X-ray or gamma ray systems or devices – Specific application – Absorption
Reexamination Certificate
2001-03-13
2002-10-01
Kim, Robert H. (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Absorption
Reexamination Certificate
active
06459762
ABSTRACT:
FIELD OF THE INVENTION
The present invention generally relates to radiation oncology, and more particularly to methods for oncological treatment with linear accelerators.
BACKGROUND OF THE INVENTION
Radiation-emitting devices are generally known and used for radiation therapy in the treatment of patients. In a typical radiation therapy device, the gantry of a linear accelerator is swivelled around a horizontal axis of rotation in the course of a therapeutic treatment of a patient. The linear accelerator generates a high-energy radiation beam (referred to herein as a “photon beam” or “photons”) for use in the therapeutic treatment.
Historically, linear accelerators used in radiation therapy applications have been equipped to provide only a single energy photon beam. In the recent past, however, some linear accelerators have been equipped to provide two different energy beams. The limited number of energies available is a continuing problem for physicians and physicists, since it is not always possible for them to give the most efficacious treatments. For example, the most commonly available dual energy linear accelerator provides six megavolt (MV) photons and either ten or fifteen MV photons. This is a typical combination since, e.g., in the treatment of breast cancer, an irradiation treatment of the whole breast is best accomplished with six MV photons. For tumors located deep within the body, however, the most commonly used energies are ten MV and fifteen MV photons. Of course, there are other tumor sites that are best treated with four MV or eight MV photons. Four MV photons are very effective for the treatment of tumors near the skin surface, while eight MV photons are excellent for the irradiation of larger breasts. Four MV and eight MV photon energies are usually not incorporated in dual energy linear accelerators because they are not needed all the time, and thus are not considered cost effective. Currently, certain manufacturers are attempting to provide linear accelerators with the capability of generating three different photon energies. Such machines, however, will still preclude many other intermediate energies that may be useful.
A therapeutic x-ray beam produced by a linear accelerator is characterized by the amount of energy that will be deposited at a treatment site by that particular x-ray beam. This characterization relates to the depth (usually measured from the surface of the skin) at which the beam's maximum energy is deposited (often referred to in the art as “dmax”). In radiation therapy, the energy deposited by ionizing radiation (absorbed dose) is typically measured in “grays” or “Gy”, instead of the traditional unit of measure of absorbed dose, the “rad.” It will be understood that a dose of one Gy will deposit one joule of energy per kilogram of matter, and that one rad is equal to 1 one-hundredth of a Gy, or one “cGy.” Thus, dmax is often represented in the form of absorbed dose in units of cGy. The absorbed dose depends upon the exposure (time and intensity) as well as the inherent characteristics of the absorbing matter, i.e., density, atomic number, etc.
For example, assuming a ten by ten cm (centimeter) field, a low energy x-ray beam (e.g., six MV) would deposit the maximum energy at one and a half cm from the surface of the skin, while a high energy beam, such as fifteen MV, would deposit the maximum energy at three cm from the surface of the skin. Thus, with a fifteen MV beam, any point closer than three cm to the surface of entry would be less than dmax and any point more than three cm from the surface of entry would also be less than dmax. At locations that are nearer to the surface or further away from the surface than the location of dmax, radiation energy is deposited to a lesser degree and at a lower rate. A significant difference between higher energy beams and lower energy beams is that the higher energy beams are more penetrating. That is, the amount of radiation deposited at a given point (say five cm) beyond the location of dmax is higher for a fifteen MV beam than it is for a six MV beam. Thus, a fifteen MV beam is more effective in treating a deep seated tumor (say fifteen cm from the surface/skin) than a six MV beam.
A standard technique for treating a breast is called “tangential beams.” Here the radiation is given by two nearly opposing beams arranged at an angle so that they just skim the chest wall (to minimize radiation exposure to the lungs) but otherwise treat the entire breast. The base of a human breast generally provides the widest separation, (i.e., the greatest transit distance through tissue, sometimes as much as twenty-five cm. With such a wide separation, a more penetrating beam is needed. However, with a fifteen MV beam, which deposits a maximum energy at three cm, the beam may not be treating the breast tissue adequately near the skin surface. However, if a six MV beam is used, it may be more appropriate in covering the breast tissue near the skin surface, but may not be treating the deep tissue adequately.
One solution in the art has been to increase the amount of radiation that is given for each treatment so that the tissue at the center of the base of the breast receives an adequate amount of radiation with a six MV beam. Unfortunately, a consequence of this method is that the tissue near the skin becomes so “hot” (receiving too much radiation) that the breast exhibits significant skin side effects (almost like a severe sun bum). In such a case, an eight MV beam would be more appropriate, since it would treat the breast tissue near the skin appropriately but yet more penetrating than the six MV beam. Unfortunately, eight MV beam machines are not commonly used at healthcare facilities.
Numerous methods and apparatus have been disclosed in the prior art for optimizing radiation therapies. For example, in U.S. Pat. No. 6,038,283, a method and apparatus for determining an optimized radiation beam arrangement for applying radiation to a tumor target volume while minimizing radiation of a structure volume in a patient is disclosed. The method uses an iterative cost function based on a comparison of desired partial volume data, which may be represented by cumulative dose volume histograms and proposed partial volume data for target tumors and tissue structures. This arrangement provides for the delivery of the optimized radiation beam arrangement to the patient by a conformal radiation therapy apparatus.
U.S. Pat. No.6,142,925 discloses a method and system for increasing resolution of a radiotherapy system to achieve virtual fractional monitor unit radiation delivery. The method identifies a desired treatment dose that exceeds the resolution of a radiation treatment device, and develops a schedule of treatment sessions for delivering the desired treatment dose that produces a combined treatment dose equaling the desired treatment dose without exceeding the resolution within each treatment session.
U.S. Pat. No. 5,880,477, discloses a method and apparatus for real time control of the dose rate of particles or ionizing radiation, especially X-ray radiation, generated from an electron linear gun and applied to polymer resins using an appropriate ionization chamber having planar electrodes placed in the field of particles. The method involves sampling continuously the current for collecting the load between the electrodes. The instantaneous dose rate of the radiation is represented by the collecting current. Using an appropriate shielded conducting system, the collecting current is directed to an amplification and measurement circuit arranged outside of the irradiation zone. The intensities of the current are translated into dose rate values. The does rate values are then processed and/or displayed and/or recorded.
U.S. Pat. No. 5,668,847 discloses a radiation emitting device for therapeutic radiation treatment which adjusts the actual radiation delivered to an object via a radiation beam, and which is dependent on the dimensions of an opening in a plate arrangement provided between a radiation source and an object.
Cheng Chee-Wai
Wong James R.
Duane Morris LLP
Ho Allen C.
Kim Robert H.
Ro Inventions I, LLC
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