Image Guided Radio
Therapy (IGRT)
The main problem with IMRT is the geometrical uncertainty related to the position and tumor morphology as well as the adjacent organs in risk in every session of the treatment; any error in this process would result in possible sub-dosage of the defined volume treatment (GTV, CTV and PTV) and/or an over-dosage of normal adjacent tissues.
For this reason Image Guided Radiotherapy techniques have been introduced to recent treatments. Their goal is to identify anatomical structures through volumetric images during treatment, where 2 independent units are installed, CT (image) and Linear Accelerator (treatment); they use a treatment board or the “cone-beam” which orthogonally incorporates a CT image in relation to the linear accelerator, as it is shown in the following image.
ELEKTA Synergy Image Accelerator
Patients and their organs move. This means that tumors also move. This motion may cause radiation to be given out of the target zone. The newest method to evaluate and correct this disarrangement is IGRT. Rearrangement is done by observing the tumor with a CT diagnosis prior to radiotherapy. If the tumor has moved then it’s necessary to redesign the radiation field. This is called Adaptive Radiotherapy.
Hence we can define IGRT like a technique where images are daily taken in order to define where and how the target volume is for each patient along all the treatment, this offers much more precise results.
Traditional radiotherapy works with static radiographies taken on the simulation day which happens some days prior the treatment starts, irradiating the zone but being unable to notice the tumor’s daily variations. All these deficiencies are dismissed with the use of this new technique because all the patient’s characteristics or temporary variations are taken into account, this is that once the patient gets on the treatment board and adopts a determinate position images of the target zone are taken and then verified so that they coincide with the plan of treatment. If they do, then the step to follow is to irradiate as planned, otherwise the plan must be redesigned or adapted to the current situation.
The possibility to adapt and direct the radiation beam in function of the images that are being obtained causes that the administered dose is so similar to the planned dose which allows a very effective treatment with the least toxic side effect.
Different image techniques can be used according to each patient’s characteristics or in case we need a very precise guidance because the patient is too unstable (he/she moves a lot or the tumor is in a quite complicated zone), we can take a CT everyday or as often as it is needed. To treat very stable patients maybe a traditional radiography is enough, but for intermediate patients, fluoroscopy or ultrasound might be the best options.
In 1993, Mackie and cols4 introduced the radiation unit called Tomo-Therapy Inc. HI-ART (Madison, WI), or Helicoidal Tomotherapy unit which for the first time integrates a guided CT radiotherapy system. The Helicoidal Tomotherapy is defined as a new modulated intensity radiotherapy technique (IMRT) which works using a rotary beam ("Fan beam") generated in a linear accelerator installed in an annular gantry similar to the CT gantry, which emits radiation in a continuous fashion while the treatment board moves longitudinally moving the patient through the radiation beam. Helicoidal Tomotherapy is a tomo-radiation administered with a simultaneous movement of the gantry and the treatment board, conceptually assimilated to a Helicoidal CT.
The use of a linear accelerator as the x-ray mega voltage source generates a tomo-image that allows us the exact location of the irradiating volume target in the patient before the treatment takes place.
HI-ART Tomotherapy unit: Description
It’s about the only helicoidal tomotherapy system of clinical use that integrates a linear accelerator which works with 6 MV photons and a detection system in the beam output. This generates a CT type image with 3.5 MV5, 6 photons. An independent jaw system integrated to the primary multileaf collimator produces the "Fan beam" with a beam width of 1 to 5 cm. It also has a binary multileaf collimator, conformed by 64 leaves which interact to the radiation beam by an open/close system defining small individual beams, being the open/close period of about 50 milliseconds; if this is translated into beams it is about 100-250 every time the system works so at the end of every session several thousands of small beams have been delivered to the volume target.
The distance between the source of radiation and the rotation axe is 85cm, and it makes it feasible to treat a 40 cm cylindrical volume in diameter by 160 cm in length in only one exposition of radiation, to a dose rate of 850cGy/minute.
The dosage calculation method is convolution/superposition in a sophisticated computerized system that uses 32 computer processors in total; this system is responsible the dosage calculation, its optimization and calculated data storage memory.
The integration of a linear accelerator to a tomograph type annular gantry grants a series of advantages:
- Similar to conventional CT, the annular gantry system of Tomo-Therapy HI-ART reproduces a precision in the Isocenter in the order of decimals of a mm that compares favorably to 1 mm to the conventional gantry of linear accelerators.
- The x-ray source used to generate the image is the linear accelerator, using an inferior energy and fluency beam; the use of the very treatment beam without modifying its trajectory assures the exact coincidence of the tomo-image to the radiated volume on real time.
- The tomograph generates CT image sections every 5 seconds and allows clearly differentiate anatomic structures (lungs, fat, muscle and bone).
A dose of only 0,5-1,5 cGy is enough to visualize structures in the tomograph having as a result a definitely lower dose tan the one received when using conventional CT imaging.
Tomotherapy: Adapted radiotherapy
The use of CT imaging in the radiotherapy treatment planning may contribute to assume the fake feeling of precision if we don’t take into account that the design, the strategy as well as the definite selection of it are based on an anatomic reality at which the study takes place.
Most of radiation treatments consider a period of several weeks and they are divided in different fractions, administering daily doses (conventional fractioning) or several daily doses (hiperfractioning). During those weeks, the patient’s anatomy, morphology, the size of the tumor and the motion of structures suffer modification in a form that the selection of the proper treatment prior to radiation might be unsatisfactory during the process and develop important mistakes that will condition the sub-dosage to the tumoral volume, recidivating to tumoral condition. Or overdosing running the high risk of toxicity to healthy organs and surrounding tissues, delicate matter to seriously considerate when using highly conformed radiation techniques (IMRT, stereotaxic RT, RTC-3D).
The optimization in a radiotherapy treatment requires not only that precise spatial geometric complex doses are obtained, but that those doses are administered to the volume target in a safe and precise way while avoiding healthy nearby organs and tissues. This is only possible through the acquisition of images of the patient’s treatment volume right before each fraction of the treatment. The prototype of tomotherapy developed in the University of Wisconsin, besides a linear accelerator also integrates a binary multileaf collimator, also a sensor which detects the radiation beam’s outflow and finally a computerized imaging tomograph. The CT sensor found in the opposite side of the linear accelerator and in the same trajectory of the radiation beam, allows the acquisition of CT mega-voltage images (tomo-image). This system also allows us confirm that the granted radiation to the patient has been given as planned.
Knowing the dosage distribution represented in CT and that corresponds to the actually received dose, the reconstructed dose can be directly compared to the distribution of the planned dose represented in the CT, providing the actual treatment verification. The reconstruction of the dose practiced with the tomotherapy could replace the needed time for dosimetry verification using dummies which are commonly considered in the standard methodology for IMRT treatments. This information is extraordinarily important and very useful because permits us correct and/or adapt the radiation treatment even fraction to fraction, contributing to achieve excellence in security and precision.
Corporación Oncológica México Americana has been the first net of radiotherapy centers which has brought the most sophisticated equipment to the reach of people in Mexico.That is why we have become the best option in radio-oncology treatments not only in our country but in Latinamerica too.
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