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

Candidate passages:
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. 
Swedish Covenant Hospital offers radiation therapy treatment, using an advanced linear accelerator, for all types of cancers.
This technology allows it to use a number of advanced techniques, including electron beam therapy for supplemental tumors and image-guided and intensity-modulated treatments for difficult-to-treat tumors.
The intensity modulated radiation therapy (IMRT) adds precision to treat highly sensitive area such as tumors of the brain, head, neck and pelvis. 
"Technology is a necessary condition, but it's not sufficient.
To get the benefits at the time that we install the new technology, we must also change the rules that recognize the existence of the limitation."
(p. 125) 
Ionizing radiation photons or x-rays (IMRT) use pencil beam scanning to precisely target a tumor.
Bone scan densitometry uses a fan scan system that replaces a pencil beam system.
Radiation treatment planning has pencil beam scanning as one of its modalities.
mean free path is involved in IMRT planning.
Also used in Monte Carlo method for photon transport.
Another three sources are: Hybrid theory for photon transport in tissue which is a combination of Diffusion theory and the Monte Carlo method all using, in part, pencil beam scanning.
Photon pencil beam scans are defined as crossing of two beams to a fine point. 
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.