Patent Number: 060524308
Section: summary

BACKGROUND OF THE INVENTION The present invention relates to radiation therapy machines, and more particularly, to a system and method for improving dose volume histograms by introducing intensity modulation in a subspace around the edges of a multi-leaf collimator defined static radiation field. DESCRIPTION OF THE RELATED ART Radiation emitting devices are generally known and used, for instance, as radiation therapy devices for the treatment of tumors. 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 directed at the tumor in a patient. It is known that the cure rates for tumors are a function of the dose delivered to the tumor. In order to control the radiation emitted toward the tumor, 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 tumor. 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 radiation dose that can be delivered to a portion of an organ of normal tissue without serious damage can be increased if the size of that portion of the organ receiving such radiation can be reduced. Avoidance of damage to the organs surrounding and overlying the tumor determines the dosage that can be delivered to the tumor. Treatment techniques provide several fields directed from different gantry angles and shaped to conform to the tumor using a multi-leaf collimator. A multi-leaf collimator employs a plurality of relatively thin plates or rods, typically opposing leaf pairs. The plates themselves are formed of a relatively dense and radiation impervious material. Multi-leaf collimators are known to be used for both static field and dynamic field applications. For a static field dose, the collimator leaves are fixed in space for a predetermined period for each selected gantry angle to deliver static fields. This technique has the advantage that applied dosages are readily determined, however, such static field dosages do not necessarily provide a particularly accurate match to the tumor volume and therefore has a less than desirable therapeutic benefit. In particular, a clinician may identify a planning target volume, which includes the tumor, may also include adjacent tissues that may contain tumor cells, and sometimes regional lymph nodes. As such, the actual treated volume, however, is designed to treat the target volume plus a surrounding region, or margin. The margin accounts for uncertainties in defining the planning tumor volume, such as dose fall-off or penumbra at the beam edge, as well as inaccuracies in defining the target volume. The larger the margin, the more healthy tissue that may be irradiated. One method of avoiding irradiating healthy tissue is through use of a dynamic multi-leaf collimator technique to delimit the applied radiation beam path. With this technique, the leaf pairs move continuously or quasi-continuously throughout the whole field. To match tumor volume, the leaves typically move throughout the entire treatment period with a variable velocity. As can be readily appreciated, a plurality of leaf pairs moving at a variety of leaf velocities requires an relatively large amount of hardware and software overhead to control, and is also very difficult to verify while in progress. Further, changing the velocity of the collimator leaves can result in undesirable forces acting on the treatment head, causing it to destabilize or go out of alignment. This causes the system to shut down or lock-out. Since the leaves are relatively heavy, and the radiation beam must be delivered at an accuracy on the order of millimeters, rapidly moving leaves combined with frequent direction changes can result in frequent lock-outs. A prior method of verification of a treatment field is through the use of port films. A port film is a radiograph taken when the treatment beam, the patient and variables, such as gantry angle, are set treatment. Typically, such films are taken only prior to the start of treatment and, due to delays necessary to take, develop and analyze the port films, cannot provide real-time verification of treatment for the dynamic techniques. In fact, evaluation of port films typically occurs only on a weekly basis. Continuous feedback control is thus necessary to verify the accuracy of radiation delivery in the treatment field. Electronic portal imaging devices have been developed which provide an image on a video monitor. While these could be used to provide such continuous feedback, the computations are known to be relatively intensive. As such, a time lag exists between analysis of actual and planned delivery. In addition, evaluation of the image is typically difficult, since the field of view is restricted to the collimator settings and a view of the surrounding anatomy cannot be made. Finally, record-and-verify systems are known in which treatment parameters are recorded, and treatment is begun only when the user-defined parameters are verified during set-up. However, the set of parameters required to define dynamic fields is generally very large, cumbersome and time-consuming. Moreover, dynamic treatments are difficult to resume if there is a power failure in the middle of treatment. Additionally, the requirement of continuously-moving leaf pairs at variable leaf velocities causes relatively more mechanical cycling of the leaves, thereby decreasing the lifetime and the reliability of the dynamic multi-leaf collimator. Accordingly, there is a need for an improved system and method for shaping treatment volumes more closely to tumor volumes in a known manner. SUMMARY OF THE INVENTION These problems in the prior art are overcome in large part by a system and method for dynamic subspace intensity modulation according to the present invention. More particularly, the margin regions at the edges of a multi-leaf collimator-defined static radiation field are intensity modulated during part or all of the delivery of each static radiation field. This is accomplished through moving the collimator leaves at a constant velocity over the subspace margin. By keeping the major portion of the field static and by only moving the leaves at one fixed velocity over a small subspace of the intensity profile, the dose volume histogram is relatively easier to determine. As such, the dose volume histogram can be calculated more accurately. Moreover, the treatment can be more readily resumed after a power failure. A method according to one embodiment of the present invention includes obtaining an intensity profile defining a histogram of radiation intensity levels to be applied over a given tumor volume. The method further includes digitizing the profile and determining the optimal way to deliver the digitized portions of the intensity map. Optimizing delivery of the digitized intensity map includes accounting for all leaf pairs and any associated constraints. The set of multiple static fields to be applied is then determined. The original intensity profile is compared with the digitized profile in order to determine whether excess slopes can be piggy-backed onto the edges of any of the static fields previously determined. The slopes of the profiles can be matched either over the distance over which the slope occurs or over the number of monitor units delivered for the given slope. For each leaf that has to deliver a slope, the number of monitor units after which the leaf should start moving is determined. If the match is by monitor units, the leaf is moved throughout the entire treatment. If the match is by distance, the number of monitor units after which the leaf should start moving is equal to the total number of monitor units in the static field minus the distance over which the leaf should move times the absolute value of the slope that a leaf can deliver. The leaves may be moved either inward or outward.