First publication on a variable energy proton Arc method: a robust and delivery-efficient spot-scanning proton arc therapy technique (aka SPArc). The aim was to present a new optimisation algorithm for proton Arc, and to probe if delivery time of SPArc can be reduced compared to a step-and-shoot multi-field delivery. Other potential advantages of this new modality are investigated. The authors present planning data comparing the standard IMPT, Arc-Multi-Field and SPArc for two separate clinical cases. SPArc seems to deliver better dose distribution compared to IMPT in terms of maximum and integral dose to healthy organs and dose conformity. The SPArc treatment delivery time is significantly reduced compared to the step-and-shoot technique.