The TUNA system requires the use of a specially designed cysto-scope that contains two independent energy delivery needles and a low-level RF generator. TUNA uses low-level RF waves (490 kHz) that allow a deeper and more uniform penetration than microwave thermo-therapy. The RF energy is delivered by two independent 22-mm needles placed at the end of the specially designed 22-Fr rigid cystoscope. The needles and protective sheaths advance and retract by controls on the catheter handle and are put into the prostate tissue at an acute angle to the catheter and at 40° to each other. The sheaths can be placed over the needles to protect the urethral lining. Thermocouples at the tip of each protective sheath and at the side of the catheter tip monitor the temperature at the intra-adenoma region and prostatic urothelium, respectively. The catheter tip can be rotated manually to direct the needles into either lateral lobe of the prostate. The RF energy can only be delivered to the tissues by direct contact, and its effects are dissipated quickly over a distance of approx 6 mm (9). The dimensions of the thermal lesion correspond to the geometry of the needles, the length of time of the energy delivery, the temperature core that is achieved, and the tissue impedance. Increased impedance such as that caused by tissue desiccation can result in tissue self-insulation and arrest thermal propagation.
The tissue effects of RF are also dramatically influenced by heat loss caused by convection (i.e., vascularity) (9). Thus, a balance must be struck: enough energy must be delivered to cause tissue necrosis, but not such a high level of energy that tissue charring is produced, leading to increased tissue impedance. In addition, the vascularity of the prostate adenoma can serve as a heat sink to draw thermal energy away from the target area (1).
Two major studies were performed to measure the feasibility of TUNA before its application in humans. Goldwasser et al. performed TUNA in dogs and demonstrated that 1-cm necrotic lesions could be created in the gland with no concurrent injury to the rectum, bladder base, or distal prostatic urethra (10). This success was replicated in studies performed by Ramon et al., who also created 1-cm necrotic lesions in ex vivo prostates (11).
Schulman et al. reported on the safety, tolerance, and efficacy of lesions created by TUNA in 25 patients undergoing radical prostatectomy (5). Patients were treated with TUNA anywhere from 1 d to 3 mo before undergoing radical retropubic prostatectomy. Macroscopic evaluation of the glands at the time of prostatectomy demonstrated the presence of defined necrotic lesions that were sharply demarcated from areas of untreated prostate (5). These investigators found hemorrhagic lesions in specimens recovered within 48 h of TUNA and discovered that necrotic lesions were maximal at 1 wk after treatment. Lesions ranged in size from 10 x 7 mm to 20 x 10 mm. Pathologic survey of the necrotic lesions by both visual inspection and immunohistochemical staining demonstrated destruction of all tissue components. Correlating the lesion size to the parameters recorded at the time of TUNA, these researchers discovered that when temperatures at the sheath (the lowest temperature of the lesion) were below 47°C, the size of the necrotic lesion was dramatically reduced (5). Rasor et al. performed mapping studies of the lesions produced by TUNA using an infrared temperature monitor and a specially designed TUNA catheter (12). They found that the lesion at the tip of the needle reached 90-100° C in an ex vivo animal model. In a study of patients undergoing TUNA before prostatectomy, temperatures of 50° C corresponded to core temperatures of 85-100°C within the target area. The extent of the lesion created by TUNA depends on the length of the needle that is used, the wattage used, the duration of treatment, the temperature achieved at the core of the lesion, and the temperature recorded at the tip of the protective sheath (12).
One of the appealing aspects of TUNA is the ability to spare the prostatic urothelium from injury at the time of therapy. Zlotta and colleagues performed neurohistochemical staining of prostate specimens treated with TUNA before radical prostatectomy (9). They found that the lesions were located 0.3-1 cm beneath the preserved urothelium, that nerve destruction was complete within the necrotic lesion, and that the normal nerve fibers were located typically within 0.5-1 cm of the urothelial lining (9). There is a higher density of innervation in the stromal portions of the prostate, under the capsule, adjacent to epithelial nodules, and just under the prostate urothelium. Sparing of these pain-sensitive areas adjacent to the prostatic urothelium and the rapid destruction of the nerve fibers within the necrotic lesions are the presumed major reasons why this procedure can usually be performed without the use of general or spinal anesthesia. Urethral sparing also may account for the low incidence of postprocedure voiding symptoms and the decreased risk of urinary retention following treatment (9).
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