximately 18% [3] and with 18.4% of the total, it is the leading cause of cancer-related death [1]. Solitary pulmonary nodules (SPNs) have a prevalence of 50% in lung cancer screening studies of high-risk smokers, which is becoming an increasing clinical problem [4, 5]. They are usually detected incidentally by low-dose helical computed tomography (CT) and, as general rule, with high probability of malignancy, such as solid and subsolid SPNs, evidence of growth, and size larger than 8 mm, the lesion should be biopsied or surgically excised for diagnosis [4, 6]. This is performed by video-assisted thoracoscopic surgery (VATS) when a nonsurgical biopsy is unavailable, not practical, or nondiagnostic. However, in the case of small, deep and solitary pulmonary nodules its localization is particularly challenging [7]. VATS falls in the group of minimally invasive procedures where the surgeon makes some small incisions in the skin and inserts several long and thin tubes into the patient body. At one end of the tubes there is either a miniaturized camera, called a laparoscope, or other laparoscopic instruments, such as dissecting forceps and scissors. A minimally invasive surgery has the advantage of a faster recovery and less postoperative pain because of the smaller incisions in the patient body. Although the position of the lung nodule was known before surgery, due to the lung collapse at thorax opening, a new localization is necessary. However mechanical palpation is not possible during laparoscopic surgery [4] and different methodologies are used. The cancer can be marked by CT-guided operations which involve the insertion of a percutaneous needle through the chest wall toward the nodule or in its close proximity. The percutaneous needle is either used to insert a guidewire with a metal hook at the end or to inject a small quantity of methylene blue. The risks associated with these procedures can be pneumothorax, hemothorax, or pulmonary hemorrhage due to the needle insertion. Or in the case of the methylene blue, uncontrolled spillage of the dye over the complete site making the subsequent localization of the tumor impossible [8]. Alternatively, intraoperative imaging with ultrasonography, fluoroscopy, or molecular imaging devices are used [4]. Nevertheless, air pockets remain in the alveoli and bronchioles even when the lung is in a deflated state producing artifacts in the ultrasound image [9; 10] and fluoroscopy and molecular imaging are unable to provide information of deep lesions [4]. Therefore an innovative methodology is sought to address the problem of localization of pulmonary lesions in operando conditions. On the other hand, for breast cancer, which is the leading case of cancer-related death for women [2], microwave imaging is widely used. This technique employs non-ionizing radiation in the frequency range between 300 MHz and 30 GHz and therefore can develop images of the internal tissues of the human body with low health risk. The imaging is based on the backscattering of the electromagnetic waves given by dissimilarities in dielectric properties of different tissues. These properties are tabulated for a large variety of human tissues [11; 12; 13; 14] and can be found online [15]. The backscattered signal (echo) is used to reconstruct the morphology of the medium through Confocal Microwave Imaging (CMI). The locations of any dielectric dissimilarity are inferred through synthetic focusing upon irradiation with an ultra-wideband (UWB) electromagnetic pulse from different physical positions. Confocal microwave imaging has been under development for two decades in the detection of breast cancer [16; 17; 18]. While only in recent years researchers moved toward lung cancer [19; 20].