IMRT Techniques

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IMRT allows for the radiation dose to conform more precisely to the three-dimensional (3-D) shape of the tumour by modulating - or controlling - the intensity of the radiation beam in multiple small volumes. IMRT also allows higher radiation doses to be focused to regions within the tumour while minimizing the dose to surrounding normal critical structures. Typically, combinations of multiple intensity-modulated fields coming from different beam directions produce a custom tailored radiation dose. During the IMRT use the inverse planning techniques, which make the optimisation of dose distribution with special calculation algorithms. In this way dose conformity can increasing and minimizing the dose to adjacent normal tissues. IMRT is the delivery of radiation to the patient via fields that have non-uniform radiation fluence. With this technique the concave PTV with surround normal tissue (prostate, head and neck) also can be irradiated.

IMRT techniques

 
IMRT can be applied in several ways. The following table summarized the different technical solutions.

Compensator is the beam modifying device which equalizes the skin surface contours, while retaining the skin-sparing effect. The material of compensator is lightweight metal, for example aluminium. Customized compensators are shaped to attenuate the open field photon fluence such that the transmitted fluence map is as designed by the dose optimisation algorithms. Disadvantages for compensator used IMRT are:
- Radiation therapist need to go into the treatment room and exchange customized compensators between treatment fields.
- Preparation of compensators is very labour-intensive.

Figure 1.: Intensity modulation with compensator: schematic diagram.

Figure 2: Intensity modulation with compensator: geometrical shape in 3D.

 
IMRT Delivery Methods Using Conventional MLC’s can be realised in several ways. The proposed IMRT delivery methods include:
– “Step-and-shoot” static IMRT using multiple MLC shapes per field (SMLC-IMRT) – includes both forward and inverse planning examples
– Dynamic IMRT with fixed gantry and moving MLC leaves (DMLC-IMRT) - includes both fully dynamic and pseudo-dynamic
– Intensity-modulated arc therapy using a full field MLC (IMAT)

“Step and Shoot” SMLC-IMRT
In the case of step and shoot techniques large number of discreet MLC shapes per field are used and the beam is turned off between segments.
• Advantages of this method of IMRT: portal verification of intensity pattern feasible, easy to understand clinically, easy to resume interrupted treatment, relatively simple accelerator control system needed, both forward and inverse planning possible
• Disadvantages of this method: complex problems require lots of segments, time required for treatment can be significant

Figure 3.: Field segments for „step and shoot” IMRT technique use conventional MLC.

Figure 4.: 2D dose intensity map receive as a result of dose distribution of all field segments.

 
Truly dynamic IMRT modulated leaf velocity with constant beam intensity - sliding window approach.
Advantages of this method of IMRT: faster beam delivery than static methods, can yield more complex dose distributions and can produce smoothly varying intensities.
Disadvantages of this method: leaf edge effects can produce dosimetric errors when field sizes are small, difficult to verify leaf patterns, potentially more difficult to recover from interruption, complex MLC control system required, inverse planning required.

Intensity Modulated Arc Therapy (IMAT) use multiple arcs delivering single intensity level, each arc consists of multiple MLC subfields - continuously changed while gantry rotates.

Figure 5.: Position of opposing moving MLC and the dose intensity map for dynamic IMRT

Figure 6.: Intensity Modulated Arc Therapy with different MLC shape for different gantry angle.

 

A special type of intensity modulated arc therapy can be used for tomotherapy. A conventional linear accelerator is equipped with a special binary collimator system (Peacock MIMiC, Nomos Corp.). The MiMiC collimator consists of a long transverse slit aperture provided with two banks of 20 leaves of each. Each leaf can be moved independently and each bank can therefore treat 1-2 cm-thick slice slices of tissue 20 cm in diameter, the couch is moved to treat the adjacent slice.
Helical tomotherapy: the linac head and gantry rotate while the patient is translated the doughnut-shaped aperture in a manner analogous to a helical CT scanner. The unit is also equipped with a diagnostic CT scanner for target localisation and treatment planning. The intensity modulation of the beam is created by collimator system like MiMiC (Figure 7.).

Figure 7.: TomoTherapy and binary collimator system

 
Cyberknife system: rotation robot arm with 6 axes, mounted on the Robot is a compact X-band linac that produces 6MV X-ray radiation. The radiation is collimated using fixed tungsten collimators (diameter: 5 - 60 mm) which produce circular radiation fields.The robotic mounting allows very fast repositioning of the source, which enables the system to deliver radiation from many different directions without the need to move both the patient and source as required by current gantry configurations.

Figure 8.: Cyberknife: rotation robot arm with 6 axes is used.

 
The race-track microtron (RTM) is an accelerator with beam recirculation which best suits applications with relatively high beam energy and low current. Due to the principle of its operation the output beam has very low energy dispersion (narrow spectrum) and low contribution of uncontrolled dark currents. These features make possible a good control of the output beam characteristics and delivered dose.
Proton therapy is a type of external beam radiotherapy using ionizing radiation. Due to their relatively large mass, protons have little lateral sidescatter in the tissue; the beam does not broaden much, stays focused on the tumour shape and delivers only low-dose side-effects to surrounding tissue. The accelerators used for proton therapy typically produce protons with energies in the range of 70 to 250 MeV. In most treatments, protons of different energies with Bragg peaks at different depths are applied to treat the entire tumour.The total radiation dosage of the protons is called the Spread-Out Bragg Peak. In scanning-beam techniques, magnets deflect and steer the proton beam. Under computer control, a narrow mono-energetic beam paints the treatment volume, voxel-by-voxel, in successive layers. The depth of penetration of the Bragg peak is adjusted by varying the energy of the beam before it enters the nozzle (Figure 9.).

Figure 9.

 
Comment: Ionnyaláb = ion beam; dipól mágnesek = dipole magnets; vízszintes szkennelés = horisontal scanning; függőleges szkennelés = vertical scanning; első réteg = first layer; utolsó réteg = last layer; daganat = tumour