This service evaluation suggests that there is no difference in overall treatment success rates between wide focus-SWL and narrow focus-SWL (treatment success 74% WF-SWL, 66% NF-SWL p = 0.20). A recent randomised control trial comparing narrow (2 mm) and wide (8 mm) focus SWL-size reported comparable stone-free rates one month after three sessions of SWL (86.6% NF-SWL vs. 86.8% WF-SWL) [13]. Energy usage per SWL treatment was comparable between wide and narrow focus groups suggesting that any improved stone fragmentation by WF-SWL is attributable to focus size rather than greater energy delivery. Experimental studies and computer modelling have shown that when the size of SWL-focus exceeds that of a stone, shockwaves travel along the outer stone surface, generating shear stress and driving fragmentation, which may account for our findings [5, 14]. Furthermore, a wider field of focus would increase the chances of shockwaves hitting a calculus in situations where there is movement. Respiratory effort is thought to account for 7.7 mm (+/- 2.9 mm) and 3.6 mm (+/- 2.1 mm) movement of renal and ureteric stones, respectively [15] and Sorensen et al. describe that up to 40% of lithotripsy shockwaves miss the targeted stone [16].

Treatment guidelines for renal and ureteric stones include SWL and endoscopic options stratified according to stone size [11, 17, 18]. While retrograde ureteroscopic and percutaneous approaches to renal and ureteric stones are available, rates of major complications in our analysis were lower than those reported in the literature for these methods [19]. No differences in rates of complications up to 31 days post-SWL were detected between wide and narrow focus patient groups. Urinary calculi may contain bacteria within their matrix; various studies have isolated bacteria from 15 to 70% of stones, with the ubiquity and organisms varying by stone composition [20, 21]. For calculi harbouring bacteria within their matrix, destruction could prompt their release into the urine, potentially increasing the risk of urinary tract infection (UTI). Our results suggest that although rates of UTI and antibiotic use were numerically higher with WF-SWL, statistical significance was not reached in this small cohort (UTI; WF-SWL = 10, NF-SWL = 2, p = 0.29; antibiotic prescribing; WF-SWL = 11, NF-SWL = 4, p = 0.46).

In contrast to previous findings describing greater pain associated with NF-SWL use [13], we identified no differences in requirement for analgesia between the treatment groups (WF-SWL = 26, NF-SWL = 12, p = 0.64).

In clinical practice, choice of focus size may be influenced by source-to-target distance, with shorter distances favouring narrow focus fields [22]. Rising levels of adiposity worldwide [23] would increasingly favour the use of wide focus settings. A subgroup analysis comparing efficacy of wide and narrow focus shockwaves in groups of patients categorised by BMI, waist circumference, or skin-to-stone distance could offer more insight.

Our findings are limited by the retrospective nature of analyses and paucity of data on key factors known to affect SWL outcomes, such as stone density, composition, and skin-to-stone distance. Including these factors may provide a more accurate assessment of the impact of focus size on stone clearance. Furthermore, pairs were also not matched for gender, age, or stone laterality [24]. We recognise that the endourological community has not reached consensus regarding the definition of clinically insignificant fragments and that some urology units would not deem the persistence of fragments < 4 mm as “treatment success”. Future studies of adequate power should randomise patients, stratified by stone location, to wide and narrow SWL treatment and prospectively collect data including energy delivery, number of shocks required, number of treatments required, image-defined stone clearance, adverse events, and requirement for requirement for further treatment after SWL to establish the relative efficacy of wide and narrow focus SWL approaches.