Plan quality effects of maximum monitor unit constraints in pencil beam scanning proton therapy for central nervous system and skull base tumors

Highlights: • The fluence of protons delivered to each spot is modulated in pencil beam scanning. • It has been hypothesized that heavy MU spot weighting may increase toxicity risk. • We evaluated the impact of MU spot weighting restrictions on treatment plan quality. • We found plan quality can be preserved while tightly restricting MU spot weighting. • Future work to correlate LET and RBE effects with spot weighting is critical. With reports of CNS toxicity in patients treated with proton therapy at doses lower than would be expected based on photon data, it has been proposed that heavy monitor unit (MU) weighting of pencil beam scanning (PBS) proton therapy spots may potentially increase the risk of toxicity. We evaluated the impact of maximum MU weighting per spot (maxMU/spot) restrictions on PBS plan quality, prior to implementing clinic-wide maxMU/spot restrictions.