issue 7
The Beamlet
Recently the University Clinic Heidelberg celebrated 100 years of radiation oncology. As part of the celebration, the Clinic held a special Symposium on TomoTherapy® radiation treatments. Thomas “Rock” Mackie, PhD, co-founder of TomoTherapy Incorporated and Professor of Medical Physics and Human Oncology at the University of Wisconsin-Madison, spoke at the event. The following is an abstract from his presentation on integral dose.
TomoTherapy and Integral Dose (Abstract)
Integral dose (ID) is defined as the volume integral of the dose deposited in a patient, and is proportional to the mean dose times volume of tissue irradiated to any dose. Some calculate ID using the mass integral, however, Dr. Mackie uses the volume integral because it can be obtained by calculating the area underneath an absolute dose-volume histogram (DVH).
While it is sometimes believed that the large number of beamlets and monitor units used in IMRT (such as TomoTherapy) leads to an increase in integral dose, Dr. Mackie points to several studies* that show integral dose is nearly invariant for photon radiotherapy, regardless of the number of beams or delivery technique used. Figure 1 represents a simple example from conventional radiotherapy, illustrating that ID is invariant with the number of beams. Figure 2 represents an example using conventional IMRT delivery. Note that intensity modulation does not change the integral dose received.
Figure 1: Integral dose is independent of the number of fields. Compared to the parallel opposed pair (left), the four-field box (right) has half the energy fluence from each of the beams and results in double the volume of normal tissue irradiated to half the dose.
Figure 2: The wedged pair is a primitive form of conventional IMRT delivery; the energy fluence values are non-uniform and dose values now refer to volume averages. Neglecting leakage, the integral dose is invariant with technique for both uniform and non-uniform delivery.
Dr. Mackie also demonstrates that, contrary to popular belief, higher energy photon beams do not substantially reduce ID even for deep-seated lesions, and may in fact be worse when taking neutrons into account (see Figure 3).
Figure 3: Why higher energy doesn’t reduce integral dose very much.
Whether it is better to deliver a small dose over a large volume of tissue or a larger dose to a small volume of tissue depends, of course, on the specific case. The best strategy, Dr. Mackie argues, is to use the power of highly modulated IMRT to avoid delivering dose to those normal tissues (such as breast and thyroid) that are most at risk for secondary malignancies.
When planning on the TomoTherapy® Hi·Art System®, it is possible to either put a high importance on avoiding the structure, or to block it completely so it cannot receive primary dose. Furthermore, the TomoTherapy Hi·Art System has less unwanted dose from leakage and head scatter compared to conventional IMRT systems, with improved primary collimation, beamline, MLC and beam stop design to reduce leakage and unwanted dose to both patient and staff.
* D’Souza WD, Rosen II. Med Phys 2003; 30: 2065-2071; Yartsev et al, Radiotherapy and Oncology 74 (2005) 49-52; Aoyama et al., ASCO Cancer Prostate Symposium, 2005.
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. So, what makes the Hi·Art system a better choice for handling integral dose?
