Editing Radiation therapy (section)

====Fractionation====
{{hatnote|This section only applies to photon radiotherapy although other types of radiation therapy may be fractionated}}
{{Main|Dose fractionation}}
The total dose is fractionated (spread out over time) for several important reasons. Fractionation allows normal cells time to recover, while tumor cells are generally less efficient in repair between fractions. Fractionation also allows tumor cells that were in a relatively radio-resistant phase of the cell cycle during one treatment to cycle into a sensitive phase of the cycle before the next fraction is given. Similarly, tumor cells that were chronically or acutely hypoxic (and therefore more radioresistant) may reoxygenate between fractions, improving the tumor cell kill.<ref>{{cite journal | vauthors = Ang KK | title = Altered fractionation trials in head and neck cancer | journal = Seminars in Radiation Oncology | volume = 8 | issue = 4 | pages = 230–236 | date = October 1998 | pmid = 9873100 | doi = 10.1016/S1053-4296(98)80020-9 }}</ref>

Fractionation regimens are individualised between different radiation therapy centers and even between individual doctors. In North America, Australia, and Europe, the typical fractionation schedule for adults is 1.8 to 2&nbsp;Gy per day, five days a week. In some cancer types, prolongation of the fraction schedule over too long can allow for the tumor to begin repopulating, and for these tumor types, including head-and-neck and cervical squamous cell cancers, radiation treatment is preferably completed within a certain amount of time. For children, a typical fraction size may be 1.5 to 1.8&nbsp;Gy per day, as smaller fraction sizes are associated with reduced incidence and severity of late-onset side effects in normal tissues.

In some cases, two fractions per day are used near the end of a course of treatment. This schedule, known as a concomitant boost regimen or hyperfractionation, is used on tumors that regenerate more quickly when they are smaller. In particular, tumors in the head-and-neck demonstrate this behavior.

Patients receiving [[palliative radiation]] to treat uncomplicated painful bone metastasis should not receive more than a single fraction of radiation.<ref name="AAHPMfive">{{Citation |author1 = American Academy of Hospice and Palliative Medicine |author1-link =  American Academy of Hospice and Palliative Medicine |title = Five Things Physicians and Patients Should Question |publisher = [[American Academy of Hospice and Palliative Medicine]] |work = [[Choosing Wisely]]: an initiative of the [[ABIM Foundation]] |url = http://www.choosingwisely.org/doctor-patient-lists/american-academy-of-hospice-palliative-medicine/ |access-date = August 1, 2013}}, which cites
* {{cite journal | vauthors = Lutz S, Berk L, Chang E, Chow E, Hahn C, Hoskin P, Howell D, Konski A, Kachnic L, Lo S, Sahgal A, Silverman L, von Gunten C, Mendel E, Vassil A, Bruner DW, Hartsell W | display-authors = 6 | title = Palliative radiotherapy for bone metastases: an ASTRO evidence-based guideline | journal = International Journal of Radiation Oncology, Biology, Physics | volume = 79 | issue = 4 | pages = 965–976 | date = March 2011 | pmid = 21277118 | doi = 10.1016/j.ijrobp.2010.11.026 | author18 = American Society for Radiation Oncology (ASTRO) | doi-access = free }}</ref> A single treatment gives comparable pain relief and morbidity outcomes to multiple-fraction treatments, and for patients with limited life expectancy, a single treatment is best to improve patient comfort.<ref name="AAHPMfive"/>