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. 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. 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. 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. some texts distinguish "tongue and groove error" from "tongue or groove error", according as both or one side of the aperture is occluded. volumetric modulated arc therapy (vmat) volumetric modulated arc therapy (vmat) is a radiation technique introduced in 2007 which can achieve highly conformal dose distributions on target volume coverage and sparing of normal tissues. the specificity of this technique is to modify three parameters during the treatment. vmat delivers radiation by rotating gantry (usually 360° rotating fields with one or more arcs), changing speed and shape of the beam with a multileaf collimator (mlc) ("sliding window" system of moving) and fluence output rate (dose rate) of the medical linear accelerator. vmat has an advantage in patient treatment, compared with conventional static field intensity modulated radiotherapy (imrt), of reduced radiation delivery times. comparisons between vmat and conventional imrt for their sparing of healthy tissues and organs at risk (oar) depends upon the cancer type. in the treatment of nasopharyngeal, oropharyngeal and hypopharyngeal carcinomas vmat provides equivalent or better protection of the organ at risk (oar). in the treatment of prostate cancer the oar protection result is mixed with some studies favoring vmat, others favoring imrt. temporally feathered radiation therapy (tfrt) temporally feathered radiation therapy (tfrt) is a radiation technique introduced in 2018 which aims to use the inherent non-linearities in normal tissue repair to allow for sparing of these tissues without affecting the dose delivered to the tumor. the application of this technique, which has yet to be automated, has been described carefully to enhance the ability of departments to perform it, and in 2021 it was reported as feasible in a small clinical trial, though its efficacy has yet to be formally studied. automated planning automated treatment planning has become an integrated part of radiotherapy treatment planning. there are in general two approaches of automated planning. 1) knowledge based planning where the treatment planning system has a library of high quality plans, from which it can predict the target and dose-volume histogram of the organ at risk. 2) the other approach is commonly called protocol based planning, where the treatment planning system tried to mimic an experienced treatment planner and through an iterative process evaluates the plan quality from on the basis of the protocol. particle therapy main article: particle therapy in particle therapy (proton therapy being one example), energetic ionizing particles (protons or carbon ions) are directed at the target tumor. the dose increases while the particle penetrates the tissue, up to a maximum (the bragg peak) that occurs near the end of the particle's range, and it then drops to (almost) zero. the advantage of this energy deposition profile is that less energy is deposited into the healthy tissue surrounding the target tissue. auger therapy main article: auger therapy auger therapy (at) makes use of a very high dose of ionizing radiation in situ that provides molecular modifications at an atomic scale. at differs from conventional radiation therapy in several aspects; it neither relies upon radioactive nuclei to cause cellular radiation damage at a cellular dimension, nor engages multiple external pencil-beams from different directions to zero-in to deliver a dose to the targeted area with reduced dose outside the targeted tissue/organ locations. instead, the in situ delivery of a very high dose at the molecular level using at aims for in situ molecular modifications involving molecular breakages and molecular re-arrangements such as a change of stacking structures as well as cellular metabolic functions related to the said molecule structures. motion compensation in many types of external beam radiotherapy, motion can negatively impact the treatment delivery by moving target tissue out of, or other healthy tissue into, the intended beam path. some form of patient immobilisation is common, to prevent the large movements of the body during treatment, however this cannot prevent all motion, for example as a result of breathing. several techniques have been developed to account for motion like this. deep inspiration breath-hold (dibh) is commonly used for breast treatments where it is important to avoid irradiating the heart. in dibh the patient holds their breath after breathing in to provide a stable position for the treatment beam to be turned on. this can be done automatically using an external monitoring system such as a spirometer or a camera and markers. the same monitoring techniques, as well as 4dct imaging, can also be for respiratory gated treatment, where the patient breathes freely and the beam is only engaged at certain points in the breathing cycle. other techniques include using 4dct imaging to plan treatments with margins that account for motion, and active movement of the treatment couch, or beam, to follow motion. contact x-ray brachytherapy contact x-ray brachytherapy (also called "cxb", "electronic brachytherapy" or the "papillon technique") is a type of radiation therapy using kilovoltage x-rays applied close to the tumour to treat rectal cancer. the process involves inserting the x-ray tube through the anus into the rectum and placing it against the cancerous tissue, then high doses of x-rays are emitted directly into the tumor at two weekly intervals. it is typically used for treating early rectal cancer in patients who may not be candidates for surgery. a 2015 nice review found the main side effect to be bleeding that occurred in about 38% of cases, and radiation-induced ulcer which occurred in 27% of cases. brachytherapy (sealed source radiotherapy) main article: brachytherapy a savi brachytherapy device brachytherapy is delivered by placing radiation source(s) inside or next to the area requiring treatment. brachytherapy is commonly used as an effective treatment for cervical, prostate, breast, and skin cancer and can also be used to treat tumours in many other body sites. in brachytherapy, radiation sources are precisely placed directly at the site of the cancerous tumour. this means that the irradiation only affects a very localized area – exposure to radiation of healthy tissues further away from the sources is reduced. these characteristics of brachytherapy provide advantages over external beam radiation therapy – the tumour can be treated with very high doses of localized radiation, whilst reducing the probability of unnecessary damage to surrounding healthy tissues. a course of brachytherapy can often be completed in less time than other radiation therapy techniques. this can help reduce the chance of surviving cancer cells dividing and growing in the intervals between each radiation therapy dose. as one example of the localized nature of breast brachytherapy, the savi device delivers the radiation dose through multiple catheters, each of which can be individually controlled. this approach decreases the exposure of healthy tissue and resulting side effects, compared both to external beam radiation therapy and older methods of breast brachytherapy. radionuclide therapy main article: radionuclide therapy radionuclide therapy (also known as systemic radioisotope therapy, radiopharmaceutical therapy, or molecular radiotherapy), is a form of targeted therapy. targeting can be due to the chemical properties of the isotope such as radioiodine which is specifically absorbed by the thyroid gland a thousandfold better than other bodily organs. targeting can also be achieved by attaching the radioisotope to another molecule or antibody to guide it to the target tissue. the radioisotopes are delivered through infusion (into the bloodstream) or ingestion. examples are the infusion of metaiodobenzylguanidine (mibg) to treat neuroblastoma, of oral iodine-131 to treat thyroid cancer or thyrotoxicosis, and of hormone-bound lutetium-177 and yttrium-90 to treat neuroendocrine tumors (peptide receptor radionuclide therapy). another example is the injection of radioactive yttrium-90 or holmium-166 microspheres into the hepatic artery to radioembolize liver tumors or liver metastases. these microspheres are used for the treatment approach known as selective internal radiation therapy. the microspheres are approximately 30 µm in diameter (about one-third of a human hair) and are delivered directly into the artery supplying blood to the tumors. these treatments begin by guiding a catheter up through the femoral artery in the leg, navigating to the desired target site and administering treatment. the blood feeding the tumor will carry the microspheres directly to the tumor enabling a more selective approach than traditional systemic chemotherapy. there are currently three different kinds of microspheres: sir-spheres, therasphere and quiremspheres. a major use of systemic radioisotope therapy is in the treatment of bone metastasis from cancer. the radioisotopes travel selectively to areas of damaged bone, and spare normal undamaged bone. isotopes commonly used in the treatment of bone metastasis are radium-223, strontium-89 and samarium (153sm) lexidronam. in 2002, the united states food and drug administration (fda) approved ibritumomab tiuxetan (zevalin), which is an anti-cd20 monoclonal antibody conjugated to yttrium-90. in 2003, the fda approved the tositumomab/iodine (131i) tositumomab regimen (bexxar), which is a combination of an iodine-131 labelled and an unlabelled anti-cd20 monoclonal antibody. these medications were the first agents of what is known as radioimmunotherapy, and they were approved for the treatment of refractory non-hodgkin's lymphoma. intraoperative radiotherapy main article: intraoperative radiation therapy intraoperative radiation therapy (iort) is applying therapeutic levels of radiation to a target area, such as a cancer tumor, while the area is exposed during surgery. rationale the rationale for iort is to deliver a high dose of radiation precisely to the targeted area with minimal exposure of surrounding tissues which are displaced or shielded during the iort. conventional radiation techniques such as external beam radiotherapy (ebrt) following surgical removal of the tumor have several drawbacks: the tumor bed where the highest dose should be applied is frequently missed due to the complex localization of the wound cavity even when modern radiotherapy planning is used. additionally, the usual delay between the surgical removal of the tumor and ebrt may allow a repopulation of the tumor cells. these potentially harmful effects can be avoided by delivering the radiation more precisely to the targeted tissues leading to immediate sterilization of residual tumor cells. another aspect is that wound fluid has a stimulating effect on tumor cells. iort was found to inhibit the stimulating effects of wound fluid. history x-ray treatment of tuberculosis in 1910. before the 1920s, the hazards of radiation were not understood, and it was used to treat a wide range of diseases. main article: history of radiation therapy medicine has used radiation therapy as a treatment for cancer for more than 100 years, with its earliest roots traced from the discovery of x-rays in 1895 by wilhelm röntgen. emil grubbe of chicago was possibly the first american physician to use x-rays to treat cancer, beginning in 1896. the field of radiation therapy began to grow in the early 1900s largely due to the groundbreaking work of nobel prize–winning scientist marie curie (1867–1934), who discovered the radioactive elements polonium and radium in 1898. this began a new era in medical treatment and research. through the 1920s the hazards of radiation exposure were not understood, and little protection was used. radium was believed to have wide curative powers and radiotherapy was applied to many diseases. prior to world war 2, the only practical sources of radiation for radiotherapy were radium, its "emanation", radon gas, and the x-ray tube. external beam radiotherapy (teletherapy) began at the turn of the century with relatively low voltage (<150 kv) x-ray machines. it was found that while superficial tumors could be treated with low voltage x-rays, more penetrating, higher energy beams were required to reach tumors inside the body, requiring higher voltages. orthovoltage x-rays, which used tube voltages of 200-500 kv, began to be used during the 1920s. to reach the most deeply buried tumors without exposing intervening skin and tissue