Using collimation can sharpen the penumbra towards surrounding normal tissue in the low energy region of the proton beam, this study from Japan thus examined how much the dose to the normal tissue was reduced when IMPT with PBS was performed using a multi-leaf collimator (MLC) for patients with maxillary sinus cancer. 26 patients' plans were compared based on D2% and the mean dose to the posterior retina, lacrimal gland, eyeball, and parotid gland were compared between with MLC and without MLC.

With MLC, this study found that D2% for the ipsilateral optic nerve was significantly reduced by 0.48 Gy, and the mean dose for the ipsilateral optic nerve was significantly reduced by 1.04 Gy, and the mean dose to the optic chiasm was significantly reduced by 0.70 Gy. The dose to most OARs and the planning at risk volumes were also reduced. This study concluded that the use of an MLC during IMPT for maxillary sinus cancer may be useful for preserving vision and preventing complications.