Nephron-sparing surgery has become the standard of care for patients with small (<4.0 cm) renal masses (SRM). For young, healthy patients with a low surgical risk, open or laparoscopic partial nephrectomy (OPN/LPN) is preferred. Renal tumor ablative techniques, however, are relatively new developments with increasing application. Such techniques were initially indicated in patients with multiple renal tumors, a solitary kidney, or signiﬁcant comorbidities that precluded higher risk surgery. In contemporary practice, however, sufﬁcient evidence indicates that ablation may be a reasonable treatment option for all patients with SRMs. The advantages of renal ablation over LPN include less blood loss, shorter hospitalization time, decreased post-operative pain, and a lower complication rate.
LAPAROSCOPIC CRYOABLATION: RETROPERITONEAL APPROACH
At present, the clinically viable ablation technologies include cryoablation and radiofrequency ablation. Either can be performed using laparoscopic or percutaneous technique. Both involve placement of probe needles directly into the renal mass.
In cryoablation, the cryoprobe needles are cooled to very low temperatures, which induces tissue necrosis. At present, such cooling is achieved by delivering pressurized argon gas to the tips of the cryoprobes. As argon gas passes through the restricted tips of the probes and then expands, it undergoes rapid cooling (a phenome-non known as the Joule-Thomson effect) and forms an iceball over the tumor. The temperature at the probe tip becomes as low as -140° to -190° C, whereas the temperature at the edge of the ice ball is just below 0° C. Since the temperature required for cell destruction is between -20° and -40° C, the efﬁcacy of the ablation process declines in a gradient radiating from the tips of the probes toward the edges of the iceball. Therefore, the iceball must involve a margin of normal tissue to ensure complete tumor destruction. Following the freeze cycle, an active thaw phase is initiated, and then a second freeze-thaw cycle is performed to further increase cell death.
In radiofrequency ablation (RFA), the probes transfer high-frequency electrical current to the target tissue, which results in the production of thermal energy. Temperatures in excess of 60° C cause tissue destruction through coagulative necrosis and thermally induced vascular thrombosis.
High-intensity focused ultrasound (HIFU) is an experimental extracorporeal procedure in which focused ultrasound waves pass through the skin and are converted to heat energy at a selected target.
Contemporary HIFU technologies, however, have not demonstrated adequate oncologic efﬁcacy and are thus not yet part of standard clinical practice.
Laparoscopic Technique. In a laparoscopic ablation, the tumor can be directly visualized, and the ablation process can be monitored in real time.
The tumor can be accessed from either a transperitoneal or retroperitoneal approach, depending on its location. In the transperitoneal approach, the colon is mobilized medially to expose the renal fascia, which is mobilized from its attachments to surrounding structures, such as the liver or spleen. Next, the renal fascia is entered over the area of the renal mass, which is targeted using preoperative imaging and intraoperative ultra-sound. The perirenal fat is then carefully removed from the renal capsule to expose the entire surface of the tumor.
Once the tumor has been adequately visualized, multiple core biopsies are acquired using a percutaneous biopsy device. Intraoperative ultrasound is then performed to further characterize the tumor’s depth and vascularity.
Finally, ablation probes are inserted through the skin and into the tumor under direct vision. The number of ablation probes that should be deployed depends both on the tumor’s characteristics and the technology being applied. The probes should enter the tumor at a right angle, and laparoscopic ultrasound should be performed to ensure the probe tips are beyond the internal margin of the tumor. Once proper positioning has been achieved, the probes are activated.
During cryoablation, laparoscopic ultrasound can be used to monitor the iceball as it forms, ensuring that it completely engulfs the mass and a 1-cm rim of normal parenchyma. Two freeze-thaw cycles are performed, and then the probes are removed. To minimize the chance of bleeding, probe extraction should not be attempted until the probes are loose enough to freely twist within the tumor.
During RFA, the ablation process cannot be visualized in real time. Instead, the RF ablation process proceeds using temperature- or impedance-based algorithms that are device-speciﬁc. Alternatively, some clinicians deploy temperature probes at selected sites around the tumor to monitor the ablation process.
After the ablation process is completed, the lesion is monitored for hemorrhage, and minor bleeding is controlled using topical hemostatic agents and gentle pressure. After hemostasis has been conﬁrmed at reduced pneumoperitoneum, the trocars are removed.
Percutaneous Technique. Percutaneous ablation offers numerous advantages over a laparoscopic procedure, including avoidance of general anesthesia, reduced complication rate, diminished postoperative pain, and expedited convalescence. The major disadvantages, however, include the lack of direct visualization during the ablation process, as well as the inability to assess for immediate postablation bleeding.
Percutaneous ablation may be performed in a CT or magnetic resonance imaging (MRI) suite. The patient is placed under conscious sedation and positioned prone. A semipermeable targeting template is positioned over the ipsilateral ﬂank, and imaging is performed to correlate the template with the renal anatomy. The targeting template is marked to indicate the location where the needles should be inserted, and then the template is removed so that the mark is visible on the patient’s skin. The site is sterilized and draped in standard fashion. An access sheath is then deployed at the
marked site, and its position in the kidney is conﬁrmed and readjusted if necessary using imaging. Several tumor biopsies are acquired through the access sheath. The ﬁrst probe is then placed through the access sheath. Subsequent probes are placed directly through the skin, with additional imaging performed to conﬁrm proper positioning. The ablation is then performed. Once completed, a ﬁnal image series is acquired using a half dose of intravenous contrast to conﬁrm successful tumor ablation.
Both cryoablation and RFA tend to have lower complication rates than OPN/LPN. Nonetheless, they carry a risk of some major complications, including bleeding from the tumor, injured intraabdominal vessels, or skin; pain at the trocar or probe sites; urinary tract infection; intraabdominal abscess; ileus; injury to adjacent organs; and tumor persistence or recurrence after treatment.