Validation and application of polymer gel dosimetry for the dose verification of an intensity-modulated arc therapy (IMAT) treatment

Polymer gel dosimetry was used to assess an intensity-modulated arc therapy (IMAT) treatment for whole abdominopelvic radiotherapy. Prior to the actual dosimetry experiment, a uniformity study on an unirradiated anthropomorphic phantom was carried out. A correction was performed to minimize deviations in the R2 maps due to radiofrequency non-uniformities. In addition, compensation strategies were implemented to limit R2 deviations caused by temperature drift during scanning. Inter- and intra-slice R2 deviations in the phantom were thereby significantly reduced. This was verified in an investigative study where the same phantom was irradiated with two rectangular superimposed beams: structural deviations between gel measurements and computational results remained below 3% outside high dose gradient regions; the spatial shift in those regions was within 2.5 mm. When comparing gel measurements with computational results for the IMAT treatment, dose deviations were noted in the liver and right kidney, but the dose-volume constraints were met. Root-mean-square differences between both dose distributions were within 5% with spatial deviations not more than 2.5 mm. Dose fluctuations due to gantry angle discretization in the dose computation algorithm were particularly noticeable in the low-dose region.     

Arc-modulated radiation therapy (AMRT): a single-arc form of intensity-modulated arc therapy

Arc-modulated radiation therapy (AMRT) is a novel rotational intensity-modulated radiation therapy (IMRT) technique developed for a clinical linear accelerator that aims to deliver highly conformal radiation treatment using just one arc of gantry rotation. Compared to fixed-gantry IMRT and the multiple-arc intensity-modulated arc therapy (IMAT) techniques, AMRT promises the same treatment quality with a single-arc delivery. In this paper, we present a treatment planning scheme for AMRT, which addresses the challenges in inverse planning, leaf sequencing and dose calculation. The feasibility and performance of this AMRT treatment planning scheme have been verified with multiple clinical cases of various sites on Varian linear accelerators.     

Intensity modulated arc therapy (IMAT) with centrally blocked rotational fields

A new technique for intensity-modulated beam (IMB) delivery that combines the features of intensity modulated arc therapy (IMAT) with the use of 'classical blocks' is proposed. The role of the blocks is to realize the high-gradient modulation of the intensity profile corresponding to the region to be protected within the body contour, while the MLC leaves or the secondary collimator defines the rest of the field and delivers intensity-modulated multiple rotational segments. The centrally blocked radiation fields are applied sequentially, in several rotations. Each rotation of the gantry is responsible for delivering one segment of the optimal intensity profile. The new IMAT technique is applied for a treatment geometry represented by an annular target volume centrally located within a circular body contour. The annulus encompasses a circular critical structure, which is to be protected. The beam opening and corresponding weight of each segment are determined in two ways. The first method applies a linear optimization algorithm to precalculated centrally blocked radial dose profiles. These radial profiles are calculated for a set of beam openings, ranging from the largest field that covers the whole planning target volume (PTV) to the smallest, which is 1 cm larger than the width of the central block. The optimization is subjected to dose homogeneity constraints imposed on a linear combination of these profiles and finally delivers the dimensions and weights of the rotational beams to be used in combination. The second method decomposes into several subfields the fluence profile of a rotational beam known to deliver a constant dose level to PTV. This fluence profile is determined by using the analytical method proposed by Brahme for the case of the annular PTV and the concentric organ at risk (OAR). The proper segmentation of this intensity profile provides the field sizes and corresponding weights of the subfields to be used in combination. Both methods show that for this particular treatment geometry, three to seven segments are sufficient to cover the PTV with the 95% dose level and to keep the dose level to the central critical structure under 30% of the maximum dose. These results were verified by experimentally delivering the calculated segments to radiotherapy verification films sandwiched between two cylindrical pieces of a pressed-wood phantom. The total beam time for a three-field irradiation was 77 s. The predicted and experimental dose profiles along the radius of the phantom agreed to within 5%. Generalization of this technique to real-patient treatment geometry and advantages over other conformal radiotherapy techniques are also discussed.     

MAGIC-type polymer gel for three-dimensional dosimetry: intensity-modulated radiation therapy verification

A new type of polymer gel dosimeter, which responds well to absorbed dose even when manufactured in the presence of normal levels of oxygen, was recently described by Fong et al. [Phys. Med. Biol. 46, 3105-3113 (2001)] and referred to by the acronym MAGIC. The aim of this study was to investigate the feasibility of using this new type of gel for intensity-modulated radiation therapy (IMRT) verification. Gel manufacturing was carried out in room atmosphere under normal levels of oxygen. IMRT inverse treatment planning was performed using the Helios software. The gel was irradiated using a linear accelerator equipped with a dynamic multileaf collimator, and intensity modulation was achieved using sliding window technique. The response to absorbed dose was evaluated using magnetic resonance imaging. Measured and calculated dose distributions were compared with regard to in-plane isodoses and dose volume histograms. In addition, the spatial and dosimetric accuracy was evaluated using the gamma formalism. Good agreement between calculated and measured data was obtained. In the isocenter plane, the 70% and 90% isodoses acquired using the different methods are mostly within 2 mm, with up to 3 mm disagreement at isolated points. For the planning target volume (PTV), the calculated mean relative dose was 96.8 +/- 2.5% (1 SD) and the measured relative mean dose was 98.6 +/- 2.2%. Corresponding data for an organ at risk was 34.4 +/- 0.9% and 32.7 +/- 0.7%, respectively. The gamma criterion (3 mm spatial/3% dose deviation) was fulfilled for 94% of the pixels in the target region. Discrepancies were found in hot spots the upper and lower parts of the PTV, where the measured dose was up to 11% higher than calculated. This was attributed to sub optimal scatter kernels used in the treatment planning system dose calculations. Our results indicate great potential for IMRT verification using MAGIC-type polymer gel.     

Monte Carlo dose verification for intensity-modulated arc therapy

Intensity-modulated arc therapy (IMAT), a technique which combines beam rotation and dynamic multileaf collimation, has been implemented in our clinic. Dosimetric errors can be created by the inability of the planning system to accurately account for the effects of tissue inhomogeneities and physical characteristics of the multileaf collimator (MLC). The objective of this study is to explore the use of Monte Carlo (MC) simulation for IMAT dose verification. The BEAM/DOSXYZ Monte Carlo system was implemented to perform dose verification for the IMAT treatment. The implementation includes the simulation of the linac head/MLC (Elekta SL20), the conversion of patient CT images and beam arrangement for 3D dose calculation, the calculation of gantry rotation and leaf motion by a series of static beams and the development of software to automate the entire MC process. The MC calculations were verified by measurements for conventional beam settings. The agreement was within 2%. The IMAT dose distributions generated by a commercial forward planning system (RenderPlan. Elekta) were compared with those calculated by the MC package. For the cases studied, discrepancies of over 10% were found between the MC and the RenderPlan dose calculations. These discrepancies were due in part to the inaccurate dose calculation of the RenderPlan system. The computation time for the IMAT MC calculation was in the range of 20-80 min on 15 Pentium-Ill computers. The MC method was also useful in verifying the beam apertures used in the IMAT treatments.     

Leaf-sequencing for intensity-modulated arc therapy using graph algorithms

Intensity-modulated arc therapy (IMAT) is a rotational IMRT technique. It uses a set of overlapping or nonoverlapping arcs to create a prescribed dose distribution. Despite its numerous advantages, IMAT has not gained widespread clinical applications. This is mainly due to the lack of an effective IMAT leaf-sequencing algorithm that can convert the optimized intensity patterns for all beam directions into IMAT treatment arcs. To address this problem, we have developed an IMAT leaf-sequencing algorithm and software using graph algorithms in computer science. The input to our leaf-sequencing software includes (1) a set of (continuous) intensity patterns optimized by a treatment planning system at a sequence of equally spaced beam angles (typically 10 degrees apart), (2) a maximum leaf motion constraint, and (3) the number of desired arcs, k. The output is a set of treatment arcs that best approximates the set of optimized intensity patterns at all beam angles with guaranteed smooth delivery without violating the maximum leaf motion constraint. The new algorithm consists of the following key steps. First, the optimized intensity patterns are segmented into intensity profiles that are aligned with individual MLC leaf pairs. Then each intensity profile is segmented into k MLC leaf openings using a k-link shortest path algorithm. The leaf openings for all beam angles are subsequently connected together to form 1D IMAT arcs under the maximum leaf motion constraint using a shortest path algorithm. Finally, the 1D IMAT arcs are combined to form IMAT treatment arcs of MLC apertures. The performance of the implemented leaf-sequencing software has been tested for four treatment sites (prostate, breast, head and neck, and lung). In all cases, our leaf-sequencing algorithm produces efficient and highly conformal IMAT plans that rival their counterpart, the tomotherapy plans, and significantly improve the IMRT plans. Algorithm execution times ranging from a few seconds to 2 min are observed on a laptop computer equipped with a 2.0 GHz Pentium M processor.     

Inverse planning for intensity-modulated arc therapy using direct aperture optimization

Intensity-modulated arc therapy (IMAT) is a radiation therapy delivery technique that combines gantry rotation with dynamic multi-leaf collimation (MLC). With IMAT, the benefits of rotational IMRT can be realized using a conventional linear accelerator and a conventional MLC. Thus far, the advantages of IMAT have gone largely unrealized due to the lack of robust automated planning tools capable of producing efficient IMAT treatment plans. This work describes an inverse treatment planning algorithm, called 'direct aperture optimization' (DAO) that can be used to generate inverse treatment plans for IMAT. In contrast to traditional inverse planning techniques where the relative weights of a series of pencil beams are optimized, DAO optimizes the leaf positions and weights of the apertures in the plan. This technique allows any delivery constraints to be enforced during the optimization, eliminating the need for a leaf-sequencing step. It is this feature that enables DAO to easily create inverse plans for IMAT. To illustrate the feasibility of DAO applied to IMAT, several cases are presented, including a cylindrical phantom, a head and neck patient and a prostate patient.     

Dynamic conformal arc therapy: transmitted signal in vivo dosimetry

A method for the determination of the in vivo isocenter dose, D(iso), has been applied to the dynamic conformal are therapy (DCAT) for thoracic tumors. The method makes use of the transmitted signal, S(t,alpha), measured at different gantry angles, a, by a small ion chamber positioned on the electronic portal imaging device. The in vivo method is implemented by a set of correlation functions obtained by the ratios between the transmitted signal and the midplane dose in a solid phantom, irradiated by static fields. The in vivo dosimetry at the isocenter for the DCAT requires the convolution between the signals, S(t,alpha), and the dose reconstruction factors, C(alpha), that depend on the patient's anatomy and on its tissue inhomogeneities along the beam central axis in the a direction. The C(alpha) factors are obtained by processing the patient's computed tomography scan. The method was tested by taking measurements in a cylindrical phantom and in a Rando Alderson phantom. The results show that the difference between the convolution calculations and the phantom measurements is within +/-2%. The in vivo dosimetry of the stereotactic DCAT for six lung tumors, irradiated with three or four arcs, is reported. The isocenter dose up to 17 Gy per therapy fraction was delivered on alternating days for three fractions. The agreement obtained in this pilot study between the total in vivo dose D(iso) and the planned dose D(iso,TPS) at the isocenter is +/-4%. The method has been applied on the DCAT obtaining a more extensive monitoring of possible systematic errors, the effect of which can invalidate the current therapy which uses a few high-dose fractions.     

Lithium formate EPR dosimetry for verifications of planned dose distributions prior to intensity-modulated radiation therapy

The objective of the present investigation was to evaluate lithium formate electron paramagnetic resonance (EPR) dosimetry for measurement of dose distributions in phantoms prior to intensity-modulated radiation therapy (IMRT). Lithium formate monohydrate tablets were carefully prepared, and blind tests were performed in clinically relevant situations in order to determine the precision and accuracy of the method. Further experiments confirmed that within the accuracy of the current method, the dosimeter response was independent of beam energies and dose rates used for IMRT treatments. The method was applied to IMRT treatment plans, and the dose determinations were compared to ionization chamber measurements. The experiments showed that absorbed doses above 3 Gy could be measured with an uncertainty of less than 2.5% of the dose (coverage factor k = 1.96). Measurement time was about 15 min using a well-calibrated dosimeter batch. The conclusion drawn from the investigation was that lithium formate EPR dosimetry is a promising new tool for absorbed dose measurements in external beam radiation therapy, especially for doses above 3 Gy.     

Multibeam tomotherapy: a new treatment unit devised for multileaf collimation, intensity-modulated radiation therapy

A fully integrated system for treatment planning, application, and verification for automated multileaf collimator (MLC) based, intensity-modulated, image-guided, and adaptive radiation therapy (IMRT, IGRT and ART, respectively) is proposed. Patient comfort, which was the major development goal, will be achieved through a new unit design and short treatment times. Our device for photon beam therapy will consist of a new dual energy linac with five fixed treatment heads positioned evenly along one plane but one electron beam generator only. A minimum of moving parts increases technical reliability and reduces motion times to a minimum. Motion is allowed solely for the MLCs, the robotic patient table, and the small angle gantry rotation of +/- 36 degrees. Besides sophisticated electron beam guidance, this compact setup can be built using existing modules. The flattening-filter-free treatment heads are characterized by reduced beam-on time and contain apertures restricted in one dimension to the area of maximum primary fluence output. In the case of longer targets, this leads to a topographic intensity modulation, thanks to the combination of "step and shoot" MLC delivery and discrete patient couch motion. Owing to the limited number of beam directions, this multislice cone beam serial tomotherapy is referred to as "multibeam tomotherapy." Every patient slice is irradiated by one treatment head at any given moment but for one subfield only. The electron beam is then guided to the next head ready for delivery, while the other heads are preparing their leaves for the next segment. The "Multifocal MLC-positioning" algorithm was programmed to enable treatment planning and optimize treatment time. We developed an overlap strategy for the longitudinally adjacent fields of every beam direction, in doing so minimizing the field match problem and the effects of possible table step errors. Clinical case studies show for the same or better planning target volume coverage, better organ-at-risk sparing, and comparable mean integral dose to the normal tissue a reduction in treatment time by more than 50% to only a few minutes in comparison to high-quality 3-D conformal and IMRT treatments. As a result, it will be possible to incorporate features for better patient positioning and image guidance, while sustaining reasonable overall treatment times at the same time. The virtual multibeam tomotherapy design study TOM'5-CT contains a dedicated electron beam CT (TOM'AGE) and an objective optical topometric patient positioning system (TOPOS). Thanks to the wide gantry bore of 120 cm and slim gantry depths of 70 cm, patients can be treated very comfortably, in all cases tumor-isocentrically, as well as with noncoplanar beam arrangements as in stereotactic radiosurgery with a couch rotation of up to +/- 54 degrees. The TOM'5 treatment unit on which this theoretical concept is based has a stand-alone depth of 40 cm and an outer diameter of 245 cm; the focus-isocenter distance of the heads is 100 cm with a field size of 40 cm x 7 cm and 0.5 cm leaves, which operate perpendicular to the axis of table motion.     

Temporal characterization and in vitro comparison of cell survival following the delivery of 3D-conformal, intensity-modulated radiation therapy (IMRT) and volumetric modulated arc therapy (VMAT)

A phantom was designed and implemented for the delivery of treatment plans to cells in vitro. Single beam, 3D-conformal radiotherapy (3D-CRT) plans, inverse planned five-field intensity-modulated radiation therapy (IMRT), nine-field IMRT, single-arc volumetric modulated arc therapy (VMAT) and dual-arc VMAT plans were created on a CT scan of the phantom to deliver 3 Gy to the cell layer and verified using a Farmer chamber, 2D ionization chamber array and gafchromic film. Each plan was delivered to a 2D ionization chamber array to assess the temporal characteristics of the plan including delivery time and 'cell's eye view' for the central ionization chamber. The effective fraction time, defined as the percentage of the fraction time where any dose is delivered to each point examined, was also assessed across 120 ionization chambers. Each plan was delivered to human prostate cancer DU-145 cells and normal primary AGO-1522b fibroblast cells. Uniform beams were delivered to each cell line with the delivery time varying from 0.5 to 20.54 min. Effective fraction time was found to increase with a decreasing number of beams or arcs. For a uniform beam delivery, AGO-1552b cells exhibited a statistically significant trend towards increased survival with increased delivery time. This trend was not repeated when the different modulated clinical delivery methods were used. Less sensitive DU-145 cells did not exhibit a significant trend towards increased survival with increased delivery time for either the uniform or clinical deliveries. These results confirm that dose rate effects are most prevalent in more radiosensitive cells. Cell survival data generated from uniform beam deliveries over a range of dose rates and delivery times may not always be accurate in predicting response to more complex delivery techniques, such as IMRT and VMAT.     

肺癌固定剂量率旋转调强和容积旋转调强的剂量学分析

目的:通过对肺癌固定剂量率旋转调强放疗（IMAT）计划和容积旋转调强放疗（VMAT）计划的剂量学分析,为临床应用中肺癌VMAT放疗剂量率方式的选取提供参考。 方法:取11例肺癌患者,用RayStation计划系统设计IMAT和VMAT计划,比较其剂量学、机器跳数（MU）和治疗时间的差异。 结果:（1）11例肺癌患者的双弧IMAT和VMAT计划均能满足临床要求,IMAT和VMAT计划的靶区最小剂量D98%、最大剂量D2%、平均剂量（Dmean）、靶区均匀性指数、靶区适形度指数相近,无明显差异。靶区覆盖率VMAT计划好于IMAT计划。（2）危及器官受量:全肺的V5、V10、V20、Dmean和心脏的V20,VMAT计划比IMAT计划低。全肺的V30、心脏的V30、脊髓的最大剂量D1%,两种计划之间无明显差异。（3）正常组织在低剂量部分V5、V10、V15和Dmean,VMAT计划低于IMAT计划;V20、V25两种计划无明显差异;接近处方剂量部分V30、V35、V40,VMAT计划高于IMAT计划。（4）出束时间和MU:VMAT计划相比于IMAT计划,治疗出束时间大大减少,VMAT计划出束时间仅为IMAT计划出束时间的62%。两者的MU无明显差异。（5）两种计划的剂量验证通过率均大于95%,达到98.72%以上,能满足治疗要求。VMAT计划的剂量验证通过率略低于IMAT计划,相差约0.44%。 结论:VMAT技术相较于IMAT技术,其计划调制能力更强,可得到更优的靶区剂量分布,提高治疗效率,可以更好地保护危及器官,尤其是减少肺的低剂量照射体积。因此,在肺癌的旋转调强放射治疗中,VMAT技术相较于IMAT技术存在较大的优势。     

A new model for computerized clinical electron beam dosimetry

Clinical electron beams consist of primary electrons, primary bremsstrahlung generated in the regular photon and electron collimator system determining the composite beam, and some short-range contaminant photon and electron scatter arising from the lower parts of the standard or regular electron applicator. Any beam-shaping insert placed inside the applicator causes some extra ("contaminant") bremsstrahlung and electron scatter. The new dose calculation model is based on separate treatment of these components. For the calculation of the primary electron dose we use experimentally determined electron scatter functions and differential electron scatter functions. The primary bremsstrahlung is treated as an unflattened but otherwise regular x-ray beam. The contaminant components arising from the rim area of the regular electron collimator and from beam-shaping inserts are considered separately. The behavior of the in-air ionization profiles is described using the concepts of effective electron source position and effective electron source diameter. The model has been tested for several electron energies.     

A novel four-dimensional image registration method for radionuclide therapy dosimetry

A novel method for registering sequential SPECT scans (4DRRT) is described, whereby all sequential scans acquired in the course of a therapy or a pre-therapy tracer study may be registered in one pass. The method assumes that a monoexponential decay function can be fitted to the series of sequential SPECT scans. Multiple volumes, presenting with different decay rates, are fitted with different mono-exponential functions. The MSSE (mean sum of squared errors in the least-squares fit algorithm), over the volume used for registration, is the cost function minimized at registration. Simulated data were used to assess the effect of thresholding, smoothing, noise and the multi-exponential nature of the four-dimensional (4D) SPECT studies on the performance of 4DRRT, resulting in three-dimensional (3D) residual registration errors <3.5 mm. The 4DRRT method was then compared to the following 3D registration methods: the correlation coefficient, the sum of absolute differences, the variance of image ratios and the mutual information. The comparisons, using both simulated and clinical data, were based on the standard deviation of the effective decay time distribution, generated from the registered 4D dataset, and showed that image registration using 4DRRT is simpler and more robust compared to the 3D techniques, especially when multiple tumour sites with different decay rates are present.     

非小细胞肺癌容积旋转调强与适形调强放疗的剂量学比较

目的:比较容积旋转调强放疗（VMAT）和适形调强放疗（IMRT）技术在非小细胞肺癌靶区和危及器官（OAR）的剂量学差异。 方法:选取首程接受放射治疗的20例非小细胞肺癌患者,分别设计5野IMRT（5F-IMRT）、7野IMRT（7F-IMRT）、双弧VMAT（D-VMAT）和部分弧VMAT（P-VMAT）计划,比较靶区剂量分布、OAR剂量体积参数。 结果:4种计划中计划靶区的Dmean比较差异无统计学意义（P>0.05）;两种VMAT计划中计划靶区的均匀性指数和适形度指数均优于两种IMRT（P<0.05）;4种计划中D-VMAT肺平均剂量高于其余3种计划（P<0.05）;P-VMAT的双肺V5、V10稍好于D-VMAT（P<0.05）,但两种VMAT计划均高于两种IMRT计划（P<0.05）;4种计划中P-VMAT的双肺V20最优,且4种计划相互间比较差异有统计学意义（P<0.05）;D-VMAT与P-VMAT双肺的V30相当（P>0.05）,但均优于两种IMRT计划（P<0.05）;4种计划双肺V40和心脏的V30、V40比较差异无统计学意义（P>0.05）。P-VMAT计划的脊髓Dmax最低,与其余计划相比差异均有统计学意义（P<0.05）。 结论:非小细胞肺癌靶区剂量分布D-VMAT和P-VMAT好于IMRT计划。P-VMAT在OAR的保护方面体现的优势更多。综合考虑,非小细胞肺癌的放疗优先推荐P-VMAT,但对于重点考虑肺低剂量区,而次要考虑靶区剂量分布的病例推荐7F-IMRT。     

术前宫颈癌容积旋转调强与固定野动态调强技术剂量对比

目的:比较容积旋转调强（VMAT）和固定野动态调强放疗（DIMRT）技术在术前宫颈癌放疗中的剂量学差别。 方法:选取10例接受放疗的术前宫颈癌患者,勾画靶区,用Eclipse 11.0计划系统设计双弧VMAT计划和5野DIMRT计划,比较两者靶区的适形度指数（CI）和均匀性指数（HI）、危及器官的剂量学差别、加速器跳数和照射时间。 结果:双弧VMAT计划与5野DIMRT计划的CI（0.78±0.05,0.84±0.03; P>0.05）和HI（0.05±0.00, 0.05±0.00; P>0.05）统计学无显著性差异,膀胱的V40、V45、V50和小肠的V20、股骨头、骨盆剂量有显著性差异之外,其它指标未有显著性差异。 结论:VMAT和DIMRT技术两者均能很好达到临床剂量学的要求,但在保护正常器官方面,VMAT比DIMRT技术更优或相当,VMAT技术在减少加速器跳数和照射时间方面优势明显。     

Just-in-time tomography (JiTT): a new concept for image-guided radiation therapy

Soft-tissue target motion is one of the main concerns in high-precision radiation therapy. Cone beam computed tomography (CBCT) has been developed recently to image soft-tissue targets in the treatment room and guide the radiation therapy treatment. However, due to its relatively long image acquisition time the CBCT approach cannot provide images of the target at the instant of the treatment and thus it is not adequate for imaging targets with intrafraction motion. In this note, a new approach for image-guided radiation therapy-just-in-time tomography (JiTT)-is proposed. Differing from CBCT, JiTT takes much less time to generate the needed tomographical, beam's-eye-view images of the treatment target at the right moment to guide the radiation therapy treatment.     

Intensity-modulated arc therapy: principles, technologies and clinical implementation

Intensity-modulated arc therapy (IMAT) was proposed by Yu (1995 Phys. Med. Biol. 40 1435-49) as an alternative to tomotherapy. Over more than a decade, much progress has been made. The advantages and limitations of the IMAT technique have also been better understood. In recent years, single-arc forms of IMAT have emerged and become commercially adopted. The leading example is the volumetric-modulated arc therapy (VMAT), a single-arc form of IMAT that delivers apertures of varying weights with a single-arc rotation that uses dose-rate variation of the treatment machine. With commercial implementation of VMAT, wide clinical adoption has quickly taken root. However, there remains a lack of general understanding for the planning of such arc treatments, as well as what delivery limitations and compromises are made. Commercial promotion and competition add further confusion for the end users. It is therefore necessary to provide a summary of this technology and some guidelines on its clinical implementation. The purpose of this review is to provide a summary of the works from the radiotherapy community that led to wide clinical adoption, and point out the issues that still remain, providing some perspective on its further developments. Because there has been vast experience in IMRT using multiple intensity-modulated fields, comparisons between IMAT and IMRT are also made in the review within the areas of planning, delivery and quality assurance.     

Volumetric modulated arc therapy: IMRT in a single gantry arc

In this work a novel plan optimization platform is presented where treatment is delivered efficiently and accurately in a single dynamically modulated arc. Improvements in patient care achieved through image-guided positioning and plan adaptation have resulted in an increase in overall treatment times. Intensity-modulated radiation therapy (IMRT) has also increased treatment time by requiring a larger number of beam directions, increased monitor units (MU), and, in the case of tomotherapy, a slice-by-slice delivery. In order to maintain a similar level of patient throughput it will be necessary to increase the efficiency of treatment delivery. The solution proposed here is a novel aperture-based algorithm for treatment plan optimization where dose is delivered during a single gantry arc of up to 360 deg. The technique is similar to tomotherapy in that a full 360 deg of beam directions are available for optimization but is fundamentally different in that the entire dose volume is delivered in a single source rotation. The new technique is referred to as volumetric modulated arc therapy (VMAT). Multileaf collimator (MLC) leaf motion and number of MU per degree of gantry rotation is restricted during the optimization so that gantry rotation speed, leaf translation speed, and dose rate maxima do not excessively limit the delivery efficiency. During planning, investigators model continuous gantry motion by a coarse sampling of static gantry positions and fluence maps or MLC aperture shapes. The technique presented here is unique in that gantry and MLC position sampling is progressively increased throughout the optimization. Using the full gantry range will theoretically provide increased flexibility in generating highly conformal treatment plans. In practice, the additional flexibility is somewhat negated by the additional constraints placed on the amount of MLC leaf motion between gantry samples. A series of studies are performed that characterize the relationship between gantry and MLC sampling, dose modeling accuracy, and optimization time. Results show that gantry angle and MLC sample spacing as low as 1 deg and 0.5 cm, respectively, is desirable for accurate dose modeling. It is also shown that reducing the sample spacing dramatically reduces the ability of the optimization to arrive at a solution. The competing benefits of having small and large sample spacing are mutually realized using the progressive sampling technique described here. Preliminary results show that plans generated with VMAT optimization exhibit dose distributions equivalent or superior to static gantry IMRT. Timing studies have shown that the VMAT technique is well suited for on-line verification and adaptation with delivery times that are reduced to approximately 1.5-3 min for a 200 cGy fraction.