Intensity Modulated Arc Therapy: Technology and Clinical Implementation

ID: 10351 Title: Advances in arc therapy Intensity Modulated Arc Therapy: Technology and Clinical Implementation Cedric X. Yu University of Maryland School of Medicine AbstractID: 10351 Title: Advances in arc therapyID: 10351 Title: Advances in arc therapy Abstract Intensity modulated arc therapy (IMAT) was proposed by Yu as an alternative to Tomotherapy. Over more than a decade, many progresses have been made. The advantages and limitations of IMAT technique have also been better understood. In recent years, single arc forms of IMAT has emerged and commercially adopted. The leading example was the Volumetric Modulated Art Therapy, a single arc form of IMAT that delivers apertures of varying weightings with a single arc rotation using dose rate variation of the treatment machine. With the commercial implementation of VMAT, wide clinical adoption is quickly taken root. However, there is a lack of a general understanding of how such arc treatments are planned, and what delivery limitations and compromises are made. Commercial promotions and competitions added further confusion for the end users. It is therefore necessary to provide a summary of this technology and some guidelines on its clinical implementation and quality assurance. The purpose of this review is to provide a summary of the works from the community leading to the wide clinical adaptation, and point to the issues still remaining, provide some perspectives on its further development in the context of increased clinical use ofIntensity modulated arc therapy (IMAT) was proposed by Yu as an alternative to Tomotherapy. Over more than a decade, many progresses have been made. The advantages and limitations of IMAT technique have also been better understood. In recent years, single arc forms of IMAT has emerged and commercially adopted. The leading example was the Volumetric Modulated Art Therapy, a single arc form of IMAT that delivers apertures of varying weightings with a single arc rotation using dose rate variation of the treatment machine. With the commercial implementation of VMAT, wide clinical adoption is quickly taken root. However, there is a lack of a general understanding of how such arc treatments are planned, and what delivery limitations and compromises are made. Commercial promotions and competitions added further confusion for the end users. It is therefore necessary to provide a summary of this technology and some guidelines on its clinical implementation and quality assurance. The purpose of this review is to provide a summary of the works from the community leading to the wide clinical adaptation, and point to the issues still remaining, provide some perspectives on its further development in the context of increased clinical use of image guidance. Because there has been vast experience in IMRT using multiple intensity modulated fields, comparisons between IMAT and IMRT are also made in the review in the areas of planning, delivery and quality assurance. AbstractID: 10351 Title: Advances in arc therapyID: 10351 Title: Advances in arc therapy I. HISTORICAL REVIEW 1, Early development leading to IMAT Although arc therapy can be traced back to the dawn of the 20 century, arc therapy involving dynamic field shaping using a multileaf collimator was first described by Takahashi in 1965 [1]. He described a method of rotational therapy, which we now referred to as conformal arc therapy – the beam aperture shaped by a multiple leaf collimator (MLC) dynamically varies to match the beam’s-eye-view of the target. In 1982, Brahme et al solved an integral equation for a hypothetical target wrapped around a critical structure and treated with arc therapy [2]. They demonstrated that to deliver a uniform dose to the target while sparing the critical structure, the beam intensities have to be modulated. In 1983, Chin et al proposed and demonstrated that with computer optimization and the freedom of computer-controlled gantry rotation, collimator motion and dose-rate variation, highly conformal dose distribution can be achieved [3]. These initial developments on arc therapy were accompanied and followed by the development and wide adoption of three-dimensional radiation therapy (3DCRT) in the 1980s [4]. The need for more convenient field shaping brought the multileaf collimator (MLC) to radiotherapy practice. In 1985, Brahme et al published a paper which showed that if the intensities of radiation can be modulated across a radiation field, the increased freedom would afford a greater ability to shape the volume of high doses to better conform to the target than 3DCRT [5]. The motorized field shaping capabilities of MLC was quickly explored to modulate the intensities within a radiation field. Intensitymodulated radiation therapy (IMRT) aims to deliver a highly conformal dose to a tumor while sparing the surrounding normal tissues and sensitive structures. In 1992, Convery AbstractID: 10351 Title: Advances in arc therapyID: 10351 Title: Advances in arc therapy and Rosenbloom derived the mathematical formula for realizing intensity modulation with the dynamic movement of collimator [6]. In 1994, more works were published to demonstrate the feasibility of using MLCs for intensity modulation [7-10]. The amount of work on this emerging technology quickly mushroomed, and clinical implementations of the IMRT technique immediately followed [11]. In 1993, another form of IMRT using rotational fan beams, called Tomotherapy, was proposed by Mackie et al [12]. The idea was quickly commercialized as the Peacock device by NOMOS Corporation [N/A]. Intensity modulation was achieved with a binary collimator, which opens and closes under computer control. As the fan beam continuously rotates around the patient, the exposure time of a small width of the fan beam, or a beamlet, can be adjusted with the opening and closing of the binary collimator, allowing the radiation to be delivered to the tumor through the most preferred directions and locations of the patient. The initial commercial system by NOMOS Corporation added the binary collimator on to a linear accelerator and delivered radiation treatments one slice at a time, and the treatment table had to be precisely indexed from one slice to the next. Helical tomotherapy was then developed by Tomotherapy, Inc. as a dedicated rotational IMRT system with a slip-ring rotating gantry. More automated delivery was achieved by continuous gantry rotation and treatment couch translation [same as 12]. The dosimetric advantages of rotational treatments are illustrated by Shepard et al [13], which summarizes results from an optimization series performed for a C-shaped target with a sensitive structure in the concavity of the C. For these simulations, all planning parameters such as percent dose constraints were held constant except for the number of beam angles. It was shown that each increase in the number of beam angles led to a more homogeneous dose in the tumor and a AbstractID: 10351 Title: Advances in arc therapyID: 10351 Title: Advances in arc therapy lower dose to the sensitive structure. Significant dosimetric improvements continued well beyond the number of beam angles typically used for fixed field IMRT. It is also noteworthy that the total integral dose is nearly independent of the number of beam angles. # Angles Obj. Funct. Value Std. Dev. in target dose d95 Mean dose to RAR Total integral dose 3 0.665 0.124 0.747 0.488 2732.5 5 0.318 0.090 0.814 0.215 2563.3 7 0.242 0.064 0.867 0.206 2596.8 9 0.222 0.064 0.855 0.192 2598.3 11 0.202 0.058 0.879 0.186 2570.2 15 0.187 0.053 0.908 0.18