SWL has been the primary treatment option for renal and upper urinary calculi since its introduction in the early 1980’s [20]. Two major international urological associations: the European association of urology and the American urological association both recommend SWL as the treatment of choice for small to intermediate renal stones. However, the treatment has certain limitations which may predict poor treatment efficacy, including steep and narrow infundibulum, long lower pole calyx, and shockwave-resistant stones [21, 22].
To maximize the efficacy and outcomes while minimizing complications of SWL, certain options regarding patient and stone characteristics have been proposed [16]. Obese patients and those with a higher body mass index (BMI) may have a larger skin to stone distance revealed during clinical examination, which could lead to unfavorable SWL success rates [12, 23]. Some studies highlighted the importance of anatomical location and the architecture of the stones as measured by CT Hounsfield units [8-11].
Other evidence showed that during SWL sessions, the energy and frequency emitted by the lithotripter notably contributed to stone disintegration. It is recommended that the treatment strategy starts from a low energy level and low frequency rate, and then gradually increases. Starting from a low energy level has been shown to pre-sensitize the kidney and cause renal vasoconstrictions [13, 24], thus reducing renal damage and improving patient tolerance. Low frequency shock wave rates have been proven to be associated with better stone fragmentation in many previous studies [14, 25-27]. Although there is no gold standard for optimal treatment frequency, it has been shown that frequency rates of 60 shocks per minute (1 Hz) and 90 shocks per minute (1.5 Hz) both had better outcomes compared with 120 shocks per minute (2 Hz) [14, 25-27]. Decreasing both the initial energy and shock wave frequency results in a reduced total number of shock waves and energy being delivered, thus reducing the possible damage to the kidney, and therefore decreasing the possibility of complications and improving patient comfort. The patient could be more easily adapted to this treatment, thus minimizing patient movement and the use of sedative agents. In this way, energy delivery could be more focused on the target and stone disruption could be improved. The training and experience of the technicians and operators performing the procedure also has an impact on the success of an SWL session, studies have shown that the stone free rate improves as the operator completes a learning curve [28, 29].
In the current study, the SWL session starts with a low energy (14KV) and low frequency setting (60Hz) in order to maximize the treatment effect. All of the SWL sessions were performed by the same two technicians who have over 10 years of experience. The authors consider this to be a strength of the current study. Using the same low-frequency, low energy machine setup and having SWL sessions performed by the same two experienced operators, the SWL efficiency could be maximized while minimizing the inter-operator bias and learning curve bias.
Last but not least, the impact of the patient’s stability during the SWL session must be considered. Patient movement and respiration during the SWL session can cause kidney movement [17], thus causing shock waves to be misdirected and focused away from the target. Excessive energy and inaccurate shockwaves may cause damage to the parenchyma and adjacent organs. Even when patients are under sedation or anesthesia during SWL, respiration related mistargeting remains inevitable. Constant tracking of the stone under a continuous fluoroscope could be the solution, however this may cause excessive ionizing radiation exposure. To overcome this problem, Chang et al. developed a real-time tracking ultrasound based system for renal stones [30]. In vitro and animal studies [31] showed the system to be efficient, and it could increase the accuracy of renal stone targeting and the efficiency of stone fragmentation.
The major advantage that ultrasound provides is real-time tracking of the stone without excessive radiation exposure to the patient. Although the dose of radiation during one SWL session may not be much and is not a worrisome dose compared to other invasive radiological procedures, the cumulative effect of ionizing radiation that needs to take into account includes the pre-treatment diagnostic imaging studies, such as CT or IVU, and possible repeat SWL treatment at a later date.
Chen et al. [19] reported on their clinical experience of renal stone treatment using an electromagnetic lithotripter integrated with an ultrasound-based real-time tracking system. Their results demonstrated increased accuracy of stone targeting with less shockwaves applied. Smith et al. [32] also compared the stone free rates between the USa localization technique and the traditional FS localization technique. The results revealed equivalent outcomes using the two different stone localization modalities but with the added benefit of no ionization exposure when using the USa technique. A randomized prospective study conducted by Van Besien et al. [33] demonstrated similar results, with the USa SWL stone-free rate not being inferior to the FS SWL, but with no need for ionizing radiation. This is probably most beneficial in pediatric patients, as children are 2-7 times more radiation-sensitive than adults [34]. In a study evaluating the outcomes and radiation exposure of children with cystine stones, Goren et al. [35] demonstrated that USa SWL was more effective and applied less ionizing radiation doses for pediatric patients. Also, in faintly radiopaque stones, such as cystine stones, USa-guided SWL has better visualization of the stones than a fluoroscope.
In the current study, the USa group had a larger baseline stone size. Despite this, the USa group had a significantly better stone-free rate, stone-disintegration rate and a lower retreatment rate; this was regardless of stone size stratification. This result indicates better stone localization and targeting with the assistance of real-time ultrasound, which causes better stone fragmentation and secondarily leads to better stone clearance and lower retreatment rates. As for safety, the USa group had a significantly lower complication rate for smaller stones, and a lower complication rate for larger stones, although this did not reach statistical significance. These results further indicate the theoretical benefit of ultrasound tracking, that is, better accuracy leads to less surrounding tissue damage, which would be more significant in smaller targets.
Previously reported stone-free rate of ESWL ranged around 47 to 92% [2-7]. In our study, the main reason of our relative lower stone-free rates is because that stone-free status was more strictly defined. Stone-free status was defined as the absence of radiographic residual stones in the follow-up imaging, any residual fragments visible on KUB was considered as non-stone-free, even the fragments were less than 2mm or 4mm, which was the most commonly used criteria in clinical practice and other studies.
Previous studies had certain limitations, such as small patient numbers or a lack of head to head comparisons [19, 32]. Despite its randomized prospective study design, Van Besein’s study failed to show statistical significance between the different localization methods, this may be related to the small number of studied patients. A strength of the current study compared to others is that we have a large number of study patients, two similar energy source lithotripters using the same SWL protocol, and all procedures were performed by the same two experienced technicians, which provides strong head-to-head comparison while minimizing bias between different institutions and operators.
However, there were several limitations to the present study that need to be addressed. First, its retrospective study design limited the bias control, such as stone composition and anatomic parameters. Second, radiation exposure was not recorded besides self-reported preference by the two technicians. Third, CT was not routinely in every patient to assess factors that affect ESWL results, such as CT values, stone-to-skin distance and BMI. Fourth, we compared stone-free rates with two different stone localization method on two electromagnetic lithotripters. The parameters of different machines may also affect the efficacy of stone disintegration. Lastly, SWL and localization techniques are very operator-dependent. The current study was conducted in one single center and only two technicians operated the machine, thus minimizing inter-operator and institutional bias. Further studies could be conducted in a larger set of patients who are prospectively randomized.