RO-20: A PLANNING APPROACH FOR LENS SPARING PROTON CRANIOSPINAL IRRADIATION IN A PEDIATRIC PATIENT

INTRODUCTION: PT is increasingly regarded as the gold standard for CSI particularly in pediatric patients. Nevertheless, lens sparing with good coverage of the cribriform plate remains a challenge. The following lens sparing CSI technique used at our Center is described. METHODS AND MATERIALS: CSI is delivered by active scanning PT with three isocenters, using three cranial beams plus two additional postero-anterior spinal beams. Cranial and caudal field junctions are planned by the ancillary-beam technique (1). The three-beams arrangement for brain irradiation includes two lateral opposed beams (gantry angle 90° and 270°), with couch angle ±15° to minimize the overlap between the cribriform plate and the lens, and an additional posterior beam. Single-field-optimization of the three equally-weighted beams is performed. During optimization, coverage of the cribriform plate is assumed as the priority goal and lens sparing as a secondary objective. RESULTS/CONCLUSIONS: The lens-sparing technique markedly decreased the dose to the lenses (Dmax 16.7%, i.e. around 6Gy) while maintaining adequate target coverage. Dose values are smaller than those reported in other studies. In our case, the opposed-lateral setup is associated to larger lens doses (56.6%) than those reported using the same technique in another study (26.4%), suggesting that our specific case was a difficult one, presumably age-related. In fact, the lens dose increased significantly with decreasing age (2). This low maximal dose to the lenses might reduce the risk of radiation-associated cataract. 1) Farace et al, Acta Oncol 2015; 2) Cochran et al, Int J Radiat Oncol Biol Phys 2008;