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3D Treatment Planning and Intensity-Modulated Radiation Therapy

3D Treatment Planning and Intensity-Modulated Radiation Therapy

ABSTRACT: Three-dimensional (3D) image-based treatment planning and new delivery technologies have spurred the implementation of external beam radiation therapy techniques, in which the high-dose region is conformed much more closely to the target volume than previously possible, thus reducing the volume of normal tissues receiving a high dose. This form of external beam irradiation is referred to as 3D conformal radiation therapy (3DCRT). 3DCRT is not just an add-on to the current radiation oncology process; it represents a radical change in practice, particularly for the radiation oncologist. Defining target volumes and organs at risk in 3D by drawing contours on CT images on a slice-by-slice basis, as opposed to drawing beam portals on a simulator radiograph, can be challenging, because radiation oncologists are generally not well trained in cross-sectional imaging. Currently, the 3DCRT approach will increase the time and effort required by physicians inexperienced with 3D treatment planning. Intensity-modulated radiation therapy (IMRT) is a more advanced form of 3DCRT, but there is considerable developmental work remaining. The instrumentation and methods used for IMRT quality assurance procedures and testing are not well established. Computer optimization cost functions are too simplistic, and thus time-consuming. Subjective plan evaluation by the radiation oncologist is still the norm. In addition, many fundamental questions regarding IMRT remain unanswered. For example, the radiobiophysical consequences of altered time-dose-fraction are unknown. Also, the fact that there is much greater dose heterogeneity for both the target and normal critical structures with IMRT compared to traditional irradiation techniques challenges current radiation oncology planning principles. However, this new process of planning and treatment delivery shows significant potential for improving the therapeutic ratio. In addition, while inefficient today, these systems, when fully developed, will improve the efficiency with which external beam radiation therapy can be planned and delivered, thus lowering costs. [ONCOLOGY 13(Suppl 5):155-168, 1999]


Introduction

Three-dimensional radiation therapy treatment
planning (3DRTTP) systems have spurred the implementation of external
beam radiation therapy techniques, in which the high-dose region can
be conformed much more closely to the cancer patient’s target
volume than was previously possible, thus reducing the volume of
normal tissues receiving a high dose. This form of external beam
irradiation is referred to as three-dimensional conformal radiation
therapy (3DCRT), and its development and clinical use are discussed
in considerable detail elsewhere.[1,2] Several groups have already
reported on their early clinical experience with 3DCRT for the
treatment of prostate cancer, and the results are encouraging.[3-5]

Three-dimensional planning is not just an addition to the current
radiation oncology planning process, but rather represents a radical
change in practice, particularly for the radiation oncologist. The
two-dimensional (2D) treatment planning approach emphasizes the use
of a conventional simulator for designing beam portals, based on
standardized beam arrangement techniques applied to whole classes of
comparable patients. Three-dimensional treatment planning emphasizes
a virtual simulation, image-based approach for objectively defining
tumor and critical structure volumes for the individual patient.

The use of the terms 2D and 3D as descriptors for the planning
process has caused some confusion in the radiation oncology
community. One should recognize that planning the cancer
patient’s treatment is, and always has been, a 3D problem, and
2D planning refers to the process and tools used. Three-dimensional
treatment planning does not require the use of “noncoplanar”
beams, a common misconception. Noncoplanar beams have been used for
years in selected sites such as breast cancer, even though 2D
treatment planning systems could not accurately account for the
geometry. Clearly, noncoplanar breast tangential fields represented
on a 2DRTTP system by a single, or at most a few, slices cannot be
considered a 3D treatment plan. Radiation oncologists will be able to
transition to 3D planning much more easily if they approach 3DRTTP as
a new treatment planning process, emphasizing image-based target
volume design rather than as a reflection of a particular beam configuration.

One of the important factors contributing to the current 3D process
is the standardization of nomenclature, published in the
International Commission on Radiation Units and Measurements (ICRU)
Report 50.[6] This report has provided a language and a way of
thinking about the problem for defining the volume of known tumor,
suspected microscopic spread, and marginal volumes necessary to
account for set-up variations and organ and patient motion. Figure
1
illustrates the ICRU 50 formal definitions for the gross tumor
volume, clinical target volume, planning target volume, and organs at
risk. Some brief discussion on the use of the ICRU 50 methodology is
presented in a later section. Also, the reader is referred to the
literature for more details regarding the use of ICRU 50 nomenclature.[7,8]

Table 1 lists the various tasks
that make up the 3D planning dose and delivery process in its current
technology state. 3DCRT treatment plans generally use an increased
number of radiation beams that are angled and shaped to conform to
the planning target volume using the 3DRTTP system’s
beam’s-eye view and room’s-eye view displays (Figures
2A and 2B
). To improve the conformity of the dose distribution,
conventional beam modifiers (eg, wedges or compensating filters) are
sometimes used. This form of 3DCRT must now be referred to as
“traditional” or “conventional” 3DCRT, because a
more advanced form of 3DCRT, called intensity-modulated radiation
therapy, is already emerging. Intensity-modulated radiation therapy
can achieve even greater conformity by optimally modulating the
radiation beam intensity (fluence) throughout each treatment field.[9]

These advances in radiation oncology technology are truly exciting
and are occurring at a very rapid pace. However, as the
implementation of 3D planning and dose delivery systems becomes more
widespread, the radiation oncology team must understand that ensuring
the safety and accuracy of this new modality is more difficult (in
its current development state) than with the standard 2D process.
Therefore, it is essential that the radiation oncology team stay well
informed on the new technologies and maintain a strong commitment to
a rigorous quality assurance program when this new treatment modality
is implemented in the clinic. This article will update a previous
review of advances in 3DRTTP and dose delivery.[10]

Advances In 3D Treatment Planning

Three-dimensional RTTP systems are currently going through a rapid
transition period, having emerged from university-developed systems
to Food and Drug Administration-approved commercially available
systems in the early 1990s. These first-generation commercial systems
provide specific functionality, such as virtual simulation or 3D
external beam dose planning. Such limited functionality has resulted
in clinics needing multiple treatment planning systems to provide for
various high-tech treatment modalities, like virtual simulation, 3D
external beam dose planning, high-dose-rate brachytherapy, prostate
seed implant, and stereotactic radiosurgery. This is clearly not very
satisfactory, resulting in increased costs and inefficiencies.

Over the next few years, the treatment planning system manufacturers
will begin to combine these planning programs into a single
integrated image-based 3DRTTP system that provides planning
capability for multiple treatment modalities, resulting in
considerable cost savings. In addition, we should soon see improved
efficiencies based on the use of “intranets.” An intranet
is a special type of internet-related technology with the potential
to dramatically improve a radiation oncology clinic’s ability to
use 3DRTTP. A radiation oncology clinic’s intranet will make use
of client server (and browser) technology that will provide
easy-to-use Windows-like displays, permitting users to easily
navigate through multiple treatment planning and other clinical
applications, data, and graphical displays using point-and-click
technology. However, prior to further discussion on information
technology, it is important to describe some of the improved features
now readily available on 3DRTTP systems.

Imaging for 3D Planning

The development of powerful but relatively inexpensive computer
systems has allowed the integration of computed tomography
(CT)–based target volume and normal tissue definition with the
process of radiation therapy treatment planning, creating an entirely
new device, the CT simulator. The CT simulator differs in both
purpose and function from the conventional simulator.[11,12] A
typical CT simulation uses a helical CT scanner with localization
software, a patient registration and laser marking system, and a
dedicated computer workstation for virtual simulation and generation
of digitally reconstructed radiographs.

Although the current generation of CT simulators provides powerful
new 3D capabilities, several improvements in the hardware are still
needed for treatment planning purposes because the CT scanner
currently used was designed for diagnostic radiology use.
Specifically, a larger CT gantry aperture is needed. Current scanners
have a 70-cm diameter, which limits some treatment set-ups. Also, the
CT reconstruction size needs to be larger (currently only a 52-cm
diameter is provided). CT simulator couch tops that mimic the
geometry of a medical linear accelerator are also needed, as are
improved registration systems for going from the real patient to the
virtual patient and then back to the real patient for treatment.
Other issues that continue to be of concern are data storage and data
transfer. Also, a satisfactory process for quantitating the effects
of respiratory organ motion or other organ movements has not yet been
developed for CT simulation.

For effective use of CT simulation, the radiation oncologist must
become familiar with CT cross-sectional anatomy and with the
appropriate use of CT contrast materials that aid in the
identification of the gross tumor volume and organs at risk. When CT
simulation/3DRTTP is first introduced in the clinic, assistance from
a diagnostic radiologist in contouring the organs at risk and gross
tumor volume/clinical target volumes can be invaluable. However,
identifying normal tissues and tumor on a treatment-planning CT can
be difficult even for an experienced diagnostic radiologist, as the
planning CT images are typically acquired in a nonstandard diagnostic
patient position. Since 3D planning requires the definition of target
volumes and normal tissue on a CT scan, strong consideration should
be given to incorporating image-based, cross-sectional anatomy
training into radiation oncology residency training programs. In the
interim, short courses on image-based anatomy are needed for
practicing radiation oncologists.

While CT is the principal source of image data for 3DRTTP, there is a
growing demand to incorporate the complementary information available
from magnetic resonance imaging (MRI). The sharply demonstrated
tumor–soft-tissue interface often seen on an MRI scan in tumors
such as in the brain can be used to better define the gross tumor
volume.[13] Several groups have also demonstrated the value of MRI in
distinguishing the prostate gland from surrounding normal
structures.[14,15] In addition, functional imaging modalities such as
single photon emission computed tomography (SPECT) and positron
emission tomography (PET) will likely prove to be important in both
the target definition phase of treatment planning and also in the
follow-up studies needed to assess efficacy. For example, Marks et al
have reported on the use of SPECT lung perfusion scans to determine
functioning regions of the lung.[16,17] The functional lung volume
data are used in calculating dose-volume histograms rather than the
CT-defined anatomy and are referred to as functional dose-volume
histograms. Radiation beams are planned that minimize irradiation of
these functioning areas. Similarly, PET imaging is used to aid in
defining the patient’s lung cancer gross tumor volume/clinical
target volumes. However, use of multimodality imaging in the
treatment planning process brings with it the need for better image
registration tools.

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