Cedric X. YDepartment of Radiation Oncology, University of Maryland School of Medicine, 22 South Greene Street,Baltimore, Maryland 21201
Christopher J. AmiesSiemens Medical Solutions USA, Inc., Oncology Care Systems Group, 4040 Nelson Avenue, Concord,California 94520
Michelle SvatosTranslational Research, Varian Medical Systems, 3100 Hansen Way M/S E263 Palo Alto,California 94304-1038
͑Received 9 May 2008; revised 11 August 2008; accepted for publication 18 September 2008;published 6 November 2008͒
Intensity modulated radiation therapy ͑IMRT͒ is an advanced form of external beam radiationtherapy. IMRT offers an additional dimension of freedom as compared with field shaping in three-dimensional conformal radiation therapy because the radiation intensities within a radiation fieldcan be varied according to the preferences of locations within a given beam direction from whichthe radiation is directed to the tumor. This added freedom allows the treatment planning system tobetter shape the radiation doses to conform to the target volume while sparing surrounding normalstructures. The resulting dosimetric advantage has shown to translate into clinical advantages ofimproving local and regional tumor control. It also offers a valuable mechanism for dose escalationto tumors while simultaneously reducing radiation toxicities to the surrounding normal tissue andsensitive structures. In less than a decade, IMRT has become common practice in radiation oncol-ogy. Looking forward, the authors wonder if IMRT has matured to such a point that the room forfurther improvement has diminished and so it is pertinent to ask what the future will hold for IMRT. This article attempts to look from the perspective of the current state of the technology to predictthe immediate trends and the future directions. This article will ͑1͒ review the clinical experience ofIMRT; ͑2͒ review what we learned in IMRT planning; ͑3͒ review different treatment deliverytechniques; and finally, ͑4͒ predict the areas of advancements in the years to come. 2008 Ameri-can Association of Physicists in Medicine. ͓DOI:
Key words: radiation therapy, intensity modulation, treatment planning, radiotherapy delivery,IMRT
I. OVERVIEW
therapy. The use of overlapping cone-beam arcstodeliver modulated beam intensities around the patient, re-
Intensity modulated radiation therapy ͑IMRT͒ has been
ferred to as intensity modulated arc therapy ͑IMAT͒, was
widely adopted as a new tool in radiation therapy to deliver
also proposed but has not been widely adopted for clinical
high doses of radiation to the tumor while providing maxi-
mal sparing of surrounding critical structures. What facili-
With nearly 10 years of clinical application, both short-
tated the quick dissemination of IMRT technology was not
term and long-term clinical results of IMRT treatments are
hard clinical evidence but rather the individually calculatedand visually illustrated dosimetric advantages. Because such
emerging. Its ability to shape the high dose volume to con-
dosimetric advantage was obvious, wide clinical adoption of
form to the shape of the target tissues allows the decrease of
IMRT preceded any randomized clinical trials. In the U.S.,
irradiation related sequelae by limiting the dose delivered to
the increased reimbursement for the technology also fueled
the surrounding normal tissues. The same dose shaping ca-
the speed of clinical dissemination. Early studies indicated
pability also allows the physicians to escalate doses to cer-
that with IMRT, radiation doses to sensitive structures could
tain tumors to enhance local control. Both dosimetric and
be reduced significantly while maintaining sufficient dose
clinical advantages have been demonstrated for almost all
coverage to the targeted tumorous tissues.Both rotational
common anatomical though the largest number of
and gantry-fixed IMRT techniques have been implemented
applications has been for prostate cancer and cancers of the
head and neck IMRT has allowed the physicians to
͑DMLC͒In gantry-fixed IMRT, multiple coplanar
escalate the dose to the prostate while reducing the toxicities
and noncoplanar beams at different orientations, each with
to the rectum and bladder, resulting in improved local control
spatially modulated beam intensities, are Rota-
and reduced complications as compared with conventional
tional IMRT, as it is practiced today, mainly employs tempo-
three-dimensional ͑3D͒ conformal therapy.IMRT has
rally modulated fan commonly known as tomo-
also shown greater capability in sparing salivary functions in
Med. Phys. 35 „12…, December 2008 0094-2405/2008/35„12…/5233/9/$23.00 2008 Am. Assoc. Phys. Med. Yu, Amies, and Svatos: IMRT planning and delivery
patients receiving radiation therapy for head and neck
further advancement in the near future. IMRT technology
Rather than delivering the prescribed dose to
consists of planning methods and delivery technologies. If
the entire pelvis, IMRT was able to spare the small bowel,
IMRT continues to advance, improvements must be made in
the bladder and the rectum, resulting in significantly lower
these two areas. It is therefore crucial to summarize what we
GI The use of IMRT instead of wedge pairs
have learned in these two key aspects of the technology.
for tangential whole breast irradiation has resulted in im-proved dose uniformitywhich in turn resulted in signifi-cantly reduced acute and chronic IMRT has also
II. IMRT TREATMENT PLANNING
been combined with stereotactic localization for deliveringradiosurgery treatments to intracranial and extracranial sites
The key technology for IMRT planning is computer opti-
using linear Other sites of application in-
mization. Although relatively new in radiation oncology,
computer optimization is not new and has been used for
operations research and other fields of theoretical and applied
These data indicate that for certain clinical cases, there is
research. Almost all the algorithms that have been used in
a direct translation between dosimetric advantage and im-
other fields, including simulated annealing,gradient
proved clinical outcome. With more emerging clinical data
search,genetic algorithms,and linear and nonlinear
pointing to the same conclusion, IMRT as a technology will
programming,have been applied to plan IMRT treat-
continue to be an important technique in external beam ra-
There have also been significant efforts on how the opti-
The way IMRT spares critical structures is by redistribut-
mization problem is formulated. Universal to all the optimi-
ing the normal tissue dose to less critical regions and to
zation methods, the objectives of each treatment plan are
reducing the high dose volume to just cover the target. For a
reduced to a single value by the objective function or cost
given integral dose to the target, the integral dose to the
function. The objectives of the treatment can be expressed
surrounding structures is roughly constant as dictated by the
according to the desired dose distributionsor in terms
physics of dose deposition.In many cases, the use of IMRT
of biological objectives.Essentially, when the objective
results in a greater volume of surrounding normal tissues
function is formulated to closely depict the desirable features
receiving a lower dose as compared with traditional three-
of the radiation dose distribution, the quality of the candidate
dimensional conformal therapy. This phenomenon raises sig-
plans is judged more accurately during the optimization pro-
nificant concerns for pediatric applications of IMRT
cess. Therefore, as long as the objectives are defined to ap-
While the long term clinical results of using IMRT for inop-
propriately reflect what is really clinically optimal, the re-
erable nonsmall lung cancer are its use for me-
sults are generally clinically meaningful and acceptable.
sothelioma following extrapleural pneumonectomy showed a
Different commercial treatment planning systems use differ-
high rate of fatal pulmonary toxicityA high mean lung
ent optimization algorithms, and slightly different dose or
dose and a higher percentage of lung volume receiving
biological dose, in the form of equivalent uniform dose, ob-
20 Gy were clear predictors of fatal pulmonary toxicity.
jectives. In general and except for certain limitations in the
Another issue of using IMRT for treating tumors in the
implementation and planner experience, there is no clinically
thorax and abdominal regions is the breathing induced target
meaningful difference in plan quality when using different
motion. As illustrated by Yu et al.the interplay between
the target motion and the movement of the beam to achieve
Complex treatment planning problems often involve mul-
intensity modulation could cause unintended hot and cold
tiple targets with many surrounding normal structures. The
spots in excess of 100% in a given fraction of treatment.
objectives specified for these structures are often in conflict.
Different motion management strategies have been proposed
In such complex situations, it is hard if not impossible to
or applied clinically, including gating the radiation beam
optimally describe what is truly desired before a plan is op-
and dynamically tracking the tumor with DMLCor with
timized. In the situation where one cannot achieve optimality
for all the objectives simultaneously, the optimal solution
Clinical use of IMRT has also brought many changes in
becomes the optimal trade-off among these objectives. There
clinical practice. Owing to its ability to reduce dose to sur-
have been several approaches proposed by different re-
rounding structures, IMRT has allowed physicians to escalate
searchers for dealing with such conflicting objectives. One of
dose to the prostate gland to improve local control.The
the approaches is to specify the treatment objective as a
reduced toxicity also encouraged the use of hypofraction-
probability density function rather than as a rigid dose
ation schemes.The ability of IMRT to paint more com-
objective.If the most desired dose is not attainable by the
plex dose distributions also allowed the increased use of con-
optimizer, other doses are acceptable but less preferred. Con-
comitant boost and the treatment of target within tar
sequently, during the optimization process, the prescription
The purpose of this article is not to provide an extensive
dose is allowed to deviate, with a certain preference level,
review of IMRT technology. Such reviews can be found in
from the most desired dose. Another approach is to build a
database of plans by varying the emphasis of different objec-
dict from what we learned in the last 10 years of develop-
tives. Using an interactive plan navigation tool, the trade-offs
ment and clinical practice of IMRT the potential direction of
between different objectives can be made explicitly based on
Medical Physics, Vol. 35, No. 12, December 2008 Yu, Amies, and Svatos: IMRT planning and delivery
the user’s clinical goals.The resultant plans from this ap-
a result, the residual room for improvements in plan quality
proach are also called Pareto-optimal plans.
under existing delivery systems has diminished over the
One of the tasks in treatment planning for external beam
years of refinements in treatment planning algorithms and
radiation therapy is to determine the number of beams to use
and their orientations. For conventional treatments, beamangles used for different treatment sites are well established. With IMRT, the task of beam angle selection is more com-
III. IMRT TREATMENT DELIVERY
plicated and less intuitive because of more complex interac-tions and mutual compensations among the different beam
Most IMRT treatment deliveries require the use of the
angles as the result of computer optimized beam intensities.
multileaf collimator ͑MLC͒, which was originally developed
As the result, beam angle optimization has been one of the
for shaping radiation fields. The ability of the MLC to easily
areas of intensive investigation. Haas et employed ge-
change and dynamically vary the field shapes was quickly
netic algorithms to search for the best beam orientations.
explored for IMRT delivery. Over the years, linear accelera-
Rowbottom et al.and Stein et al.used simulated anneal-
tor vendors have not only improved the reliability of MLCs
ing algorithms to perform beam angle optimization by com-
but also made the leaf width smaller. Smaller leaf widths
paring thousands of sets of fixed number of beams sampled
allow the treatment planning system to use finer beamlet size
from a constrained or unconstrained space of beam orienta-
during optimization for achieving better plan quality.
tions. Pugachev et al.also reported different schemes of
IMRT treatments are delivered using either fixed beam
beam angle optimization with the simulated annealing algo-
angles or rotational beams. Both forms of delivery were pro-
rithm. Although all these studies demonstrated some effects
posed and developed at the start of the IMRT technology.
of beam angle selection on plan quality, the degree of im-
Although the technology is termed “intensity modulation,”
provements has been small. It is thus reasonable to conclude
the intensity variation is actually achieved by temporal
that the mutual interactions and compensations among the
modulation, allowing different beamlets to be irradiated for
beams under intensity optimization also made beam angle
selection less influential on plan quality as compared with
Most of the IMRT treatments delivered today use a few
three-dimensional conformal therapy.
fixed beam directions. The treatment planning system has
In spite of years of refinements in IMRT planning, argu-
converted the optimal intensity distributions into deliverable
ably, the quality of treatment plans has not improved much
overlapping field segments. Because the conversion is not
from that of the early days. The explanation may be rooted in
within the optimization process and MLC motion constraints
the basic principles of inverse planning. For each patient, the
have to be considered during such conversion, it may intro-
anatomy dictates a unique set of preferred beam orientations
duce differences between the treatment plans using the opti-
and, in each beam orientation and radiation field, the pre-
mal intensity distributions and that achievable with overlap-
ferred locations through which radiation is directed to the
ping field These field segments, also called
tumor. If more freedom is given to make use of these intrin-
subfields, can be delivered dynamically, i.e., MLC transition-
sic preferences in treatment planning, either performed by a
ing through the shapes of the subfields dynamically when the
human planner or by a computer optimization system, better
beam is on. They can also be delivered one segment at a time
plans can be generated. IMRT provides an additional free-
such that the MLC is not moving during irradiation.
dom as compared with 3D conformal radiation therapy be-
Up to now, rotational IMRT treatments have primarily
cause the radiation intensities within a radiation field can be
been delivered using tomotherapy,an approach where a
varied according to the location preferences. However, there
binary collimator ͑open/close͒ is used to control the amount
is a fundamental limit to which the quality of treatment plans
of exposure time of a small width of the fan beam, or a
can be improved. This limit is defined by the physics of the
beamlet. The fraction of time a leaf is in the “open” position
radiation dose deposition and by the degrees of freedom that
at a given beam angle determines the relative “intensity” of
are given to the computer optimization routine. There are
radiation at that angle. Because the fan beam can only irra-
indications that the required degree of intensity variation for
diate a slice of the patient at a given time, larger tumors are
achieving the optimal plan quality for a large majority of
treated by either stepping the table one slice at a timeor
cases is relatively small. For example, Ludlum et al.opti-
continuously.The latter is called helical tomotherapy be-
mized IMRT plans for different challenging cancer sites us-
cause the trajectory of the radiation source relative to the
ing different number of intensity levels. They found that re-
patient is a helix. Tomotherapy™ ͑Madision, WI͒ offers a
ducing the number of intensity levels from 10 to 3 only cause
“turn-key” approach to IMRT implementation with the plan-
minor degradations in dose distribution. Comparisons among
ning system specifically designed for the delivery unit. To-
different treatment planning and delivery
motherapy utilizes all coplanar beam angles and the intensity
have not yield any approach with clinically meaningful su-
variations of the beamlets are not constrained by the me-
periority. These studies and our own experience led us to
chanical limits of the binary MLC. Therefore, theoretically
believe that with the current state of the radiation delivery
speaking, tomotherapy provides a planning system with the
systems and without new advances to provide additional de-
freedom to use highly modulated beams to create more con-
grees of freedom, IMRT has approached its limits in the
formal treatment plans then MLC delivery with fixed cone
quality of treatment plans that can be physically achieved. As
Medical Physics, Vol. 35, No. 12, December 2008 Yu, Amies, and Svatos: IMRT planning and delivery
Another rotational IMRT approach, called IMAT, was
proaches to metabolic imaging, and the emergence of a new
suggested by YuInstead of rotating fan beams around the
generation of radiation treatment delivery systems.
patient, IMAT uses cone beams shaped with conventionalMLCs. IMAT delivers optimized intensity distributions for
IV.A. Quality improvements
large number of beams spaced every 5 – 10 deg around the
Based on 10 years of experience with IMRT, we have
patient. Optimized intensity distributions are translated into a
learned that the opportunities in improving plan quality are
stack of superimposed irregular fields of uniform beam in-
limited within the constraint of present linac/MLC delivery.
tensities and delivered by overlapping arcs with synchro-
To improve the quality of IMRT treatment plans, we must
nized gantry rotation and field shape variations. As the gan-
inject new degrees of freedom. This may require an overhaul
try is rotating around the patient and the radiation beam is
of existing technologies. A new generation of treatment de-
on, it is important that the subfields of adjacent beam angles
livery systems is emerging. One of such machines is pro-
do not require the MLC leaves to travel very long distances.
posed by Kamino et Besides the IMRT capability and
Ensuring such connectedness of adjacent subfields for
improved image guidance features, a notable feature is that
smooth leaf motion is of great concern in the leaf sequencing
the linear accelerator head can be pivoted. By offering non-
algorithm for IMAT.Effective planning tools for IMAT
coplanar beams easily without couch rotations, such machine
provides new degrees of freedom into the solutions, and,
Comparisons between IMRT plans for different delivery
therefore, has the potential of improving the qualities of
methods have been conducted. In a comparison of tomo-
therapy and MLC delivery, Mavroidis et al. found that linear
Several new machines currently under development incor-
accelerator delivery with a MLC has slight advantage over
porate image guidance from Magnetic resonance imaging
tomotherapy for most sites other than head and neck.Simi-
͑MRI͒. The design of the Renaissance™ ͑ViewRay, Inc.,
lar results have been found by Muzik et al.Cao et al.
Cleveland, OH͒ incorporates real-time MR imaging and mul-
compared the treatment plan quality of IMAT plans and to-
tihead Co-60 rotational delivery. The use of MRI for image
motherapy plans for ten cases including head and neck, lung,
guidance may provide improved soft tissue contrast for im-
brain, and prostate. It was found that these two kinds of
rotational delivery methods are also equivalent for most
These new technologies are aimed for delivering radiation
cases. For cases where noncoplanar beams are desirable,
treatments to all sites. As the room for quality improvements
such as for intracranial tumors and some head and neck
is further squeezed, technologies specific for particular treat-
cases, the use of partial noncoplanar arcs in IMAT was found
ment sites may also emerge to take advantage of the anatomy
to be more Shepard et al. compared IMAT
plans with IMRT 84 and found that the employment of rota-
As machines increase in complexity, it becomes harder for
tional IMRT is advantageous for most of the cases.
a generic treatment planning system to fully utilize the ma-
It is important to note that there are many other issues
chines capabilities. Diversity in delivery systems forces the
besides plan quality that are associated with different deliv-
planning system to be more specifically tailored to the treat-
ery techniques. These include the efficiency of planning, de-
ment machine. Treatment planning systems designed specifi-
livery, and quality assurance, the complexity and reliability
cally for a delivery device as a turn-key solution will become
of delivery, and the total monitor units required to deliver the
IMRT is not limited to the use of photons. It has been
demonstrated that the IMRT principle can also be applied to
IV. FUTURE DIRECTIONS OF ADVANCEMENTS
electronsand protons.The fast introduction of proton
beams in radiation therapy would allow us to push the limitof achievable plan quality higher than using only photons.
Revolutionary advances in biology and genomics have
brought a wave of new molecular therapeutic agents for
IV.B. Efficiency improvements
fighting cancer and the continued acceleration in the direc-tion will mark a new era of cancer therapies. However, mo-
The fact that treatment plans of different complexity de-
lecular agents have not been shown to eliminate local thera-
signed for different delivery methods can achieve similar
pies. Therefore, we do not see that the role of radiation
quality of treatments suggest that there is substantial room
therapy will diminish in this new era. The dose shaping ca-
for improving the workflow and efficiency. Past attempts in
pabilities of IMRT make it an essential tool for better inte-
improving treatment efficiency include the use of direct ap-
gration of radiation with chemotherapy and molecular thera-
erture optimizationand the development of single arc
The areas in which IMRT technology can be improved
Instead of optimizing intensity maps and then translating
include ͑1͒ the quality of treatment plans, ͑2͒ the accuracy of
the intensity maps into deliverable segments, Shepard et
treatment delivery, and ͑3͒ the efficiency of both planning
proposed and developed a method called “direct aperture op-
and delivery of IMRT treatments. We predict that the ad-
timization” ͑DAO͒ that directly optimizes the shapes and
vances in these areas will be driven by factors including
weights of MLC segments simultaneously in one step. The
on-line and real-time image guidance, individualized ap-
planner determines the number of beams to use and the num-
Medical Physics, Vol. 35, No. 12, December 2008 Yu, Amies, and Svatos: IMRT planning and delivery
ber of apertures desired at each of the beams. The planning
IV.C. Improving the accuracy of IMRT delivery with
system optimizes the aperture shape and aperture weights
image guidance
simultaneously. Physical constraints of the MLC are consid-
Besides improving efficiency, great opportunities also ex-
ered in the optimization process. With a small number of
ist for improving the accuracy of treatment delivered by us-
apertures, the resulting plan quality generally rivals that
ing on-board image guidance. The advent of on-board cone
based on intensity optimization which requires large number
beam computed tomography ͑CBCT͒ and in-room CT with
of apertures. Earl et al.and Kim et al.also demon-
high soft tissue contrast opens new opportunities for high-
strated that even with overlapping rectangular fields formed
precision radiation therapyThe success of the image
by independent jaws, IMRT type of dose distribution can be
guided radiation therapy ͑IGRT͒ lies not only in the ability to
achieved for prostate, breast, and even head and neck cases,
acquire the images but largely in how we use the images to
if the aperture shapes and weights are optimized with DAO.
achieve our goal of controlling more cancers while decreas-
Earl et al. illustrated that DAO can also be applied for the
ing the normal tissue toxicities. In many cases, the tumor not
optimization of IMAT plans.The possibility of varying
only changes its location but also its shape and the relative
dose rates during gantry rotation and irradiation also opens
geometrical relationship with its surrounding normal struc-
the opportunity for delivering IMRT-like plans with a single
tures. Consequently, simply shifting the patient does not
arc rotation. In proposing the IMAT idea, Yhas predicted
guarantee optimal treatments. Therefore, reoptimizing the
that, with reasonable number of beam angles, the quality of
IMRT plan or adapting an existing plan to the “anatomy of
an IMRT or IMAT plan depends on the number of total seg-
the day” will increasingly become an active area of research.
ments, or aperture variations. Given enough aperture varia-
Recognizing the impracticality of a full-fledged daily re-
tions, a single arc should be able to achieve IMRT-like plan
planning with current technology, several researchers have
quality. Beam intensity modulation is not a fundamental re-
recently proposed methods for incorporating the deformable
quirement for achieving optimal treatment plans. The ability
component into an online correction strategy. Wu et al.have shown that for tomotherapy delivery, intensities can be
to take advantage of the preferred angles and locations,
modified online with given transformation parameters or
through which the radiation is directed to the tumor, is the
with a quick reoptimization. Mohan et proposed to de-
key to achieve optimal dose distributions. For example, the
form the intensity patterns as an online correction strategy.
CyberKnife™ system͑Accuray, Sunnyvale, CA͒, which
Feng et have proposed a scheme that deforms the ap-
uses small circular x-ray beams generated by an x-band lin-
erture shapes of the IMRT segments by using the deforma-
ear accelerator mounted on a robot to deliver the radiation to
tion matrix derived from the planning image set and the im-
the target, does not explicitly modulate the intensity of a
ages of the day. Ahunbay et developed a similar
beam, but is able to deliver highly conformal treatments.
scheme but added aperture weight optimization after aperture
The feasibility of delivering IMRT with a single arc was
deformation. These schemes have shown to be able to ap-
first demonstrated by Cameron et al.Using a DAO scheme
proach the plan quality resulting from reoptimization using
as Earl et al. but employing a more efficient way for select-
ing the initial aperture shapes, Ulrich et al. also showed that
As computer technology continue to advance, such short
IMRT-like dose distributions can be achieved with a single
cuts as described above may no longer be needed. Replan-
When optimizing a large number of beam apertures
ning using the images of the day will become a reality. The
from large number of beam angles, the scheme by Ulrich and
advance of IGRT will make imaging, planning and treatment
Earl can take a long time for the optimization to converge.
delivery all performed in a single session. Under the new
Otto devised a coarse-to-fine optimization scheme that starts
“image-plan-treat” process, initial imaging and planning will
with a small number of beams with large angular spacing and
become merely a first look into what degree of dose confor-
gradually inserts new beam angles to be optimized.The
mity is achievable. With experience and confidence, the ini-
computer optimizes the aperture shapes and weights simul-
tial planning may even be eliminated. Such a procedure,
taneously as in DAO. Tang et al.illustrated that a single arc
which has been the norm for radiosurgery, will allow further
IMAT plan can be derived from a multiarc IMAT plan by
shrinkage of PTV margins and encourage radiation oncolo-
spacing the stacked apertures to the neighboring angles. Us-
gists to revise the current dose fractionation schemes. There
ing graph algorithms, Wang et al.have shown that IMRT
are three technologies that will enable for this image-plan-
plans optimized with 36 fields can be sequenced into and
delivered with a single arc. The pressure for more efficient
͑1͒ Imaging systems integrated with delivery machines will
IMRT delivery will encourage linear accelerator vendors to
be faster and provide greater image quality before and
offer different single arc IMAT solutions.
during treatment. Currently, the gantry speed is setting
In addition to rotational delivery, the use of higher dose
the limit on the speed of image acquisition for imaging
rates, either by the improvements in acceleration technology
systems mounted on the treatment gantry. The image
or by eliminating the flattening filter, could also be explored
quality achievable with CBCT also does not match that
for efficiency As IMRT becomes easier to plan and
of fan beam CT units because of the limitations of the
to deliver, the need for traditional beam modifiers such as the
reconstruction algorithms for CBCT and the added ra-
use of physical wedges will diminish.
diation scatter contributions. These limitations will be
Medical Physics, Vol. 35, No. 12, December 2008 Yu, Amies, and Svatos: IMRT planning and delivery
corrected in the long-term by the emergence of new de-
such as DAO and single arc IMAT. The rapid adaptation of
signs of on-line imaging systems, new imaging recon-
IGRT will encourage the integration of IMRT planning and
struction and scatter subtraction algorithms, and/or new
delivery with on-line imaging. Strategies of on-line plan ad-
aptation and on-line replanning will be the emphasis in the
͑2͒ Dose calculation and optimization can be performed
near future. The adoption of biology and molecular imaging
within a couple of minutes. Computational speeds have
guidance will push IMRT into dose painting paradigm in the
been increasing rapidly according to Moore’s law. Other
innovations, such as the use of special hardware for dose
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We have reviewed and described the current state of
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Medizinische Informationen zu HIV und AIDSVorsorge- und Kontroll unter-suchungen bei HIV-InfektionHIV-Positive müssen regelmäßige Arzttermine wahrnehmen und zwar unabhängig davon, ob sie HIV-Medikamente nehmen oder nicht. Diese Broschüre gibt einen Überblick über die Routineuntersuchungen, die jeder HIV-Positive in Anspruch nehmen sollte. Da HIV-Positive heute dank der verbesserten T