Intensity-modulated radiotherapy (IMRT) - KCE

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Intensity-modulated radiotherapy (IMRT) - KCE

6 Intensity-modulated radiotherapy KCE reports 62

For verification purposes the patient’s plan can be applied to a CT study of a phantom,

in which dose measurements can be made using ion chambers and/or film. Compared

with conventional radiotherapy in vivo dosimetry for IMRT is more complex and still a

challenge to perform.

The dose distribution within the target can be made more homogeneous using IMRT,

but inhomogeneity will often be observed due to the overriding need to protect organsat-risk

and limitations of the planning systems. On the other hand inhomogeneity can be

the aim as in ongoing IMRT research targeting an increased dose to specific tumour

area’s. In theory, IMRT also allows for a reduction in the margin for dose fall-off at the

beam edges (“penumbra”) by the use of compensating rinds of increased beam intensity.

2.3 DELIVERY TECHNIQUES

IMRT can be produced through numerous delivery methods.

1. Fixed gantry during irradiation, adding different sub-multileaf collimator

(MLC) fields to each field (multiple static field or step-and-shoot MLC

technique)

2. Fixed gantry, changing the dwell time for each MLC leaf during a treated field

by moving the MLC leaves with the radiation on (dynamic MLC technique)

3. Moving gantry with the treatment beam on, using an arcing or tomotherapy

(serial or spiral delivery) method with dynamic collimation.

In MLC-based IMRT the orientations of the multiple beams still have to be manually preselected,

while in fully rotational approaches such as tomotherapy, individual beams do

not exist, nor the possibility to select beam angles. In the future it is expected all

radiation treatment delivery machines will be optimized to also deliver IMRT. Ongoing

product enhancements by accelerator vendors (Varian, Elekta, TomoTherapy, Siemens)

and treatment planning companies should lead to improvements in efficiency in planning

and delivery, safety (less radiation leakage) and quality assurance.

The U.S. Food and Drug Administration (FDA) has approved a number of medical

charged-particle radiation therapy system devices and radiation therapy treatment

planning system devices. A few examples include: the TomoTherapy Hi•Art System®

(TomoTherapy Inc., Madison, WI); the Peacock System (NOMOS Corp., Sewickley,

PA); and SmartBeam IMRT (Varian Medical Systems, Inc. Palo Alto, CA).

An instrument which cannot be classified under IMRT but shows some similarities is the

Cyberknife (Accuray, Sunnyvale, CA). This system is used for stereotactic radiosurgery

of intracranial and extracranial tumours.

2.4 PRECAUTIONS

The process of IMRT implementation and delivery remains complex. It requires a much

expanded emphasis on quality assurance procedures to guarantee its proper

implementation. In the US and Europe, the evolution is towards more and more

radiotherapy departments with limited physics and dosimetry support starting IMRT.

The possibility that patient safety will be compromised is of great concern. 3 Training of

physicists and dosimetrists is essential in this regard. Using inverse planning for IMRT

will not guarantee an optimum treatment plan. It has been recommended to assess the

difference between dose-volume histograms obtained after planning optimisation and

the final calculation used for dose delivery which take into account the optimization of

the apertures. 3 The issue is compounded by the multitude of combinations possible of

inverse planning approaches and dose delivery methods, each requiring their own

quality assurance procedures. Also error free data communication between systems

requires attention, since information transfer is a common source of treatment error. It

has been advised to start with a single technique in routine practice. 3

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