X-ray or gamma ray systems or devices – Specific application – Absorption
Reexamination Certificate
1997-09-25
2001-05-29
Church, Craig E. (Department: 2876)
X-ray or gamma ray systems or devices
Specific application
Absorption
Reexamination Certificate
active
06240161
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.
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 radiation therapist'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. Typical optimization engines optimize the dose volume histograms by considering the oncologist's prescription, or three-dimensional specification of dosage to be delivered. In such optimization engines, 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. However, such optimization engines do not account for the beam shielding device itself; the output intensity map is simply a level of dosage to be applied at a particular cell.
More particularly, once the optimization routines define a plurality of intensity levels at each cell, the beam shielding device settings must be chosen according to the output number of fields. Often, however, the output of such routines define a number of fields that would require a prohibitive amount of time to deliver, or which is physically impossible for the beam shielding device to achieve. Thus, to provide a realizable dosage, fewer intensity levels must be provided, or fewer fields, and the dose volume histograms are thereby degraded. While methods are known for efficiently arranging leaf positions to deliver dosages according to the intensity maps (e.g., U.S. Pat. No. 5,663,999, assigned to Siemens Medical Systems, Inc.), such systems may still cause a degradation of the dose volume histogram.
Accordingly, there is a need for a system and method for determining radiation delivery which accounts for the system's physical constraints.
SUMMARY OF THE INVENTION
These problems in the prior art are overcome in large part by a system and method for radiation therapy delivery according to the present invention. More particularly, rather than determining a number of beam shielding device settings after one or more optimized intensity maps have been determined, the present invention accounts for the physical attributes of a beam shielding device when determining an optimal radiation treatment. These include, for example, realizable positioning of plates and/or collimator leaves. Thus, an optimal set of fields and intensity levels for those fields are chosen, and a sequence of beam shielding device settings which are optimized to the actual device are output.
According to one embodiment of the invention, a user selects a number of beams and collimator and/or plate fields. The user also selects a dose volume histogram, or prescription, for each volume of interest. The routine then selects an initial set of leaf positions and a field intensity for each field. The initial selection is determined by shaping the leaf field to the tumor and choosing higher intensities where there is a higher tumor volume. The dose volume histograms are calculated and a figure of merit is determined which measures how closely the calculated dose volume histograms match the specified dose volume constraints for the region of interest. The leaf positions and the field intensities for each field are then varied according to an optimization algorithm such as simulated annealing and the figure of merit is evaluated again. Several iterations occur until either the figure of merit no longer improves or reaches a predetermined threshold. The fields for each beam are then arranged in an autosequence that will reduce the amount of leaf travel.
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Church Craig E.
Siemens Medical Systems Inc.
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