e-newsletter 
Know Your Tomo
e-newsletter Know Your Tomo
Integral Dose roughly corresponds to the total amount of energy deposited in the patient by all the beams used to irradiate the tumor. Ideally, energy would only be deposited within the designated tumor volume, but with any type of external beam radiation there is energy deposited along the entire beam path.
For a given type of radiation and patient geometry, integral dose depends mostly on the total energy directed into the patient through the beam apertures. There may be just a few apertures, as with conventional RT, or thousands of small apertures, as created by open MLC leaves in a TomoTherapy delivery. Total energy entering the patient through these apertures is minimized when the treatment is optimized to keep the “high dose” region confined to the tumor volume. That is, incident energy is not wasted in creating a high dose region larger than it needs to be. The TomoTherapy Hi·Art System can create the most conformal dose distributions of any x-ray treatment modality. Correspondingly, TomoTherapy radiation treatments direct a smaller amount of energy into the patient through its beam apertures than is the case with less conformal treatment techniques.
Integral dose depends to a lesser extent on “leakage radiation”, in which energy enters the patient due to radiation not being completely absorbed by shielding devices such as MLC leaves and beam jaws. Fortunately the TomoTherapy Hi·Art System is built with a very high level of shielding, so leakage radiation is small despite the relatively long “beam-on” times inherent to all IMRT treatments. Conventional systems, whose shielding is designed for the shorter beam-on times of conventional RT, contribute a larger leakage radiation component to the integral dose when operating in IMRT mode.
Because of the rotational nature of a TomoTherapy delivery, there is a high degree of “focusing” of the incident beam energy within the tumor and consequently a rapid and even fall-off of dose away from the tumor. This is in contrast to the use of a small number of fixed beam directions, where relatively high doses extend along the beam directions out to the patient surface. Thus there is generally a larger volume of low dose with TomoTherapy, but a smaller volume receiving a high dose.
Which is better: A large volume receiving a low dose or a smaller volume receiving a high dose? A pragmatic answer is that while it is a matter of debate whether the so-called “low dose bath” characteristic of rotational treatments can cause secondary malignancies, it is not a matter of debate whether high doses to normal tissues are dangerous. In fact, reduction of the volume of normal tissue receiving a high dose is the essential premise of conformal radiotherapy. This is what TomoTherapy technology excels at.
It is worth pointing out that just as a TomoTherapy radiation treatment delivery is able to keep high doses away from selected regions close to the tumor, plan parameters can be entered so as to further reduce the dose to regions far from the tumor and thus reduce the integral dose within those regions. It is likely that reduction of integral dose within specific organs susceptible to secondary malignancies is the more pertinent and achievable goal than general reduction of integral dose. With the TomoTherapy Hi·Art System, you have the power to achieve this goal due to the flexibility in how energy is directed at the tumor.
Issue 2: What is a sinogram?A sinogram, in the context of TomoTherapy® radiation treatment, usually refers to the pattern of MLC leaf opening times throughout successive rotations. It is easiest to think of a sinogram as a stack of 1-dimensional maps, each describing the time for which each leaf will be left open at a given gantry angle during a given rotation. So, a single element of the sinogram represents the intensity of a single beamlet, which as we learn below is one of thousands involved in a treatment delivery, and is thus one of thousands of pixels in a sinogram.
The stack of beamlet intensities in the sinogram usually has a repetitive pattern that resembles one or more sine-waves, hence the name. The sinusoidal pattern is due to the variation in which leaves are responsible for irradiating a given target object, or blocking radiation from hitting a given organ at risk, as the gantry progresses through successive 360 degree rotations. An object far from the gantry axis will cause a sine-wave of large amplitude to appear in the sinogram.
More than just a catchy title for our e-newsletter, the beamlet is a key distinguishing feature of TomoTherapy® radiation treatment.
A beamlet is a single "element" of a beam, which in turn is one of hundreds of beams within a typical helical TomoTherapy delivery. A single beamlet corresponds to the radiation emitted through a single open MLC leaf, with the gantry at any given angle during rotation. It has a width in the transverse direction of approximately 0.6 cm (the projected leaf width) and a length dependent on the jaw setting selected for treatment. Each beamlet makes an optimized contribution to the total dose of magnitude proportional to the time for which the leaf is opened.
To put the role of an individual beamlet in context, let's say a 5 cm-long cylindrical target volume is approximately 5 cm in diameter and a 2.5 cm beam width is used with a "pitch" of 0.2. This treatment would usually involve approximately 8 cm of couch travel, which in this case would require 8 cm/(2.5 cm x 0.2) = 16 gantry rotations. Each gantry rotation includes 51 separate modulated beams comprising approximately 5 cm/0.6 cm = 9 beamlets. The number of beamlets contributing to the dose distribution is therefore a grand total of 16x51x9 = 7344. That's a lot of individual dose contributions to accurately calculate, store, optimize opening times for, and efficiently deliver—but that's why you have that powerful rack of 32 processors and our patented binary MLC!
