Protons versus photons: a status assessment at the beginning of the 21st Century.

At present modern photon delivery techniques permit high dose isodose conformality similar to protons in many cases. However, a proton advantage appears to still be present for target volumes of higher degrees of complexity and concavity. Also, for selected histologies and sites, notably skull base tumors, protons have established a "gold standard" and photon data have yet to duplicate those proton results in clinical practice. Proton radiation therapy offers superior dose distribution by reduced low-dose integral irradiated volume. The avoidance of functional and cosmetic side effects in children by protons is at present acknowledged by the radiation oncology community and is expected to gain similar recognition in adult patients. This advantage of protons and disadvantage for photons constitutes an "inherent physical gap" that will likely be long lasting. Although the priorities of proton irradiation advantages have shifted over the decades, clinical advantages remain and are of sufficient significance and importance to justify further development and installation of proton radiation facilities worldwide.