most from proton therapy. Clinical trials are underway to examine the comparative efficacy of proton therapy (vs photon radiation) for the following: Pediatric cancers—by St. Jude Children's Research Hospital, Samsung Medical Center Base of skull cancer—by Heidelberg University Head and neck cancer—by MD Anderson, Memorial Sloan Kettering and other centers Brain and spinal cord cancer—by Massachusetts General Hospital, Uppsala University and other centers, NRG Oncology Hepatocellular carcinoma (liver)—by NRG Oncology, Chang Gung Memorial Hospital, Loma Linda University Lung cancer—by Radiation Therapy Oncology Group (RTOG), Proton Collaborative Group (PCG), Mayo Clinic Esophageal cancer—by NRG Oncology, Abramson Cancer Center, University of Pennsylvania Breast cancer—by University of Pennsylvania, Proton Collaborative Group (PCG) Pancreatic cancer—by University of Maryland, Proton Collaborative Group (PCG) X-ray radiotherapy The figure at the right of the page shows how beams of X-rays (IMRT; left frame) and beams of protons (right frame), of different energies, penetrate human tissue. A tumor with a sizable thickness is covered by the IMRT spread out Bragg peak (SOBP) shown as the red lined distribution in the figure. The SOBP is an overlap of several pristine Bragg peaks (blue lines) at staggered depths. Megavoltage X-ray therapy has less "skin scarring potential" than proton therapy: X-ray radiation at the skin, and at very small depths, is lower than for proton therapy. One study estimates that passively scattered proton fields have a slightly higher entrance dose at the skin (~75%) compared to therapeutic megavoltage (MeV) photon beams (~60%). X-ray radiation dose falls off gradually, needlessly harming tissue deeper in the body and damaging the skin and surface tissue opposite the beam entrance. The differences between the two methods depends on: Width of the SOBP Depth of the tumor Number of beams that treat the tumor The X-ray advantage of less harm to skin at the entrance is partially counteracted by harm to skin at exit point. Since X-ray treatments are usually done with multiple exposures from opposite sides, each section of skin is exposed to both entering and exiting X-rays. In proton therapy, skin exposure at the entrance point is higher, but tissues on the opposite side of the body to the tumor get no radiation. Thus, X-ray therapy causes slightly less damage to skin and surface tissues, and proton therapy causes less damage to deeper tissues in front of and beyond the target. An important consideration in comparing these treatments is whether the equipment delivers protons