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Current Techniques in Three-Dimensional CT Simulation and Radiation Treatment Planning

Current Techniques in Three-Dimensional CT Simulation and Radiation Treatment Planning

ABSTRACT: The modern CT simulator is capable of interactive three-dimensional (3D) volumetric treatment planning; this allows radiation oncology departments to operate without conventional x-ray simulators. Treatment planning is performed at the time of virtual simulation by contouring the organs or volumes of interest and determining the isocenter. A digitally reconstructed radiograph (DRR) provides a beam's-eye-view display of the treatment field anatomy and contoured areas of interest. Conformal and noncoplanar teletherapy is facilitated for patients with prostate cancer, lung cancer, and brain tumors. Ongoing developments include 3D dose calculation, dose-volume histogram analysis, and tumor dose escalation. [ONCOLOGY 9(11):1225-1240, 1995]

Introduction

With the advent of high-speed, high-capacity computers, commercial
CT simulators are now available that permit three-dimensional
(3D), volumetric, conformal radiation therapy techniques to be
used on a routine basis in busy, modern radiation therapy departments.
Three-dimensional radiation therapy simulation allows multiple
cross-sectional CT slices to be inputted into the treatment planning
equation as part of a large, complex data set. With these sophisticated
computers, the cross-sectional slices can be synthesized into
a unified 3D volume [1].

Furthermore, interactive 3D treatment planning at the time of
CT simulation provides a graphic display of the 3D anatomy of
both normal tissue and a discrete tumor or tumor-bearing region.
Consequently, complex relationships between structures can be
appreciated within the treatment display; this facilitates
the design of an optimal treatment plan through the rapid scanning
of various treatment beams and field arrays.

Most significantly, the use of noncoplanar beams can now be quickly
assessed and integrated into the treatment plan, and the radiation
oncologist is no longer confined to simple coplanar anterior-posterior(AP) or opposed lateral fields. A third dimension is
readily and clinically available with superior and inferior, as
well as superior-oblique and inferior-oblique, fields. These new
fields may be used to optimize the therapeutic plan and to minimize
the dose to normal tissue structures [2-4].

Another important feature of 3D CT simulation is the ability to
generate a digitally reconstructed radiograph (DRR), or beam's-eye-view
display, which allows for verification of the treatment field
using conventional port film techniques [5]. CT simulation permits
multiplanar reconstruction, so that virtually any planar treatment
field can be calculated using standard two-dimensional (2D) treatment
planning systems.

Three-dimensional treatment planning techniques are now approved
by the FDA, which will facilitate the use of multiple noncoplanar
fields. Three-dimensional treatment planning technology also allows
for dose-volume histogram analysis. This is a more accurate and
optimal way of comparing (in a 3D sense) one treatment plan to
the other, which compares the tumor volume and the volume of normal
tissue that are irradiated by displaying the dose received according
to the percent volume of tissue [6].

This article will briefly describe the 3D CT simulation procedure
and the various types of images it can produce. (Table 1 defines
key terms used in the article.) Clinical results to date with
conformal radiation therapy in various carcinomas, including prostate,
lung, and brain tumors, will also be summarized, as well as recent
technologic developments.

3D CT Simulation Procedure

CT Data Acquisition

Computed tomographic scanning is performed with a 70-cm ring aperture
and flat couch, which is integrated into a computer-based virtual
simulator capable of 3D volumetric reconstruction and multiplanar
reconstruction for treatment planning. A laser isocenter projection
system is mounted in the room; the system combines two fixed and
one movable sagittal laser with a variance of only 2 mm over a
6-month period. Digitally reconstructed radiographs (DRRs) with
superimposed target volume information are generated by the computer
and transferred to a commercially available CT film processor.
Analysis of DRRs reveals acceptable bone detail and anatomic agreement
with port films taken on the treatment unit.

The best quality DRRs are obtained with a nonspiral CT slice thickness
of 2 mm and table increment of 2 mm. A slice thickness of 5 mm
and table increment of 3 mm also yield DRRs of excellent quality.
For larger fields up to 40 cm in length, a 5-mm slice with 5-mm
table increment is used most commonly, and results in lesser detail
but clinically acceptable positioning accuracy and results. For
patients who cannot stay in position for more than 15 to 20 minutes,
a 10-mm slice and 10-mm table increment provide acceptable quality
for treatment of bone metastases and other simple treatment set-ups.
Plastic mask immobilization facilitates brain, head, and neck
positioning and stability.

The average length of CT simulation, including patient positioning;
CT data acquisition with an average of 40 cuts; physician contouring
of the target organ, volume of interest, or isocenter definition;
and projection of the isocenter on the patient, is less than 1
hour. The amount of time that the patient spends on the CT table
is consistently less than with conventional simulators for intact
breast or prostate.

We have developed CT urethrography, which improves definition
of the relationship of the prostate apex to the urogenital diaphragm
and external urethra. An average of three CT simulation procedures
daily, with a maximum of eight, have been performed. The ring
aperture of 70 cm requires special arm positioning for breast
and mantle simulation and treatment. Patients up to 350 pounds
have been simulated on the rated couch.

Treatment Planning

Computed tomographic simulation requires a different perspective
and expectations of simulation. With conventional simulation,
the physician determines the edges of a field, whereas with CT
simulation, the physician contours the treatment volume, and the
computer determines the isocenter of a field. As such, with CT
simulation and interactive 3D treatment planning, the physician
identifies an organ or target of interest, such as the prostate
or a brain tumor, along with a ring of normal tissue to account
for patient positioning and microscopic extension of disease beyond
the enhancing lesion seen on CT scan. When contouring of a discrete
volume is performed, computerized selection of an isocenter can
be performed at the time of virtual simulation and an isocenter
placed into the middle of a 3D volume. In radiation therapy, however,
a discrete organ or tumor nodule is not always treated with a
small field. Therefore, a volume of interest must be defined,
such as the intact breast, a long bone (Figure 1), a lymph node
region, or multiple lymph node regions, such as a mantle field
(Figure 2).

Three-dimensional CT simulation with accurate positioning and
immobilization allows for rapid data acquisition, contouring of
key structures, the selection of an isocenter, either manually
or calculated by the computer, and 3D reconstruction, with the
generation of a DRR in which the target volume is superimposed
on the film. Once the isocenter has been determined, treatment
planning can take minutes or hours, depending on the complexity
desired. Exact field angles can be determined after the patient
has left the simulation room with an isocenter marked on the patient's
skin.

Digitally Reconstructed Radiograph

Using traditional simulation, the radiation oncologist obtains
a radiograph from the region in which radiation is to be given.
The simulator allows a field size with an isocenter to be projected
upon the patient's skin, which can also be drawn on the skin.
Once the radiograph with this information is obtained, specialized
metal (cerrobend) blocking can be produced from the film and custom
shape the radiation beam for daily treatment.

A disadvantage of conventional simulation is that it provides
detail only of bone. In the case of the lung, conventional simulation
can visualize soft-tissue masses if they are within the lung parenchyma.
However, this type of simulation generally does not visualize
mediastinal and hilar involvement as well as does a 3D CT simulation
rendering (Figure 3). When conventional simulation is used for
the abdomen or pelvis, contrast agents may allow the organ to
be delineated more precisely. When treating the brain, the surgeon
occasionally will leave a bone flap, which may help localize the
tumor volume. If the treatment volume cannot be visualized, a
larger field may be required than if CT simulation were utilized.

With a DRR, the contoured tumor volume is superimposed on the
radiograph [7]. This radiograph takes into account beam divergence,
and is an accurate representation of the treatment beam as it
can be imaged with a port film [8]. Traditional simulation allows
for radiographs to be obtained in anterior-posterior (AP), posterior-anterior
(PA), oblique, or lateral directions. Treatment with superior
or inferior fields, such as a vertex field in therapy for brain
tumors (Figure 4) or inferior-oblique fields in therapy for prostate
cancer, is difficult to simulate unless optimized with 3D simulation
(Figures 5A and5B). This therapeutic approach, termed "noncoplanar
therapy," refers to treatment that is given in anything other
than the standard transaxial or CT plane.

Using interactive virtual simulation, beams can now be established
from virtually any direction or angle. This allows critical structures,
such as the spinal cord, kidney, eyes, or liver, to be shielded,
and consequently, higher doses to be delivered to the tumor volume
[9]. While noncoplanar therapy allows for the use of multiple
beams (in fact, there has been a tendency for radiation oncologists
to use multiple beams to treat a lesion), we have been surprised
at the ability of just three noncoplanar fields to dramatically
reduce doses to crucial structures. An example is opposed lateral
and anterior-inferior oblique fields for treating the prostate.
Using dose-volume histograms and comparing a three-field set-up
with a six-field coplanar set-up, one can achieve a dramatic reduction
in the rectal dose with the use of an anterior-inferior field
(see "Dose-Volume Histogram" below).

3D Dose Calculations

Using 3D CT simulation, it is possible to do multiplanar reconstruction
through virtually any plane of treatment. In this way, it is possible
to obtain very good dose calculation using standard 2D treatment
planning systems that are FDA approved and have a proven track
record for clinical use. With currently available 3D treatment
planning systems, one can look through the entire volume being
treated and plan the superior and inferior aspects, rather than
just the mid-plane dose calculation that a 2D system facilitates
for noncoplanar treatment. Most 3D planning systems still display
the isodoses on a slice through the treatment plane, although
3D isodose renderings are now available in which an isodose can
be selected; ie, if a portion of the tumor volume is being underdosed,
it projects through the 3D rendering of the isodose [10].

Dose-Volume Histogram

Dose-volume histograms display the dose received according to
the percent volume of tissue and can be generated by the computer
looking at the target volume and comparing it to tissues that
are at risk and that are also contoured at the time of treatment
planning [11]. Three-dimensional treatment planning allows for
volumetric analysis of the area being treated, and as such, can
aid in field design that may be difficult to visualize at the
time of virtual simulation [12,13]. A dose-volume histogram can
compare a six-field coplanar prostate set-up with a three-field
noncoplanar prostate set-up and reveal a significant reduction
in dose to a portion of the rectal volume (Figures 6A and 6B).

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