newer and more flexible delivery method is pencil beam scanning, using a beam that sweeps laterally over the target so that it delivers the needed dose while closely conforming to the tumor's shape. This conformal delivery is achieved by shaping the dose through magnetic scanning of thin beamlets of protons without needing apertures and compensators. Multiple beams are delivered from different directions, and magnets in the treatment nozzle steer the proton beam to conform to the target volume layer as the dose is painted layer by layer. This type of scanning delivery provides greater flexibility and control, letting the proton dose conform more precisely to the shape of the tumor. Delivery of protons via pencil beam scanning, in use since 1996 at the Paul Scherrer Institute, allows for the most precise type of proton delivery: intensity-modulated proton therapy (IMPT). IMPT is to proton therapy what IMRT is to conventional photon therapy—treatment that more closely conforms to the tumor while avoiding surrounding structures. Virtually all new proton systems now provide pencil beam scanning exclusively. A study led by Memorial Sloan Kettering Cancer Center suggests that IMPT can improve local control when compared to passive scattering for patients with nasal cavity and paranasal sinus malignancies. Application It was estimated that by the end of 2019, a total of ~200,000 patients had been treated with proton therapy. Physicians use protons to treat conditions in two broad categories: Disease sites that respond well to higher doses of radiation, i.e., dose escalation. Dose escalation has sometimes shown a higher probability of "cure" (i.e. local control) than conventional radiotherapy. These include, among others, uveal melanoma (ocular tumor), skull base and paraspinal tumor (chondrosarcoma and chordoma), and unresectable sarcoma. In all these cases proton therapy gives significant improvement in the probability of local control, over conventional radiotherapy. For eye tumors, proton therapy also has high rates of maintaining the natural eye. Treatment where proton therapy's increased precision reduces unwanted side effects by lessening the dose to normal tissue. In these cases, the tumor dose is the same as in conventional therapy, so there is no expectation of increased probability of curing the disease. Instead, emphasis is on reducing the dose to normal tissue, thus reducing unwanted effects. Two prominent examples are pediatric neoplasms (such as medulloblastoma) and prostate cancer. Pediatric Irreversible long-term side effects of conventional radiation therapy for pediatric cancers are well documented and include growth