When beams of high energy x-rays or electrons are used for radiotherapy, it is important to direct the beams to a tumor within the patient, while restricting the beams from striking healthy tissue outside the tumor region. Tumors commonly have irregular shapes and it is necessary to shape the beam cross-section to the corresponding irregular shape. It is common for a treatment plan to prescribe the beam to be directed at the tumor from a number of different angles, where the beam profile is unique for each corresponding angle.
In an attempt to alleviate the need to fabricate a unique aperture for each exposure, multi-leaf collimators (MLC) have been implemented as a way to shape the radiation beam cross-section for radiotherapy treatments. These devices include a set of flat, thin leaves made from a high-density material, such as tungsten, where each leaf is moved transversely in and out of the radiation field to selectively attenuate portions of the beam to create a unique beam cross-section. The shape of the beam can be altered dynamically during the therapy session using motorized controls connected to each leaf. By dynamically attenuating select portions of the beam, intensity-modulated radiotherapy (IMRT) has been made possible, where by moving the leaves during beam exposure, the beam can be delivered in a manner such that the spatial fluence of the irradiation is not constant over the irradiated area.
IMRT can also be accomplished by making multiple irradiations, each with a different field shape, the sum of which creates a field of non-uniform intensity. The leaves must be thick enough to highly attenuate the beam.
In accordance with some accelerators, the MLC has been used to replace the standard field-shaping jaws of a beam accelerator. The shape of the portion of the leaf that defines the edge of the field is designed for minimum penumbra to create the sharpest edge of the beam as possible between the irradiated and protected areas.
The goal of radiotherapy is to deliver a radiation dose to the tumor while minimizing radiation-induced damage to surrounding normal tissue and organs. Accomplishing these tasks includes providing an imaging scheme to plan the treatment and an imaging scheme to ensure accurate delivery of the planned treatment.
The effectiveness of radiation treatment depends on the accuracy of providing an appropriate radiation dose to the target using geometric and dosimetric configurations, and upon the precision of repeated dose delivery. Radiotherapy is typically performed in multiple sessions spread over a period of several weeks, with each session having multiple dose fractions delivered from different beam angles. In an ideal situation, the patient's internal anatomy as well as their placement with respect to the beam would be constant throughout the course of the treatment and identical to the geometry that was recorded for generating the treatment plan. In practice, significant deviations in patient geometry can occur both between dose fractions (interfraction) and within a single fraction (intrafraction).
Interfraction deviations can cause interfaction errors during the beam delivery in a time span as short as between dose fractions, where the deviations occur from systematic changes in the internal anatomy such as tumor shrinkage and/or tissue shrinkage, or patient-initiated movement to achieve a more comfortable position. Various forms of in-room, 2D imaging (e.g., MV x-ray imaging, orthogonal kV x-ray imaging) and 3D imaging (e.g., cone-beam kV and MV CT, 3D ultrasound, and optical position monitoring) strategies have been employed in order to monitor and minimize such interfraction deviations.
Deviations in intrafraction geometry occur primarily from some form of physiological process such as respiration, cardiac motion, bladder filling, and movement of rectal gas, or by patient-initiated motion during system operation, referred to as “Beam-ON”. Such deviations can result in significant changes in the position and the shape the tumor target as well as the surrounding anatomical structures, causing significant geometric and dosimetric uncertainties in both treatment planning and dose delivery. Thus, insufficient dose may be delivered to the tumor target(s) and/or high levels of dose may inadvertently be delivered to healthy tissue and critical organs. While these problems have been long recognized, it is only recently that increasing attention has been directed toward the effective management of intrafraction motion, particularly, in the context of treating thoracic and abdominal tumors.
Currently, the approach used to account for all forms of geometric uncertainty in radiotherapy is to add a margin around the volume to be irradiated, also called the clinical target volume (CTV) to create a larger planning target volume (PTV) that accommodates the geometric uncertainties due to intrafraction and interfraction related deviations. In the case of intrafraction motion, the range of motion can be estimated using fluoroscopy or 4D CT scans in order to determine the required margins so as to maintain the target in the beam trajectory at all times.
While the use of 4D image-guidance in defining the PTV margin is a useful approach, this strategy has two limitations. First, even with image-guidance, a motion inclusive margin results in significant volumes of healthy tissue around the target receive unnecessary radiation. Furthermore, this “no-feedback” strategy incorrectly assumes that motion observed in the 4D planning images is reproducible and correctly reflects motion during dose delivery.
A prominent intrafraction geometric deviation arises from patient respiration when treating thoracic and abdominal tumors. Some intrafraction motion management strategies have been implemented in an attempt to minimize or eliminate these deviations, such as abdominal compression, or instructing the patient to breath-hold during a chosen respiratory phase or to perform shallow breathing. All of these techniques have been reported to reduce target motion and, thereby, yield lower geometric uncertainty in planning and delivery. However, the success of these techniques is heavily predicated on patient compliance with instructions, which may not be possible for lung cancer patients exhibiting compromised lung function or having significant discomfort.
In order to address some of the limitations of the strategies described above, many groups have worked on approaches that are based on allowing intrafraction motion and adapting treatment planning and/or treatment delivery to accommodate the resulting geometric deviations. In these approaches, the problem of motion management involves two independent tasks: real-time estimation of target position and shape and, real-time beam adaptation through reshaping of the treatment beam relative to the estimated target position/shape.
In this discussion, the term real-time is used to denote a time duration that is much less than the time scale of the motion being studied. Additionally, the terms beam adaptation and adaptive radiotherapy are used to indicate a dynamic change in the delivered fluence through modifying the geometric shape of the beam aperture.
Real-time motion monitoring of the tumor target(s) can be performed using external markers, internal markers and/or internal anatomical features. Systems that use external markers are based on infrared LEDs or reflective markers placed on the patient and imaged continuously with a video camera, or by using systems that monitor respiratory motion using spirometry or a wraparound strain gauge. Most of the systems that use internal markers are based on frequent monoscopic or stereoscopic x-ray imaging of radio-opaque markers that are surgically implanted near the tumor target. A system based on electromagnetically-excitable, implanted transponders has been reported for real-time monitoring of prostate motion.
While external markers have been found to be well correlated with internal anatomy within an imaging session, there is no guarantee that these correlations will continue to exist and be constant throughout the course of the therapy. For example, even with precise patient localization, uncertainties in external marker-based monitoring may arise from (a) changes in patient anatomy (e.g., weight loss) and (b) day-to-day variations in marker placement. Further, it is difficult to use external markers to provide information about target deformation, or about dissimilar motion of multiple targets. In general, implanted, radio-opaque seeds have been found to be more reliable than external markers. However, in order to obtain high-quality real-time information about the target from radio-opaque markers, it is necessary to perform fluoroscopic x-ray imaging, resulting in increased patient dose. For this reason “hybrid strategies” that primarily use external surrogates and periodically update the position information using x-ray images of implanted markers are being attempted to overcome these issues. The situation is somewhat better with implanted electromagnetic transponders, which do not involve ionizing radiation. However, implantation of fiducials, whether radio-opaque or electromagnetic, is necessarily invasive and carries with it the risk of associated complications—an issue that becomes especially important for cancer patients with weakened systems.
Techniques attempting to perform beam adaptation are based on beam gating or continuous motion tracking. Gating is almost exclusively employed to account for respiratory motion, which tends to be periodic or quasi-periodic. From the information provided by the position monitoring system, the treatment beam is switched on during a pre-defined gating “window”. This temporal window (typically, 30 to 50% of the respiratory cycle) may be based on the time-varying positional changes as recorded by the monitoring system or on the respiratory phase calculated from the position information. Currently, gating, based on continuous position monitoring external, is considered the method of choice for the management of respiratory motion, however dosimetric variations remain as yet an unresolved problem due to variations in the breathing cycle by the patient.
Among the various strategies for beam adaptation, respiratory gating, used in conjunction with respiratory-correlated 4D CT, is arguably the most widely implemented. Despite promising results, there remains a major weakness in that gating assumes that target motion observed in planning images (typically acquired over one to three respiratory cycles) accurately and consistently represents motion over several respiratory cycles during delivery. This assumption need not hold true even within the same treatment session due to changes in patient's breathing pattern. Such changes include increase/decrease in breathing volume, or changes in the displacement and phase relationship between the surrogate and the tumor. Furthermore, the relatively low efficiency of gating (˜30 to 50%) when combined with that of IMRT delivery (˜20 to 50%) can result in 4 to 15 times increase in delivery time. Consequently, the tumor control efficacy of such prolonged treatments can potentially be compromised due to intrafraction repair of sub-lethally damaged tumor cells.
Tracking, in contrast to gating, uses a radiation beam for irradiating a target through a multiple leaf collimator (MLC), for collimating and adjusting the shape of the radiation beam projected onto the target. An image detector detects the image beam and generates an image signal of the target. The target signal is used to generate a beam adjustment signal for controlling the beam adjuster, thereby enabling the radiation beam generated by the radiation source to track the target. Other employed (x-ray) beam tracking strategies are based on a combination of a movable remote-controlled patient couch, a light-weight linear accelerator mounted on a robotic arm, and moving aperture(s) created by adjusting the MLC. However real-time tracking during treatment remains a real problem due to an inherent latency of the combined systems.
Limitations in the utility of current motion monitoring techniques include reliance on surrogates, assuming that these accurately represent internal target motion, monitoring only the target centroid for a single target and, in imposed “cost” upon the patient due to increased imaging dose and/or interventional complications.
Tumor tracking using a MLC is of much interest for intrafraction motion management in thoracic and abdominal cancer radiotherapy to overcome the significant geometric and dosimetric uncertainties in radiation treatment planning and dose delivery. Thus the need for achieving greater geometric precision in treatment planning and delivery is well recognized. It is highly desirable to routinely achieving sub-millimeter targeting accuracy during dose delivery.
Accordingly, there is a need to develop a robust, dynamic multi-leaf collimator (DMLC) tracking algorithm that obtains real-time information of the location shape, size and orientation of the target and surrounding anatomy from one or more independent position monitoring systems, and dynamically repositions the beam to account for real-time 3D motion and deformation of one of more targets and the surrounding anatomy including one or more critical organs, for the purpose of conformal intensity modulated radiotherapy (IMRT) and/or dynamic volumetric arc-based radiation delivery using x-ray or particle beams to account for the above-mentioned motion and deformation of the targets and the surrounding anatomy.