Editing Radiation therapy (section)

==== Intensity-modulated radiation therapy (IMRT) ====
[[File:Varian TruBeam.jpg|thumb|[[Varian Medical Systems|Varian]] TrueBeam [[Linear Accelerator]], used for delivering IMRT]]
Intensity-modulated radiation therapy (IMRT) is an advanced type of high-precision radiation that is the next generation of 3DCRT.<ref name="Galvin">{{cite journal | vauthors = Galvin JM, Ezzell G, Eisbrauch A, Yu C, Butler B, Xiao Y, Rosen I, Rosenman J, Sharpe M, Xing L, Xia P, Lomax T, Low DA, Palta J | display-authors = 6 | title = Implementing IMRT in clinical practice: a joint document of the American Society for Therapeutic Radiology and Oncology and the American Association of Physicists in Medicine | journal = International Journal of Radiation Oncology, Biology, Physics | volume = 58 | issue = 5 | pages = 1616–1634 | date = April 2004 | pmid = 15050343 | doi = 10.1016/j.ijrobp.2003.12.008 }}</ref> IMRT also improves the ability to conform the treatment volume to concave tumor shapes,<ref name="camphausen" /> for example when the tumor is wrapped around a vulnerable structure such as the spinal cord or a major organ or blood vessel.<ref>{{cite web |url=http://www.irsa.org/imrt.html |title=Intensity Modulated Radiation Therapy |publisher=Irsa.org |access-date=2012-04-20 |archive-date=2017-05-04 |archive-url=https://web.archive.org/web/20170504072943/http://www.irsa.org/imrt.html |url-status=dead }}</ref> Computer-controlled X-ray accelerators distribute precise radiation doses to malignant tumors or specific areas within the tumor. The pattern of radiation delivery is determined using highly tailored computing applications to perform [[Optimization (mathematics)|optimization]] and treatment simulation ([[Treatment Planning]]). The radiation dose is consistent with the 3-D shape of the tumor by controlling, or modulating, the radiation beam's intensity. The radiation dose intensity is elevated near the gross tumor volume while radiation among the neighboring normal tissues is decreased or avoided completely. This results in better tumor targeting, lessened side effects, and improved treatment outcomes than even 3DCRT.

3DCRT is still used extensively for many body sites but the use of IMRT is growing in more complicated body sites such as CNS, head and neck, prostate, breast, and lung. Unfortunately, IMRT is limited by its need for additional time from experienced medical personnel. This is because physicians must manually delineate the tumors one CT image at a time through the entire disease site which can take much longer than 3DCRT preparation. Then, medical physicists and dosimetrists must be engaged to create a viable treatment plan. Also, the IMRT technology has only been used commercially since the late 1990s even at the most advanced cancer centers, so radiation oncologists who did not learn it as part of their residency programs must find additional sources of education before implementing IMRT.

Proof of improved survival benefit from either of these two techniques over conventional radiation therapy (2DXRT) is growing for many tumor sites, but the ability to reduce toxicity is generally accepted. This is particularly the case for head and neck cancers in a series of pivotal trials performed by Professor [[Christopher Nutting]] of the Royal Marsden Hospital. Both techniques enable dose escalation, potentially increasing usefulness. There has been some concern, particularly with IMRT,<ref name="pmid12694826">{{cite journal | vauthors = Hall EJ, Wuu CS | title = Radiation-induced second cancers: the impact of 3D-CRT and IMRT | journal = International Journal of Radiation Oncology, Biology, Physics | volume = 56 | issue = 1 | pages = 83–88 | date = May 2003 | pmid = 12694826 | doi = 10.1016/S0360-3016(03)00073-7 }}</ref> about increased exposure of normal tissue to radiation and the consequent potential for secondary malignancy. Overconfidence in the accuracy of imaging may increase the chance of missing lesions that are invisible on the planning scans (and therefore not included in the treatment plan) or that move between or during a treatment (for example, due to respiration or inadequate patient immobilization). New techniques are being developed to better control this uncertainty – for example, real-time imaging combined with real-time adjustment of the therapeutic beams. This new technology is called image-guided radiation therapy or four-dimensional radiation therapy.

Another technique is the real-time tracking and localization of one or more small implantable electric devices implanted inside or close to the tumor. There are various types of medical implantable devices that are used for this purpose. It can be a magnetic transponder which senses the magnetic field generated by several transmitting coils, and then transmits the measurements back to the positioning system to determine the location.<ref>{{cite journal | vauthors = Maleki T, Papiez L, Ziaie B | title = Magnetic tracking system for radiation therapy | journal = IEEE Transactions on Biomedical Circuits and Systems | volume = 4 | issue = 4 | pages = 223–231 | date = August 2010 | pmid = 23853368 | doi = 10.1109/TBCAS.2010.2046737 | s2cid = 25639614 }}</ref> The implantable device can also be a small wireless transmitter sending out an RF signal which then will be received by a sensor array and used for localization and real-time tracking of the tumor position.<ref>{{Cite journal | vauthors = Pourhomayoun M, Fowler M, Jin Z |title=A Novel Method for Tumor Localization and Tracking in Radiation Therapy |journal=IEEE Asilomar Conference on Signals, Systems and Computers, 2012. }}</ref><ref>{{Cite journal | vauthors = Pourhomayoun M, Fowler M, Jin Z |title=Robustness Analysis of Sparsity Based Tumor Localization under Tissue Configuration Uncertainty |journal=IEEE Signal Processing in Medicine and Biology Symposium (SPMB12), 2012. }}</ref>

A well-studied issue with IMRT is the "tongue and groove effect" which results in unwanted underdosing, due to irradiating through extended tongues and grooves of overlapping MLC (multileaf collimator) leaves.<ref name="Webb 2004">{{Cite book| vauthors = Webb S |title=Contemporary IMRT: Developing Physics and Clinical Implementation|date=1 October 2004|publisher=CRC Press|isbn=978-1-4200-3453-0|pages=77–80}}</ref>  While solutions to this issue have been developed, which either reduce the TG effect to negligible amounts or remove it completely, they depend upon the method of IMRT being used and some of them carry costs of their own.<ref name="Webb 2004" />  Some texts distinguish "tongue and groove error" from "tongue or groove error", according as both or one side of the aperture is occluded.<ref name="Atallah and Blanton 2009">{{Cite book| vauthors = Atallah MJ, Blanton M |title=Algorithms and Theory of Computation Handbook, Volume 2: Special Topics and Techniques|url=https://books.google.com/books?id=SbPpg_4ZRGsC&pg=SA7-PA5|date=20 November 2009|publisher=CRC Press|isbn=978-1-58488-821-5|pages=7}}</ref>