Jake Van Dyk, et al., London Regional Cancer Centre and University of Western Ontario (resume)
Cancer and Radiation Therapy
Cancer is the most significant health care problem in the western world surpassing heart disease as the leading cause of potential years of life lost. In Canada, about 134,000 people are diagnosed annually with cancer. This represents more than one in three people who will develop cancer during their lifetimes. Radiation will be used to treat approximately 66,000 new cancer patients per year of whom 33,000 will be treated with an attempt to cure the disease.
The radiation therapy process is complex and involves multiple steps.
The process begins with patient diagnosis and three-dimensional (3-D) imaging, through various steps that prepare the patient for treatment and, finally, to treatment verification and actual radiation dose delivery. Patients who are treated for cure receive high radiation doses of 60 to 70 Gy, given in 30 to 40 daily fractions at the rate of 5 fractions per week.
There are several critical steps in this process. One of these is the use of sophisticated 3-D imaging using computerized tomography (CT), magnetic resonance imaging (MRI), single photon emission tomography (SPECT), or positron emission tomography (PET). These imaging modalities have evolved dramatically over the last decade and provide information about tumour location and tumour extent, with each modality providing unique information that is especially relevant for specific tumour types.
With such image data, sophisticated dose calculations can be performed using shaped radiation beams from various directions to yield optimized treatment plans.
In addition to imaging for therapy planning, there are a number of requirements in order to deliver a prescribed radiation dose to the patient with a sufficient control and accuracy. These relate to the technologies used to deliver the dose to the patient and the computerized calculational procedures that are required to optimize the treatment technique and to predict precisely the dose that will be given to the patient using complex radiation delivery technologies.
Modern Dose Delivery
One of the unique features of radiation therapy, compared to other forms of cancer treatment, is that the radiation can be delivered in an anatomically and geometrically specific fashion by using radiation field collimation and beam shaping.
Today, linear accelerators (linacs), generating electron energies between 4 and 25 MeV, are generally used for producing x-ray beams for the treatment of tumours. Conventionally, these machines have collimators that produce rectangular fields between 4 x 4 cm2 to 40 x 40 cm2.
A collimator is a device that narrows a beam of particles or waves. To «narrow» can mean either to cause the directions of motion to become more aligned in a specific direction or to cause the spatial cross section of the beam to become smaller.
The newer machines have collimators which are divided into multiple segments from two opposite sides. The «leaves» in these «multileaf collimators» are motor-driven and computer-controlled and can project shadows at the level of the patient that are 0.5 or 1 cm in width.
In addition to simple field shaping, computer-controlled multileaf collimators provide the capability of defining multiple field shapes either for individual directions or for multiple fields aimed at the tumour from different directions. This, combined with “automated” optimization programs using “inverse” dose calculations, allows control of the beam intensity pattern at the patient such that a well-defined and uniform dose can be delivered to the target and normal tissue doses can be minimized. This process has become known as segmented field, intensity modulated radiation therapy (IMRT) or when using moving leaves and a moving machine gantry, it is known as dynamic IMRT.
Tomotherapy, literally translated, means “slice therapy». The first implementation of this concept was performed by NOMOS Corporation and was provided as an add-on accessory to existing linear accelerators. The add-on feature consists of a set of multileaf collimators that provide a narrow “fan” beam shape (Figure 1) projecting a maximum width at the patient of about 20 cm. The fan beam thickness can be either 0.8 or 1.6 cm and each leaf projects a shadow of about 1 cm width at the patient. When the leaves are in the beam, that portion of the beam is fully shielded except for a minor (~0.5%) transmission component.
Figure 1. (a) Schematic of a binary multileaf collimator with a fan beam geometry. This schematic example shows leaves that move from one side. (b) Picture of TomoTherapy single slice multileaf collimator the interdigitated leaves and how they move from both sides.
Either the leaf is open or closed for that slice providing “binary” dose delivery, i.e., for that portion of the beam, the beam is either on or off. The open beam components are generally referred to as «beamlets» or «pencil beams». Radiation delivery consists of a machine that rotates around the patient while the beam is on and the leaves rapidly move in and out depending on whether that beamlet is aimed at the target or at normal tissues.
After two simultaneous slices have been delivered, the patient is translated by two slice thicknesses and the next two slices are delivered until the total treatment volume is covered, hence the nomenclature, “serial tomotherapy”.
General Design Considerations
Traditional linear accelerators are currently limited to serial tomotherapy due to the limited rotation possible (~370°) and the inability to move the couch smoothly and automatically during radiation delivery. Furthermore, serial tomotherapy is unable to image the patient in treatment position and, therefore, unable to assure the accurate placement of the high dose volume with respect to the malignant region. The new tomotherapy units are capable to remove these limitations.
As can be seen in Figure 2, the helical tomotherapy machine is a combination of a helical CT scanner and a linear accelerator. It uses the slip ring technology of diagnostic CT scanners and, therefore, the unit is capable of continuous rotation around the patient while the couch is moving into the gantry, thus providing smooth helical delivery.
Mounted on the rotating gantry and attached to the slip ring is a compact linear accelerator generating a 6 MV photon beam. The beam from the accelerator is collimated by a multileaf collimator consisting of 64 leaves each of which project a shadow of 6.25 mm at the patient generating a total fan beam width of 40 cm. By using a separate collimation («jaws») system above the multileaf collimators, the «slice thickness» can range between 0.5 to 5 cm. Since it is a specially designed machine for helical, fan beam delivery, the multileaf collimation system is specifically designed to minimize leaf transmission and interleaf leakage – important considerations for narrow beam, multislice delivery procedures.
The Process of Helical Tomotherapy
Due to the integration of several technologies into a single piece of equipment, helical tomotherapy allows the development of a number of processes that are either very difficult or simply not possible with other radiation therapy devices.
Dose Delivery Capabilities
The next examples will be used to illustrate the kinds of dose distributions that can be delivered using the tomotherapy technology. Figure 3 is a schematic example of a «U»- shaped target that encompasses a critical normal tissue. This represent a nasopharyngeal tumour around the spinal cord and a schematic example of a «U»-shaped high dose region (red for tumour) surrounding a critical structure (spinal cord) that receives a low dose (blue).
The principal advantage of this type of treatment is the reduction of the damage to other organs and reduces the radiation collateral effects.
Helical tomotherapy mounted on a ring gantry provides significant advantages over today’s state-of-the-art radiation treatment. First, it provides on-line imaging which allows for treatment adaptation on a daily basis accounting for the tissue locations on each set-up. The dose reconstruction capabilities provide an ability to determine the dose actually delivered to the patient, also on a daily basis. The tomotherapy unit fits into a significantly smaller room compared to modern linear accelerators since it does not involve a couch rotation.
The technology of radiation oncology is evolving at a rapid rate, primarily as a result of the evolution of computer applications and their integration into diagnostic imaging and radiation therapy dose delivery equipment. The ring-mounted helical tomotherapy concept combines state-of-the art imaging and treatment capabilities. Perhaps the tomotherapy development represents the greatest advance in radiation therapy since the first use of cobalt-60 in the 1950s.
These advances will provide a radiation treatment technology that allows daily adaptation of the treatment technique to match the location of the tumour and the normal tissues. With the better ability to focus the radiation beams, higher doses can be delivered to the tumour resulting in higher cure rates. In addition, lower doses will be delivered to normal tissues resulting in lower complication rates. The net result should be an overall improvement in the quality of life of the cancer patient.
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