Question: How is IMRT technology limited?
About the article "Radiation therapy", section "Virtual simulation, 3-dimensional conformal radiation therapy, and intensity-modulated radiation therapy".

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The planning of radiation therapy treatment has been revolutionized by the ability to delineate tumors and adjacent normal structures in three dimensions using specialized CT and/or MRI scanners and planning software.
Virtual simulation, the most basic form of planning, allows more accurate placement of radiation beams than is possible using conventional X-rays, where soft-tissue structures are often difficult to assess and normal tissues difficult to protect.
An enhancement of virtual simulation is 3-dimensional conformal radiation therapy (3DCRT), in which the profile of each radiation beam is shaped to fit the profile of the target from a beam's eye view (BEV) using a multileaf collimator (MLC) and a variable number of beams.
When the treatment volume conforms to the shape of the tumor, the relative toxicity of radiation to the surrounding normal tissues is reduced, allowing a higher dose of radiation to be delivered to the tumor than conventional techniques would allow.
Intensity-modulated radiation therapy (IMRT) is an advanced type of high-precision radiation that is the next generation of 3DCRT.
IMRT also improves the ability to conform the treatment volume to concave tumor shapes, for example when the tumor is wrapped around a vulnerable structure such as the spinal cord or a major organ or blood vessel.
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 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 tissue is decreased or avoided completely.
The customized radiation dose is intended to maximize tumor dose while simultaneously protecting the surrounding normal tissue.
This may result 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 program 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.
Both techniques enable dose escalation, potentially increasing usefulness.
There has been some concern, particularly with IMRT, 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 (IGRT) 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.
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.
Radiotherapy can be delivered in many ways but is most commonly produced by a linear accelerator.
This usually involves treating the whole breast in the case of breast lumpectomy or the whole chest wall in the case of mastectomy.
Lumpectomy patients with early-stage breast cancer may be eligible for a newer, shorter form of treatment called "breast brachytherapy".
This approach allows physicians to treat only part of the breast in order to spare healthy tissue from unnecessary radiation.
Improvements in computers and treatment delivery technology have led to more complex radiotherapy treatment options.
One such new technology is using IMRT (intensity modulated radiation therapy), which can change the shape and intensity of the radiation beam making "beamlets" at different points across and inside the breast.
This allows for better dose distribution within the breast while minimizing dose to healthy organs such as the lung or heart.
However there is yet to be a demonstrated difference in treatment outcomes (both tumor recurrence and level of side effects) for IMRT in breast cancer when compared to conventional radiotherapy treatment.
In addition, conventional radiotherapy can also deliver similar dose distributions utilizing modern computer dosimetry planning and equipment.
External beam radiation therapy treatments for breast cancer are typically given every day, five days a week, for five to 10 weeks.
Within the past decade, a new approach called accelerated partial breast irradiation (APBI) has gained popularity.
APBI is used to deliver radiation as part of breast conservation therapy.
It treats only the area where the tumor was surgically removed, plus adjacent tissue.
APBI reduces the length of treatment to just five days, compared to the typical six or seven weeks for whole breast irradiation.APBI treatments can be given as brachytherapy or external beam with a linear accelerator.
These treatments are usually limited to women with well-defined tumors that have not spread.
In breast brachytherapy, the radiation source is placed inside the breast, treating the cavity from the inside out.
There are several different devices that deliver breast brachytherapy.
Some use a single catheter and balloon to deliver the radiation.
Other devices utilize multiple catheters to deliver radiation.
A study is currently underway by the National Surgical Breast and Bowel Project (NSABP) to determine whether limiting radiation therapy to only the tumor site following lumpectomy is as effective as radiating the whole breast.
New technology has also allowed more precise delivery of radiotherapy in a portable fashion — for example in the operating theatre.
Targeted intraoperative radiotherapy (TARGIT) is a method of delivering therapeutic radiation from within the breast using a portable X-ray generator called Intrabeam.
The "TARGIT-A trial" was an international randomised controlled non-inferiority phase III clinical trial led from University College London.
28 centres in 9 countries accrued 2,232 patients to test whether TARGIT can replace the whole course of radiotherapy in selected patients.
The TARGIT-A trial results found that the difference between the two treatments was 0.25% (95% CI -1.0 to 1.5) i.e., at most 1.5% worse or at best 1.0% better with single dose TARGIT than with standard course of several weeks of external beam radiotherapy.
In the "TARGIT-B trial", as the TARGIT technique is precisely aimed and given immediately after surgery, in theory it could be able provide a better boost dose to the tumor bed as suggested in phase II studies. 
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Intensity modulated radiation therapy (IMRT) is an advanced radiotherapy technique used to minimize the amount of normal tissue being irradiated in the treatment field.
In some systems this intensity modulation is achieved by moving the leaves in the MLC during the course of treatment, thereby delivering a radiation field with a non-uniform (i.e.
modulated) intensity.
With IMRT, radiation oncologists are able to break up the radiation beam into many "beamlets."
This allows radiation oncologists to vary the intensity of each beamlet.
With IMRT, doctors are often able to further limit the amount of radiation received by healthy tissue near the tumor.
Doctors have found this sometimes allowed them to safely give a higher dose of radiation to the tumor, potentially increasing the chance of a cure. 
TrueBeam is a radiotherapy device, a linear accelerator, manufactured by Varian.
The system dynamically synchronizes imaging, patient positioning, motion management, and treatment delivery.
It is a versatile platform which can be used for all forms of advanced radiotherapy modalities including Image Guided Radiotherapy (IGRT)and Image Guided Radiosurgery (IGRS), Intensity-Modulated Radiotherapy (IMRT), Volumetric Intensity Modulated Arc Therapy (Rapid Arc) and Stereotactic Body Radiotherapy (SBRT) along with conventional and 3-D conformal radiotherapy.
The new technology is currently functioning in only a handful of centers around the world, such as the Hospital of the University of Pennsylvania, University Hospital Zurich in Europe, John Stoddard Cancer Center, Stanford University in USA and Hôtel-Dieu de France, University Hospital of Saint-Joseph University in Beirut, Lebanon, The Luckow Pavilion, Paramus, New Jersey.