Lei Xing and Arthur Boyer
Department of Radiation Oncology, Stanford University
Stanford, CA 94305-5847, USA
Developments in computer plan optimization and computerized delivery of radiotherapy using multileaf collimators (MLC) have led to a new treatment modality referred to as intensity-modulated radiation therapy (IMRT). The advent of IMRT represents one of the most importance technical advancement in the history of radiation therapy. Institutions worldwide are attempting or planning to integrate this new technology into their clinics. The purpose of this talk is to present an overview of physics aspects of the state-of-the-art IMRT technology.
IMRT consists the following major steps: patient immobilization and treatment planning CT/MRI image acquisition, tumor target volume and sensitive structure delineation, inverse treatment planning, pre-treatment quality assurance (QA) and simulation on, on-treatment QA, and IMRT dose delivery. Among them, the treatment planning, QA and dose delivery are distinctly different from the conventional 3D conformal radiation therapy (CRT). Instead of using X-ray beams with either uniform or wedged-shaped photon fluence distributions as in 3D CRT, the fluence profile of an IMRT beam can take an arbitrary form and is optimally chosen on a patient specific basis. For a given patient, the task of IMRT treatment planning is to determine the fluence profiles of the incident beams (typically 5-9 beams from different directions). This is usually achieved by using a special computer optimization technique named inverse treatment planning, during which the computer iteratively computes and compares different competing IMRT plans based on a mathematical model that mimics the decision-making process of a physician in ranking different competing treatment plans. This process continues until a satisfactory plan is obtained. Several commercial companies now offer inverse treatment planning systems for IMRT and much research efforts are being made in both academic institutions and industries to improve the performance of the currently available computing techniques.
Upon the completion of dose optimization, each optimal fluence profile is converted into a stack of segmented MLC-shaped fields through a leaf sequencing software implemented in the treatment planning system. The MLC sequences specified by their shapes and relative weights are then compiled into an electronic file and the execution of the file at the treatment delivery machine will realize the intended intensity modulation. Intensity modulation is achieved with computer-controlled MLC using either static, in which the MLC movement from a segment to another and the dose delivery are done at different instances, or dynamic delivery techniques, in which the MLC movement and dose delivery are realized simultaneously.
A rigorous but practical QA procedure is critically important to ensure that the planned dose distribution can be achieved safely and accurately. The QA of IMRT has three natural aspects; commissioning and testing of the system, verification of treatment plans, and routine QA of the MLC delivery system. Principles and practice of the QA for radiotherapy can be found in the AAPM Task Group Report 40. The tasks of patient specific QA can be divided into geometric verification and dosimetric verification. The former is concerned with the geometric accuracy of IMRT beams, typically including the isocenter verification and the portal verification. A pair of orthogonal digital reconstruction radiographs (DRRs) are used to verify the patient position by comparison with simulation films from a conventional simulator. A ˇ°conventionalˇ± portal image for an IMRT field can be created as well, using the MLC boundary as the port of the radiation field. AAPM IMRT Subcommittee (Medical Physics 30, 2089-2115, 2004) and ASTRO IMRT Scope Committee (International Journal of Radiation Oncology, Biology, Physics 58, 1616-1634, 2004) have made a series of recommendations regarding what dosimetric quantities need to be examined in order to dosimetrically validate an IMRT treatment plan.