Advances in Radiation Therapy
Daniel Low Professor, Department of Radiation Oncology and Director, Division of Medical Physics,
Washington University School of Medicine
Radiation therapy has made significant advances over
the past two decades. The beginning of the 1980s saw
radiation therapy as a prescriptive medicine.Treatments
using megavoltage X-rays were planned using the crude
imaging tools available in the day. Radiation portals
were typically defined using radiographic planar
imaging, where only bony anatomy and large air
cavities were visualized. The radiation oncologist used
previous experience based on the tumor type, location,
and extent (determined using planar imaging and
palpation), to define the portals. Significant
customization of the radiation portals based on a
quantitative understanding of the patient’s tumor
geometry was not possible due to the poor access to
three-dimensional (3-D) imaging modalities.
The advent of broader access to computed tomography
(CT) and computing technology led to the first
technological revolution in the 1980s. 3-D radiotherapy
treatment planning (3DRTP), where the tumor is
defined in 3-D using CT scans and the radiation portals
are designed based on the 3-D tumor and normal organ
shapes, was introduced and made rapid inroads into
radiation therapy practice. The use of 3-D imaging
allowed the radiation oncologist to customize the
radiation dose distribution to the patient’s specific
tumor and normal organ geometry. Studies showed that
in many cases, the earlier (called the 2-D approach)
often underestimated the required beam portal size,
causing underdosing of the tumor.
3DRTP also allowed a quantitative understanding of
the radiation dose to critical structures. Physicians were
provided with feedback on the radiation dose
throughout the normal organs, and dose summary
tools were invented that enabled the physicians to
efficiently determine if the doses were within clinically
acceptable limits.
3DRTP also allowed a quantitative understanding of
the radiation dose to critical structures. Physicians were
provided with feedback on the radiation dose
throughout the normal organs, and dose summary
tools were invented that enabled the physicians to
efficiently determine if the doses were within clinically
acceptable limits.
As an example, the position of the prostate depends
greatly on the fill status of the bladder and rectum.A CT
acquired with a full rectum will show the prostate in a
more anterior position than when the rectum is empty.
Consequently, the treatment plan will be designed with
the dose more anterior than the average tumor position
and a ‘geometric miss’ could occur. Other similar tumorpositioning
errors included patient positioning
reproducibility and gradual tumor shrinking or tissue
swelling that modified the size, shape, and position of the
tumor. In order to combat these limitations, the concept
of a tumor margin was defined. A margin was a 3-D
expansion of the imaged tumor (gross tumor volume
(GTV)) that was sufficiently large that the tumor would
be positioned within the margin surface on a daily basis.
The radiation dose was designed to encompass the
margin volume (planning target volume (PTV)) rather
than the imaged tumor volume. While this presumably
guaranteed full irradiation of the tumor, it also led to
treatment portals that irradiated more normal tissues than
necessary given the tumor size and shape.
Secondly, the CT image could not show the entire
extent of the tumor. Tumors typically invade with
microscopic extensions, often infiltrating many
centimeters from the bulky disease. The microscopic
extensions cannot be visualized using CT imaging, so a
method for identifying the suspected occult disease
volume was developed, termed the clinical target volume
(CTV). By definition, the CTV encompassed suspected
tumor cells and its design was therefore less quantitative
and required more clinical judgment to define than the
GTV. Because the tumor burden was less in the CTV
than the GTV, the dose required to sterilize the tumor in
the CTV was assumed to be less than in the GTV. CTV
doses were therefore often smaller than the GTV doses.
The PTV margin concept was also applied to the CTV.
The second technological revolution occurred from
treatment planning simulation studies conducted in the
late 1980s. During that time, scientists identified the fact
that the radiation dose distributions could be made
significantly more conformal if the radiation fluence
(intensity) from each beam could be optimized using
software-based algorithms. Luckily, the linear accelerators of the day were capable of delivering these
optimized fluence patterns.This technology was labeled
intensity modulated radiation therapy (IMRT), and it
allowed the design of very conformal dose distributions.
IMRT provided for the capability of irradiating tumors
while sparing nearly surrounding normal organs.
