Volumetric Modulated Arc Therapy 1
Volumetric Modulated Arc Therapy: Benefiting Patients
Samantha L. Read
English 289
5 June 2009
Volumetric Modulated Arc Therapy (VMAT) is an improved radiation cancer
treatment that not only includes old methods, but also improves them by increasing the
amount of healthy tissue spared and decreasing the time it takes to treat patients. The
old methods of treatment are not bad, but in the field of cancer treatment, it is important
to always be improving methods in order to benefit the patient. It is necessary to stay
up-to-date on the latest cancer treatment technology. VMAT technology should be
utilized because it increases conformal dose to tumor site, decreases damage to
healthy tissue and organs at risk, and decreases the time it takes to treat patients,
which increases the number of patients therapists can treat; it increases the benefits to
patients without making the planning and simulation more difficult for oncologists,
dosimetrists, and radiation therapists.
VMAT is a new and advanced form of
Intensity Modulated Radiation Therapy (IMRT)
treatment where the dose volume is delivered
during a single 360° gantry arc.1-2 The picture to
the left demonstrates the continuous rotation of
Figure 1: This picture demonstrates
the continuous 360° rotation of the
linear accelerator around a patient
during treatment.3
the gantry during a VMAT treatment. The target
is continuously irradiated while the source of
radiation rotates around it in a single or multiple arcs. In traditional IMRT, patients are
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treated from only a few select angles, causing a longer treatment time and increased
radiation exposure to healthy tissue.2 If you
treat from many angles you can use a lower
energy to destroy the tumor, which lowers the
amount of radiation healthy tissue is receiving,
but treating multiple angles using IMRT can
take a significant amount of time, which may
cause the patient to become restless and
move off their alignment marks and waste
more time realigning them or cause harm to
Figure 2: This picture demonstrates a 5field IMRT, which allows high doses of
radiation to conform to tumor cells while
sparing normal surrounding tissues.4
healthy tissue that may not have been at risk to begin with. The picture to the right is an
example of a 5-field IMRT treatment; while IMRT is the more widely used form of
radiation therapy because of its accuracy, VMAT will allow more fields to be treated in a
single gantry arc, rather than having to adjust the gantry angle to treat multiple fields, it
will also be more accurate in delivering radiation to the tumor while sparing healthy
tissue, and it will lower the amount of time it takes to treat a patient.
IMRT is not the only radiation treatment that VMAT can be compared to. VMAT
also shares similarities with three-dimensional conformal radiotherapy (3D-CRT). The
VMAT technique is similar to 3D-CRT because they both utilize a 360° beam rotation,
but VMAT is different because the dose is delivered to the entire planning target volume
(PTV) in a single gantry rotation and 3D-CRT delivers treatment in a slice by slice
maneuver, which also takes a considerable amount of time.5 Using VMAT, which
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utilizes a whole 360° rotation would lower the amount of radiation exposed to
healthy tissue and decrease treatment time dramatically.
During a clinical investigation
performed by David Palma, M.D., Emily
Vollans, M.Sc., Kerry James, R.T.T.,
Sandy Nakano, R.T.T., Vitali Moiseenko,
Ph.D., Richard Shaffer, F.R.C.R., Michael
McKenzie, F.R.C.P.C., James Morris,
F.R.C.P.C., and Karl Otto, Ph.D., constant
dose rate (cdr-VMAT) and variable dose
rate (vdr-VMAT) for the delivery of
prostate radiotherapy was compared to
IMRT and 3D-CRT.5 Using these four
forms of treatment, plans were created for
ten computed tomography data sets from
Figure 3: Field arrangements for (a) 3DCRT, (b) five-field IMRT, and (c) VMAT
with a circular field that indicates
continuous 360° arc of delivery, with
each intersecting line indicating angle of
optimization. Planning target volume
indicated in red.5
patients undergoing radiotherapy for
prostate cancer. During the investigation,
several important factors were evaluated,
doses to organs at risk, equivalent uniform doses, dose homogeneity and
conformality, and monitor units required for a 2-Gy fraction.5 The results of this
investigation showed that both forms of VMAT and IMRT resulted in lower doses
to normal critical structures than 3D-CRT plans for nearly all dosimetric endpoints
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analyzed.5 The vdr-VMAT was the better technique out of the four; it resulted in
more favorable dose distributions than the IMRT or cdr-VMAT techniques, and
reduced the monitor units required compared with IMRT.5 The picture on the
previous page provides a visual example of the conformality of the VMAT beam
compared to 3D-CRT and IMRT. Thus far, results have shown that treatment
plans generated with VMAT optimization have dose distributions that are
equivalent to or superior to a commercial static gantry IMRT system.1
VMAT is a new method of operating a linear accelerator that requires new
methods of commissioning and quality assurance.6 James L. Bedford, Ph. D., and Alan
P. Warrington, M. Sc. describe a series of commissioning tests and their outcomes for a
commercial linear accelerator.6 Although these tests were performed for the Elekta
PreciseBeam VMAT with MLCi
and Beam Modulator heads, they
can be applied to any
manufacturer’s linear accelerator.
The picture to the right is the
machine used to perform the
tests, which tested for beam
flatness and symmetry at the
variable dose rates required for
Figure 4: This is the linear accelerator used to perform
the commissioning tests.6
VMAT were performed, multileaf collimator (MLC) calibration was investigated using
dynamic prescriptions, rotational accuracy was evaluated using dynamic prescriptions
which required accurate correlated motion of both gantry and MLC leaves, and
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measured and calculated dose distributions for complete VMAT treatment plans were
compared and evaluated.6 Beam symmetry was found to be better than VMAT’s
predecessors, with dose rates down to 75 MU/min.6 MLC calibration provided better
continuity of dose at match planes, tests for rotational accuracy showed uniformity of
peripheral dose, and a two-arc prostate case showed an absolute lower dose
difference.6 The results of these tests concluded that VMAT has been successfully
commissioned and has been introduced into clinical use.
There are a few options for cancer treatment centers to choose from after
decided to utilize VMAT technology. Depending on how new their current model is, they
can choose to simply upgrade their linear accelerator or have a brand new machine
installed.7 There are downsides to both of these options. Both options will shut down
the machine that is being upgraded or replaced for a period of time, making it difficult or
impossible to treat patients, especially if the center only operates one machine. 7
Obviously, upgrading a linear accelerator will be cheaper than having a new one
installed, however both choices run in the million dollar range.7 Companies such as
Varian Medical Systems and Elekta Ltd. manufacture linear accelerators and each have
their own form of VMAT. Most cancer patients are not able to pay for radiation
treatment out of pocket, therefore patients who are currently being treated with IMRT or
3D-CRT may be concerned with an increase in cost to them if their cancer center
upgrades to VMAT, however if their insurance is currently covering their treatment then
it should not be an issue.7 Although cost is always an issue, the benefits of VMAT
outweigh the downfalls.
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VMAT is worth the cost because of the huge advantages it provides to the cancer
patients. In the past, advances in dose distribution quality have come at the cost of
longer treatment times; longer treatment times require patients to lie in positions that
may not be comfortable, for extended amounts of time.1 This has been the case for
fixed gantry IMRT. One of the best features of VMAT is that high-quality dose
distributions can be delivered in less time than IMRT and 3D-CRT.1 With VMAT
radiation delivery times have been reduced from 15-30 minute appointments down to
1.5-3 minutes for a 200 cGy fraction. VMAT is also extremely more efficient than IMRT
and 3D-CRT because the entire three-dimensional dose distribution is created in a
single rotation.1 Another advantage of VMAT is the continuous MLC leaf positioning
provides more precision in beam aperture shaping.1 This leads to an extremely
conformal beam of radiation, that is easier to manipulate in order to avoid healthy tissue
and organs at risk. VMAT also has the ability of reducing the number of monitor units
(MU), which has the dual benefit of a lower treatment time and less total body scatter
dose.1 Less total body scatter dose means less radiation exposure to healthy tissue
and organs at risk. These advantages alone are more than enough reason to upgrade
current linear accelerators in order to provide the best care possible for cancer patients.
As time goes on, more cancer centers will begin to upgrade or install VMAT
systems. Cancer patients deserve to be treated with the best radiation units on the
market. By decreasing their treatment times, conforming the radiation beam to the
tumor, which spares their healthy tissue and organs at risk, and lowering the monitor
units used to treat, they will be given a better quality of life. Patient care is the number
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one priority of oncologists, dosimetrists, and radiation therapists; by utilizing VMAT
technology providing this care is made easy.
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References
1. Otto K. Volumetric modulated arc therapy: IMRT in a single gantry arc. Med. Phys.
January 2008; 35:310-317.
2. Nucletron, RaySearch extend collaboration. Medical Device Daily. 2009;13:7-7.
3. Imaging Technology News. Elekta’s Infinity Delivers VMAT. Updated July 2008.
http://www.itnonline.net/node/21845/. Accessed June 1, 2009.
4. Advanced Oncology Services. Advanced Oncology Center, Inc. 2009. Available at
http://www.aoctr.com/Services.html. Accessed April 23, 2009.
5. Palma D, Vollans E, James K, Nakano S, Moiseenko V, Shaffer R, McKenzie M,
Morris J, Otto K. Volumetric Modulated Arc Therapy for Delivery of Prostate
Radiotherapy: Comparison with Intensity-Modulated Radiotherapy and ThreeDimensional Conformal Radiotherapy. Int. J. Radiation Oncology Biol. Phys. 2008;
72:996-1001.
6. Bedford JL, Warrington AP. Commissioning of Volumetric Modulated Arc Therapy
(VMAT). Int. J. Radiation Oncology Biol. Phys. 2009; 73:537-545.
7. R.J. Read, R. Read [richard.read@varian.com], email, May 25, 2009.