disorders, neurocognitive toxicity, ototoxicity with subsequent effects on learning and language development, and renal, endocrine and gonadal dysfunctions. Radiation-induced secondary malignancy is another very serious adverse effect that has been reported. As there is minimal exit dose when using proton radiation therapy, dose to surrounding normal tissues can be significantly limited, reducing the acute toxicity which positively impacts the risk for these long-term side effects. Cancers requiring craniospinal irradiation, for example, benefit from the absence of exit dose with proton therapy: dose to the heart, mediastinum, bowel, bladder and other tissues anterior to the vertebrae is eliminated, hence a reduction of acute thoracic, gastrointestinal and bladder side effects. Eye tumor Proton therapy for eye tumors is a special case since this treatment requires only relatively low energy protons (~70 MeV). Owing to this low energy, some particle therapy centers only treat eye tumors. Proton, or more generally, hadron therapy of tissue close to the eye affords sophisticated methods to assess the alignment of the eye that can vary significantly from other patient position verification approaches in image guided particle therapy. Position verification and correction must ensure that the radiation spares sensitive tissue like the optic nerve to preserve the patient's vision. For ocular tumors, selecting the type of radiotherapy depends on tumor location and extent, tumor radioresistance (calculating the dose needed to eliminate the tumor), and the therapy's potential toxic side effects on nearby critical structures. For example, proton therapy is an option for retinoblastoma and intraocular melanoma. The advantage of a proton beam is that it has the potential to effectively treat the tumor while sparing sensitive structures of the eye. Given its effectiveness, proton therapy has been described as the "gold standard" treatment for ocular melanoma. Base of skull cancer When receiving radiation for skull base tumors, side effects of the radiation can include pituitary hormone dysfunction and visual field deficit—after radiation for pituitary tumors—as well as cranial neuropathy (nerve damage), radiation-induced osteosarcoma (bone cancer), and osteoradionecrosis, which occurs when radiation causes part of the bone in the jaw or skull base to die. Proton therapy has been very effective for people with base of skull tumors. Unlike conventional photon radiation, protons do not penetrate beyond the tumor. Proton therapy lowers the risk of treatment-related side effects from when healthy tissue gets radiation. Clinical studies have found proton therapy to be effective for skull base tumors. Head and