X-ray or gamma ray systems or devices – Specific application – Absorption
Reexamination Certificate
1998-10-23
2001-05-29
Porta, David P. (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Absorption
C378S108000
Reexamination Certificate
active
06240162
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a radiation emitting device, and more particularly, to a system and method for efficiently delivering radiation treatment.
DESCRIPTION OF THE RELATED ART
Radiation emitting devices are generally known and used, for instance, as radiation therapy devices for the treatment of patients. A radiation therapy device generally includes a gantry which can be swiveled around a horizontal axis of rotation in the course of a therapeutic treatment. A linear accelerator is located in the gantry for generating a high energy radiation beam for therapy. This high energy radiation beam can be an electron beam or photon (X-ray) beam. During treatment, this radiation beam is trained on one zone of a patient lying in the isocenter of the gantry rotation.
In the case of an electron beam, for example, the electron accelerator typically includes an electron gun, accelerating cavities, an exit window, and a radio frequency input. A trigger system generates modulator and injector signals and supplies them to an injector and a high voltage modulator. The modulator generates the radio-frequency pulses and the injector generates the injector pulses. The injector pulses control the quantity of the electrons that will be emitted by the electron gun. The radio frequency creates an electromagnetic field in the accelerator which accelerates the electron beam toward the exit window. The injector and the radio frequency pulses must be synchronized; otherwise, beam acceleration will not occur.
To control the radiation emitted toward an object, a beam shielding device, such as a plate arrangement or a collimator, is typically provided in the trajectory of the radiation beam between the radiation source and the object. An example of a plate arrangement is a set of four plates that can be used to define an opening for the radiation beam. A collimator is a beam shielding device which could include multiple leaves, for example, a plurality of relatively thin plates or rods, typically arranged as opposing leaf pairs. The plates themselves are formed of a relatively dense and radiation impervious material and are generally independently positionable to delimit the radiation beam.
The beam shielding device defines a field on the object to which a prescribed amount of radiation is to be delivered. The usual treatment field shape results in a three-dimensional treatment volume which includes segments of normal tissue, thereby limiting the dose that can be given to the tumor. The dose delivered to the tumor can be increased if the amount of normal tissue being irradiated is decreased and the dose delivered to the normal tissue is decreased. Avoidance of delivery of radiation to the organs surrounding and overlying the tumor determines the dosage that can be delivered to the tumor.
The delivery of radiation by a radiation therapy device is prescribed and approved by an oncologist. The prescription is a definition of, for example, a particular volume and the level of radiation permitted to be delivered to that volume. Actual operation of the radiation equipment, however, is normally done by a therapist. When the therapist administers the actual delivery of the radiation treatment as prescribed by the oncologist, the radiation-emitting device is programmed to deliver that specific treatment. When programming the treatment, the therapist has to take into account the actual radiation output and has to adjust the dose delivery based on the plate arrangement opening to achieve the prescribed radiation treatment at the desired depth in the target.
The oncologist's challenge is to determine the best number of fields and delivered intensity levels to optimize the dose volume histograms, which define a cumulative level of radiation which is to be delivered to a specified volume. To optimize dose volume histograms to the prescriptions, the three-dimensional volume is broken into cells, each cell defining a particular level of radiation to be administered. The outputs of the optimization engines are intensity maps, which are determined by varying the intensity at each “cell” in the map. The intensity maps specify a number of fields defining desired (optimized) intensity levels at each cell. The fields may be statically or dynamically modulated, such that a different accumulated dosage is received at different points in the field. Once radiation has been delivered according to the intensity map, the accumulated dosage at each cell, or dose volume histogram, should correspond to the prescription as closely as possible.
One technique used in conjunction with intensity modulation is auto-sequencing. In an auto-sequencing technique, the field segments are delivered in via a verify and record system in a rapid and fully automated manner. An important component of auto-sequencing is the ability to cycle the radiation beam on and off quickly and accurately during an intensity modulation radiation treatment.
With both single beam and auto-sequencing techniques, the ion chambers and analog and digital dosimetry circuitry can introduce delays which can cause inaccuracies in the applied radiation. These delays are not generally significant with regard to single beam cycle beam treatments, since their effect on linearity is only about 2%. However, when the treatment field is split into many segments, each with its own beam cycle, the error may be orders of magnitude greater, since the amount of the error increases with the number of segments being treated.
In addition, linearity errors between delivered and programmed dosages have been found to be particularly significant for prescribed treatments of less than 10 MUs. Such errors are in compliance with current linearity specifications since linearity is not specified below 50 MUs. However, since IMRT can require the delivery of as little as 1 MU, there is a need for an improved linearity performance below 50 MU.
Accordingly, there is a need for a method for compensating for dosimetry delays in a radiation treatment device.
SUMMARY OF THE INVENTION
These problems in the prior art are overcome in large part by a system and method for control of radiation therapy delivery according to the present invention. In particular, known delays are compensated for by applying a compensation factor to the dose at the start of the beam cycle. According to one embodiment of the invention, a dosimetry controller is configured to sense radiation on (RAD ON) and monitor the dose rate at the beginning of the beam cycle. The dosimetry controller then multiplies the dose rate by a compensation factor. Thus, for each beam cycle, the dosimetry controller resolves the magnitude of the lost dose rate data and compensates each segment accordingly.
According to one embodiment of the invention, the compensation factor is a multiplication factor. According to another embodiment, particularly useful for low dosages, the compensation factor is an offset.
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Alaifai Hussein
Calderon Edward Lewis
Forknall Simon John
Hernandez-Guerra Francisco M.
Pond David L.
Porta David P.
Siemens Medical Systems Inc.
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