Immobilization Systems

New teletherapy techniques like Intensity Modulated Radiation Therapy (IMRT) facilitate gradients of doses with very small margins, in other words, the dose of radiation given in two different sites separated for 5mm may vary in a quite radical way, obtaining differences up to the 50% or more. For this reason, the daily positioning and localization of the patient plays a crucial role, therefore all tools and immobilization devices available in the oncologic center to minimize all possibilities of imprecision are of vital interest to achieve the highest quality in treatments.

ALIGNMENT SYSTEMS

In order to reproduce immobilization systems properly it is helpful to use at least a system of 3 reference alignment points (coordinates) for properly position the patient.

• The first system of coordinates by axes is defined in the treatment room and it is related to the gantry and the collimator rotation axes and it is kept identified by a laser beam system.
  

The visualization of the laser alignment system is identified by the blue arrows.

• The second system of coordinates is somehow arbitrary but it is and has been the conventional tool to verify the correct position of the patient according to the simulation images. This system defines the anatomical position of the body section to be treated; this system is customary recognized by marks on the skin of the patient (tattoos), and/or in the immobilization device. The correct alignment of these systems is the standard technique.

• The third alignment system refers to the alignment coordinates of the volume to treat; this is the most difficult system to establish in a customary way during the development of treatment, examples of how a routinary development is established is through an alignment system using ultrasound, setting radio-opaque markers or with tomography when placing the patient. The use of these alignment systems is called Image Guided Radiotherapy.

Treatment Isocenter

IMMOBILIZATION SYSTEMS

As it has been previously mentioned, the daily positioning of the patient is a crucial point in the success of any radiation treatment and more importantly when more modern techniques like IMRT, where treatment fields tend to be smaller, are applied. For this purpose individualized protections are performed with the multileaf collimator, then the dose is scaled by fraction and totally and major dose gradients are granted in one very volume.

A very easy way to assure this fact is by individualizing the patient's collocation bracket and by using other rigid or semi-rigid systems which can be placed and replaced before every radiotherapy session. There's no doubt that a wide variety of systems have been developed aiming this purpose, we will only mention some of them and more importantly you'll be able to find them available in our Louis y Lucille Grado Radiotherapy Center.

• Thermoplastic Masks: Plastic material, windowed, malleable when heated on bain-marie, allows adaptation of the patient’s body. This device can be fixed to flat surfaces to assure the proper immobilization of the patient, in few minutes when it becomes rigid again keeps shape and immobilizes small parts of the body of the patient like the face, neck, limbs, etc. There are bigger sizes to immobilize pelvis and abdomen
  
• Individualized Vacuum Sealed Bags: In this case this devices are hermetically sealed bags which are filled by small Styrofoam balls. It is placed on the treatment board and then the patient lays on it performing the proper position for treatment, after this a special vacuum machine is used to extract al the air in the bag and finally it seals it. The final result is a crushproof mould which will always be used to place the patient on it before each session.


When these two systems are combined for fixation, the proper daily reproducibility of the patient's collocation is guaranteed.

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