Effect of Focal Zone Size on Treatment Outcomes and Renal Injury Following Extracorporeal Shockwave Lithotripsy of Renal Calculi: A Prospective Randomized Study

Background The narrower focal zone (FZ) size of modern lithotripter was considered as one of the factors that resulted in suboptimal treatment result of shockwave lithotripsy (SWL). Therefore, we would like to investigate the efficacy and safety of standard narrow or extended (FZ) sizes in SWL for patients with renal stones. Material and Methods: In this prospective study conducted between April 2018 and October 2022, patients with renal stones were randomized to receive SWL with either standard or extended FZ. Treatment was delivered using a Modulith SLX-F2 lithotripter with a maximum of 3000 shocks at 1.5 Hz. The primary outcome was treatment success 12 weeks after a single SWL session, defined as the lack of a stone or stone fragment <4 mm on computed tomography. Secondary outcomes included the incidence of perinephric hematoma, stone-free rate and changes in the urinary levels of acute renal injury markers. Results Three hundred twenty patients were recruited, and 276 patients were randomized into the two groups. The two groups had similar baseline parameters. The treatment success rate was significantly better for standard FZ (74.3%) than the extended FZ group (59.3%) (p = 0.009). Standard FZ also had a significantly better stone-free rate (Grade-A, 36.8% vs 23.0%, p = 0.013) and less pain after treatment. Both groups had similar perinephric haematoma formation rates, unplanned hospital admission rates, and changes in urinary acute renal injury markers. Conclusions: The standard narrow focal zone has better treatment efficacy and similar safety compared to the extended focal zone during SWL for renal stones.