Lasers in Urology

The workhorse lasers of urology are primarily CO2, argon ion, Nd:YAG, and dye lasers. CO2 lasers are best in precise cutting of tissue as already discussed in Sect. 3.2. Argon ion lasers and Nd:YAG lasers are used for the coagulation of highly vascularized tumors or malformations. Among these two lasers, the Nd:YAG laser is preferably applied for the coagulation of large tissue volumes because its radiation deeply penetrates into tissues. Moreover, Q-switched Nd:YAG lasers which interact in the photodisruptive mode have become a standard tool in lithotripsy beside ultrasound fragmentation. Dye lasers have not been investigated until recently in lithotripsy and in photo-dynamic therapy.

After the development of the first fiberoptic endoscope by Nath et al. (1973), Staehler et al. (1976) performed initial experimental studies with the argon ion laser in urology. Meanwhile, the indications for urologic laser treatments have significantly increased. They extend from the external genital, the lower urinary tract (urethra), the bladder, the upper urinary tract (ureter), all the way up to the kidneys as shown in Fig. 4.47. In addition, very promising results have already been achieved in treating benign hyperplasia of the prostate which embraces the urethra. Various laser therapies for all these different organs require specific strategies and parameters. They shall now be discussed in the above order.

Fig. 4.47. Scheme of male urinary tract

The most frequent malformations of the external genital are called condylomata acuminata. These benign warts must be treated as early as possible, because they tend to be very infectious and degenerating. After circumcision and application of 4 % acetic acid to suspected areas, they are coagulated with either Nd:YAG or CO2 lasers. Occasional recurrences cannot be excluded, especially in the treatment of intraurethral condylomata. With both laser types, however, the rate of recurrence is less than 10% as reported by Baggish (1980) and Rosemberg (1983). Hemangiomas of the external genital should be treated with radiation from Nd:YAG lasers because of its higher penetration depth. Hofstetter and Pensel (1993) stated that additional cooling of the tissue surface may even improve the procedure. Carcinoma of the external genital are best treated with a Nd:YAG laser if they are at an early stage. This significantly reduces the risk of having to perform a partial amputation. According to Eichler and Seiler (1991), powers of 40 W and focal spot sizes of 600 |im are usually applied. At an advanced stage, the tumor is first mechanically extirpated. Afterwards, the remaining tissue surface should be additionally coagulated.

Frequent diseases of the lower urinary tract are stenoses induced by either inflammation, tumor growth, or unknown origins. In these cases, urethrotomy by endoscopic control is usually performed as proposed by Sachse (1974). During this conventional technique, stenotic material is removed with a cold scalpel. Unfortunately, restenoses often occur due to scarring of the tissue. Further urethrotomies are not of great help, since they only enhance additional scarring. The first recanalizations of urethral stenoses with an argon ion laser were performed by Rothauge (1980). However, the results obtained were not as promising as initially expected. Then, no further progress was made until Wieland et al. (1993) recently published first results using a Ho:YAG laser. Meanwhile, follow-up periods of 20 months after Ho:YAG laser treatment were reported by Nicolai et al. (1995). They concluded that this technique is a considerable alternative to mechanical urethrotomy in virgin stenoses as well as restenoses. The probability for the occurrence of laser-induced restenoses is approximately 10 % only. In Figs. 4.48a-b, the effects of the Ho:YAG laser on the urethra and ureter are shown, respectively. In both samples, thirty pulses with an energy of 370 mJ and an approximate duration of 1 ms were applied.

Tumors of the bladder are very difficult to treat, since they tend to recur after therapy. It is yet unknown whether this is due to metastasation induced either prior to or by the treatment. Unfortunately, bladder tumors also easily break through the bladder wall. Thus, a treatment is successful only if it completely removes the tumor, does not perforate the bladder wall, and does not damage the adjacent intestine. Frank et al. (1982) have compared the effects of CO2, Nd:YAG, and argon ion lasers on bladder tissue. Among these, the Nd:YAG laser has proven to be best suited in coagulating bladder tumors. Argon ion lasers are applicable only in superficial bladder tumors.

Fig. 4.48. (a) Effect of thirty pulses from a Ho:YAG laser (pulse duration: 1ms, pulse energy: 370 mJ, bar: 250 |im) on the urethra. (b) Effect of thirty pulses from a Ho:YAG laser (pulse duration: 1ms, pulse energy: 370 mJ, bar: 250 |m) on the ureter. Photographs kindly provided by Dr. Nicolai (Regensburg)

Fig. 4.48. (a) Effect of thirty pulses from a Ho:YAG laser (pulse duration: 1ms, pulse energy: 370 mJ, bar: 250 |im) on the urethra. (b) Effect of thirty pulses from a Ho:YAG laser (pulse duration: 1ms, pulse energy: 370 mJ, bar: 250 |m) on the ureter. Photographs kindly provided by Dr. Nicolai (Regensburg)

According to Hofstetter et al. (1980), the rate of recurrence after laser treatment is approximately 1-5 %, whereas it ranges from 40-60 % if conventional transurethral resection (TUR) is performed. Even advanced tumors can be efficiently removed with Nd:YAG lasers, since the hemostatic treatment guarantees best vision. Pensel (1986) suggests the application of 30-40 W of laser power and a working distance between 1 mm and 2 mm. The tumor should be irradiated until it visibly pales. Afterwards, coagulated necrotic tissue is mechanically removed. For safety reasons, the remaining tissue surface should be coagulated, as well.

It was emphasized by Hofstetter and Pensel (1993) that tumors can still be graded and staged by biopsies after coagulation. Usually, control biopsies should be obtained within the next 3-6 months. The laser treatment itself is extremely safe, since perforations of the bladder wall are very unlikely, and the function of the bladder remains unaffected. All transurethral treatments are performed with a rigid cytoscope and a flexible fiber. In most cases, local anesthetization is sufficient.

Recently, photodynamic therapy (PDT) has gained increasing significance in the treatment of bladder tumors. First endoscopic applications of HpD have already been investigated by Kelly and Snell (1976). Several clinical reports on PDT are available, e.g. by Benson (1985), Nseyo et al. (1985), and Shumaker and Hetzel (1987). A complete treatment system including in vivo monitoring and dose control was decribed by Marynissen et al. (1989). A list of potential complications arising when using dihematoporphyrin ether was given by Harty et al. (1989). Today, photodynamic therapy is considered as a useful supplement to other techniques, since it enables the resection of tumors which are not visible otherwise. The ability of simultaneous diagnosis

- by means of laser-induced fluorescence - and the treatment of tumors is thus one of the key advantages of photodynamic therapy. So far, red dye lasers at 630 nm and energy densities between 10-50 J/cm2 are usually applied. In most cases, laser treatment is still restricted to superficial tumors due to the limited penetration depth at the specific wavelength. However, the recent discovery of novel photosensitizers like 5-aminolaevulinic acid (ALA)

- as already discussed in Sect. 3.1 - will certainly improve photodynamic therapy in urology during the next few years.

Lithotripsy of urinary calculi is often based on ultrasound techniques. However, not all calculi are equally indicated for such an external therapy. In particular, those calculi which are stuck inside the ureter are in an extremely inconvenient location. In these cases, laser-induced lithotripsy offers the advantage of directly applying energy to the vicinity of the calculus by means of a flexible fiber. First experiments regarding laser lithotripsy have already been performed by Mulvaney and Beck (1968) using a ruby laser. From today's perspective, though, it is quite obvious that these initial studies had to be restricted to basic research, since they were associated with severe thermal side effects. Watson et al. (1983) first proposed the application of a Q-

switched Nd:YAG laser. Shortly after, pulsed dye lasers were investigated by Watson et al. (1987). With the decrease in pulse durations, additional complications arose concerning induced damage of the fiber. Extensive calculations of the limits of fiber transmission were published by Hering (1987). The advantages of different approaches like bare fibers or focusing fiber tips were studied by Dorschel et al. (1987) and Hofmann and Hartung (1987). Furthermore, a review of 20 years of laser lithotripsy experience was given by Dretler (1988).

Today, dye lasers and Nd:YAG lasers are preferably used for lithotripsy of urinary calculi inside the ureter. A detailed description of the procedure was given by Hofstetter et al. (1986). Typically, pulse energies of 50-200 mJ and pulse durations between 10 ns and 1 |is are applied. The diameter of the optical fiber varies between 200 | m and 600 | m. With these parameters, optical breakdown is achieved close to the target. As described in Sect. 3.5, plasma formation at high pulse energies is associated with shock waves, cavitations, and jets. This photodisruptive interaction finally leads to the fragmentation of urinary calculi.

Since the 1980s, research in urology has increasingly focused on various treatments of the prostate. This very sensitive organ embraces the urethra. Diseases of the prostate, e.g. benign hyperplasias or carcinoma, thus often tend to handicap the discharge of urine. A profound analysis of the development of benign prostatic hyperplasia (BPH) was given by Berry et al. (1984). Several conventional therapies are available, e.g. the initial application of phytopharmaka or transurethral resection in severe cases. Other techniques such as cryotherapy or photodynamic therapy have also been investigated, e.g. by Bonney et al. (1982) and Camps et al. (1985). A complete list of potential treatment methods was provided by Mebust (1993). During the first few years, research was restricted to the treatment of prostatic carcinoma. Bowering et al. (1979) were the first to investigate the effect of Nd:YAG laser radiation on tumors of the prostate. Shortly after, several detailed reports followed, e.g. by Sander et al. (1982) and Beisland and Stranden (1984). The latter study pointed out the extreme importance of temperature monitoring of the adjacent rectum. Extensive clinical results were reported by McNicholas et al. (1988). Usually, indication for laser treatment is given only if the tumor cannot be completely resected otherwise.

At the beginning of the 1990s, the demand for minimally invasive techniques significantly increased. In the treatment of BPH, two milestones were achieved with the development of improved surgical techniques called transurethral ultrasound-guided laser-induced prostatectomy (TULIP) and laser-induced interstitial thermiotherapy (LITT). The idea of TULIP was proposed by Roth and Aretz (1991) and Johnson et al. (1992). Detailed clinical results were published by McCullough et al. (1993). The key element of TULIP is to position a 90° prism inside the urethra by ultrasound control. Thereby, the precision in aiming at the target is strongly enhanced.

In other studies, Siegel et al. (1991) have shown that hyperthermia alone, i.e. temperatures up to 45°C, is not sufficient in treating BPH. This has led to the idea of LITT as already described in Sect. 3.2. During LITT, the tissue is completely coagulated, i.e. temperatures above 60°C are obtained. The technical realization of suitable ITT fibers was discussed by Hessel and Frank (1990). In urology, initial experimental results with LITT were published by McNicholas et al. (1991) and Muschter et al. (1992). With typical laser powers of 1-5 W, coagulation volumes with diameters of up to 40 mm are achieved. Meanwhile, Muschter et al. (1994) have reported on clinical studies with approximately 200 patients. Roggan et al. (1994) have determined the optical parameters of prostatic tissue for diode lasers at 850 nm and Nd:YAG lasers at 1064 nm, respectively. Their data are found in Table 2.3. Moreover, they observed that the scattering coefficient of prostatic tissue increases during coagulation by an appoximate factor of two. With these data and appropriate computer simulations, they were able to optimize the parameters for an efficient procedure.

In Fig. 4.49, the most significant postoperative results of LITT in the treatment of BPH are summarized. According to Muschter et al. (1993), the peak urinary flow rate increased from 6.6ml/s to 15.2ml/s two months after treatment, whereas the residual urinary volume decreased from 206 ml to 38 ml. The mean weight of the prostate dropped from 63 g to 44 g during the same period. These data are based on mean values obtained from 15 patients. Severe complications were not observed. From these results, it can be concluded that LITT is an excellent therapy for BPH.

Fig. 4.49. Peak urinary flow and mean residual urinary volume after benign prostatic hyperplasia. The data represent mean values from 15 Data according to Muschter et al. (1993)

LITT of patients.

Fig. 4.49. Peak urinary flow and mean residual urinary volume after benign prostatic hyperplasia. The data represent mean values from 15 Data according to Muschter et al. (1993)

LITT of patients.

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