Published data for this review have been identified by careful search of the major journals covering this subject, including Physics in Medicine and Biology, the British Journal of Radiology, Medical Physics, Radiotherapy and Oncology, the International Journal of Radiation, Oncology, Biology and Physics and other journals referenced therein. The article is also based on extraction of key data and material from my three books (references 1, 2, 29) and references therein.
ReviewAdvances in three-dimensional conformal radiation therapy physics with intensity modulation
Section snippets
Intensity-modulated radiation therapy
The development of imrt has had the most significant impact on improving the physical basis of radiation therapy during the past 5 to 10 years. imrt has become a mantra for the late 20th century. At any conference on radiotherapy or the physics of radiotherapy, 30–40% of papers will cover some aspect of imrt. This is because it offers the potential for substantial improvement in tumour control probability, at fixed normal tissue complication probability, thanks to its ability to sculpt
The multiple-static-field technique
The MSF technique relies on the observation that any two-dimensional intensity modulation can be created by a superposition of open fields with different field shapes. The field shapes are conveniently defined by a multileaf collimator. The principle is shown in figure 3 for a one-dimensional intensity-modulated beam. This is created by sequentially delivering open fields of different widths, where each field is formed by one leaf pair of the multileaf collimator. This can be done independently
The dynamic multileaf collimator technique
The DMLC technique to deliver imrt is a related method to that described above. As its name suggests, it makes use of a multileaf collimator, attached to a linear accelerator in which the field is defined by a series of leaves or fingers of attenuating lead. Here, the leading leaves start moving ahead of the trailing leaves, exposing the field to radiation. Later, this is followed by the commencement of movement of the left leaves. At some given time during the irradiation, the right and left
Tomotherapy
Tomotherapy is an entirely different way of delivering imrt. It does not make any use of the conventional multileaf collimator. Instead, the radiation is collimated to a narrow fan and the fan beam rotates isocentrically around the patient. As it does so, the one-dimensional intensity modulation is created and continually modified by shooting vanes of attenuating lead into the aperture at right angles to the direction of the slit. Two versions of the technique have been proposed. One has been
Swept pencil beams
The concept of sweeping a pencil beam of radiation is attractive, since this promises ultimate control over beam delivery. The idea was proposed some time ago for the Racetrack Microtron.25 But, at that time, the beam had a very large full-width-at-half-maximum of many centimetres and so the idea did not seriously rival the other imrt techniques. Recently, it has been suggested that a much finer beam could be generated using an optimised bremsstrahlung source and a short isocentric distance.26
The moving bar technique
A one-dimensional intensity-modulated beam can be created by sweeping an attenuating bar through the field (figure 8). By varying the dwell time of the bar at different locations, the one-dimensional profile receives an intensity modulation. The disadvantage here is that not all modulations are achievable and the modulations are not two-dimensional. Clearly, the sum of the dwell times must not come to more than the total irradiation time. The method therefore works best for shallow modulations,
Intensity-modulated arc therapy
Intensity-modulated arc therapy (IMAT) was invented by Cedric Yu,28 and is based on the observation that any one-dimensional intensity-modulated beam can be created by a set of mlc-shaped fields in an enormous number of ways.13, 14 If we imagine just one two-dimensional slice irradiated by a set of one-dimensional intensity-modulated beams, arranged to be delivered by gantry rotation, the basis of the method is to decompose the one-dimensional intensity modulated beams into multiple
The future of imrt
imrt represents a unique opportunity to improve the physical basis of radiotherapy. It has generated an enormous amount of literature, and seven methods have been proposed and demonstrated, at least in principle. These have been briefly discussed here and detailed reviews can be found elsewhere. Readers may draw their own conclusions about which are likely to succeed clinically.
However, a new proposal has been put forward by this author, namely that the ultimate imrt could be delivered by a
Search strategy and selection criteria
Glossary of terms
- intensity-modulated radiation therapy (imrt)
- the delivery of non-uniform radiation fluence.
- Inverse-planning
- the technique that starts from the required dose distribution and creates the required fluence maps.
- Intersegment deadtime
- the time between segments of a multiple-static field IMRT treatment.
- Organs-at-risk
- tissues to shield during irradiation.
- Tongue-and-groove underdose
- the underdose that sometimes results from passage of radiation through multileaves of partial depth. In the DMLC technique,
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