A Retrospective Study of Renal Growth Changes after Proton Beam Therapy for Pediatric Malignant Tumor

The purpose of this study was to analyze renal late effects after proton beam therapy (PBT) for pediatric malignant tumors. A retrospective study was performed in 11 patients under 8 years of age who received PBT between 2013 and 2018. The kidney was exposed in irradiation of the primary lesion in all cases. Kidney volume and contour were measured on CT or MRI. Dose volume was calculated with a treatment-planning system. The median follow-up was 24 months (range, 11–57 months). In irradiated kidneys and control contralateral kidneys, the median volume changes were −5.63 (−20.54 to 7.20) and 5.23 (−2.01 to 16.73) mL/year; and the median % volume changes at 1 year were −8.55% (−47.52 to 15.51%) and 9.53% (−2.13 to 38.78%), respectively. The median relative volume change for irradiated kidneys at 1 year was −16.42% (−52.21 to −4.53%) relative to control kidneys. Kidneys irradiated with doses of 10, 20, 30, 40, and 50 GyE had volume reductions of 0.16%, 0.90%, 1.24%, 2.34%, and 8.2% per irradiated volume, respectively. The larger the irradiated volume, the greater the kidney volume was lost. Volume reduction was much greater in patients aged 4–7 years than in those aged 2–3 years. The results suggest that kidneys exposed to PBT in treatment of pediatric malignant tumor show continuous atrophy in follow-up. The degree of atrophy is increased with a higher radiation dose, greater irradiated volume, and older age. However, with growth and maturation, the contralateral kidney becomes progressively larger and is less affected by radiation.