via the scattering method (historically, the most common) or a spot scanning method. Spot scanning can adjust the width of the SOBP on a spot-by-spot basis, which reduces the volume of normal (healthy) tissue inside the high dose region. Also, spot scanning allows for intensity modulated proton therapy (IMPT), which determines individual spot intensities using an optimization algorithm that lets the user balance the competing goals of irradiating tumors while sparing normal tissue. Spot scanning availability depends on the machine and the institution. Spot scanning is more commonly known as pencil-beam scanning and is available on IBA, Hitachi, Mevion (known as HYPERSCAN which became US FDA approved in 2017) and Varian. Surgery Physicians base the decision to use surgery or proton therapy (or any radiation therapy) on tumor type, stage, and location. Sometimes surgery is superior (such as cutaneous melanoma), sometimes radiation is superior (such as skull base chondrosarcoma), and sometimes are comparable (for example, prostate cancer). Sometimes, they are used together (e.g., rectal cancer or early stage breast cancer). The benefit of external beam proton radiation is in the dosimetric difference from external beam X-ray radiation and brachytherapy in cases where use of radiation therapy is already indicated, rather than as a direct competition with surgery. In prostate cancer, the most common indication for proton beam therapy, no clinical study directly comparing proton therapy to surgery, brachytherapy, or other treatments has shown any clinical benefit for proton beam therapy. Indeed, the largest study to date showed that IMRT compared with proton therapy was associated with less gastrointestinal morbidity. Side effects and risks Proton therapy is a type of external beam radiotherapy, and shares risks and side effects of other forms of radiation therapy. The dose outside of the treatment region can be significantly less for deep-tissue tumors than X-ray therapy, because proton therapy takes full advantage of the Bragg peak. Proton therapy has been in use for over 40 years, and is a mature technology. As with all medical knowledge, understanding of the interaction of radiations with tumor and normal tissue is still imperfect. Costs Historically, proton therapy has been expensive. An analysis published in 2003 found that the cost of proton therapy is ~2.4 times that of X-ray therapies. Newer, less expensive, and dozens more proton treatment centers are driving costs down and they offer more accurate three-dimensional targeting. Higher proton dosage over fewer treatments sessions (1/3 fewer