Intensity-modulated radiation therapy (IMRT) represents one of the most significant technical advances in radiation therapy since the advent of the medical linear accelerator. It allows the clinical implementation of highly conformal nonconvex dose distributions. This complex but promising treatment modality is rapidly proliferating in both academic and community practice settings. However, these advances do not come without a risk. IMRT is not just an add-on to the current radiation therapy process; it represents a new paradigm that requires the knowledge of multimodality imaging, setup uncertainties and internal organ motion, tumor control probabilities, normal tissue complication probabilities, three-dimensional (3-D) dose calculation and optimization, and dynamic beam delivery of nonuniform beam intensities. Therefore, the purpose of this report is to guide and assist the clinical medical physicist in developing and implementing a viable and safe IMRT program. The scope of the IMRT program is quite broad, encompassing multileaf-collimator-based IMRT delivery systems, goal-based inverse treatment planning, and clinical implementation of IMRT with patient-specific quality assurance. This report, while not prescribing specific procedures, provides the framework and guidance to allow clinical radiation oncology physicists to make judicious decisions in implementing a safe and efficient IMRT program in their clinics.

The desire to improve local tumour control and cure more cancer patients, coupled with advances in computer technology and linear accelerator design, has spurred the developments of three-dimensional conformal radiotherapy techniques. Optimized treatment plans, aiming to deliver high dose to the target while minimizing dose to the surrounding tissues, can be delivered with multiple fields each with spatially modulated beam intensities or with multiple-slice treatments. This paper introduces a new method, intensity-modulated arc therapy (IMAT), for delivering optimized treatment plans to improve the therapeutic ratio. It utilizes continuous gantry motion as in conventional arc therapy. Unlike conventional arc therapy, the field shape, which is conformed with the multileaf collimator, changes during gantry rotation. Arbitrary two-dimensional beam intensify distributions at different beam angles are delivered with multiple superimposing arcs. A system capable of delivering the IMAT has been implemented. An example is given that illustrates the feasibility of this new method. Advantages of this new technique over tomotherapy and other slice-based delivery schemes are also discussed.

IMRT treatment plans for step-and-shoot delivery have traditionally been produced through the optimization of intensity distributions (or maps) for each beam angle. The optimization step is followed by the application of a leaf-sequencing algorithm that translates each intensity map into a set of deliverable aperture shapes. In this article, we introduce an automated planning system in which we bypass the traditional intensity optimization, and instead directly optimize the shapes and the weights of the apertures. We call this approach "direct aperture optimization." This technique allows the user to specify the maximum number of apertures per beam direction, and hence provides significant control over the complexity of the treatment delivery. This is possible because the machine dependent delivery constraints imposed by the MLC are enforced within the aperture optimization algorithm rather than in a separate leaf-sequencing step. The leaf settings and the aperture intensities are optimized simultaneously using a simulated annealing algorithm. We have tested direct aperture optimization on a variety of patient cases using the EGS4/BEAM Monte Carlo package for our dose calculation engine. The results demonstrate that direct aperture optimization can produce highly conformal step-and-shoot treatment plans using only three to five apertures per beam direction. As compared with traditional optimization strategies, our studies demonstrate that direct aperture optimization can result in a significant reduction in both the number of beam segments and the number of monitor units. Direct aperture optimization therefore produces highly efficient treatment deliveries that maintain the full dosimetric benefits of IMRT.

Computer-optimized treatment plans, aimed to enhance tumour control and reduce normal tissue complication, generally require non-uniform beam intensities. One of the techniques for delivering intensity-modulated beams is the use of dynamic multileaf collimation, where the beam aperture moves and the field shape changes during irradiation. When intensity-modulated beams are delivered with dynamic collimation, the problem of intra-fraction organ motion can cause distortions to the desired beam intensities. Unlike static field treatments, where intra-fraction organ motion only affects the boundaries creating broad dose penumbra, the interplay of the movement of the beam aperture and the movement of the patient anatomy can create 'hot' and 'cold' spots throughout the field. The mechanism for creating these effects is not well understood. This paper provides a simple analytical model which illustrates the fundamental mechanism for creating the dosimetric variations in the target when both the beam aperture and the target move during irradiation. Numerical analysis was carried out which calculates the cumulative primary photon fluence, or beam intensity, received by each point in the target, for a given pattern of motion. The results show that, for clinically realistic parameters, the magnitude of intensity variations in the target can be greater than 100% of the desired beam intensity. The magnitude of the photon intensity variations is strongly dependent on the speed of the beam aperture relative to the speed of the target motion, and the width of the scanning beam relative to the amplitude of target motion. The effects of fractionation as well as methods of minimizing and eliminating the dosimetric effects of intra-fraction organ motion are discussed.

Significant differences between planned and delivered treatments may occur due to respiration-induced tumour motion, leading to underdosing of parts of the tumour and overdosing of parts of the surrounding critical structures. Existing methods proposed to counter tumour motion include breath-holds, gating and MLC-based tracking. Breath-holds and gating techniques increase treatment time considerably, whereas MLC-based tracking is limited to two dimensions. We present an alternative solution in which a robotic couch moves in real time in response to organ motion. To demonstrate proof-of-principle, we constructed a miniature adaptive couch model consisting of two movable platforms that simulate tumour motion and couch motion, respectively. These platforms were connected via an electronic feedback loop so that the bottom platform responded to the motion of the top platform. We tested our model with a seven-field step-and-shoot delivery case in which we performed three film-based experiments: (1) static geometry, (2) phantom-only motion and (3) phantom motion with simulated couch motion. Our measurements demonstrate that the miniature couch was able to compensate for phantom motion to the extent that the dose distributions were practically indistinguishable from those in static geometry. Motivated by this initial success, we investigated a real-time couch compensation system consisting of a stereoscopic infra-red camera system interfaced to a robotic couch known as the Hexapod, which responds in real time to any change in position detected by the cameras. Optical reflectors placed on a solid water phantom were used as surrogates for motion. We tested the effectiveness of couch-based motion compensation for fixed fields and a dynamic arc delivery cases. Due to hardware limitations, we performed film-based experiments (1), (2) and (3), with the robotic couch at a phantom motion period and dose rate of 16 s and 100 MU min(-1), respectively. Analysis of film measurements showed near-equivalent dose distributions (