Dual-gated volumetric modulated arc therapy
© Fahimian et al.; licensee BioMed Central Ltd. 2014
Received: 30 October 2013
Accepted: 29 August 2014
Published: 25 September 2014
Gated Volumetric Modulated Arc Therapy (VMAT) is an emerging radiation therapy modality for treatment of tumors affected by respiratory motion. However, gating significantly prolongs the treatment time, as delivery is only activated during a single respiratory phase. To enhance the efficiency of gated VMAT delivery, a novel dual-gated VMAT (DG-VMAT) technique, in which delivery is executed at both exhale and inhale phases in a given arc rotation, is developed and experimentally evaluated.
Arc delivery at two phases is realized by sequentially interleaving control points consisting of MUs, MLC sequences, and angles of VMAT plans generated at the exhale and inhale phases. Dual-gated delivery is initiated when a respiration gating signal enters the exhale window; when the exhale delivery concludes, the beam turns off and the gantry rolls back to the starting position for the inhale window. The process is then repeated until both inhale and exhale arcs are fully delivered. DG-VMAT plan delivery accuracy was assessed using a pinpoint chamber and diode array phantom undergoing programmed motion.
DG-VMAT delivery was experimentally implemented through custom XML scripting in Varian’s TrueBeam™ STx Developer Mode. Relative to single gated delivery at exhale, the treatment time was improved by 95.5% for a sinusoidal breathing pattern. The pinpoint chamber dose measurement agreed with the calculated dose within 0.7%. For the DG-VMAT delivery, 97.5% of the diode array measurements passed the 3%/3 mm gamma criterion.
The feasibility of DG-VMAT delivery scheme has been experimentally demonstrated for the first time. By leveraging the stability and natural pauses that occur at end-inspiration and end-exhalation, DG-VMAT provides a practical method for enhancing gated delivery efficiency by up to a factor of two.
KeywordsDual gating IGRT VMAT SBRT
Respiratory induced tumor motion is the major complicating factor in radiotherapy of thoracic and upper abdominal targets. A variety of techniques have been developed for the clinical management of organ motion, each with distinct advantages and drawbacks [1–5]. These techniques can be generally categorized in order of approximate increased technical complexity as motion encompassing irradiation, breath-hold methods, compression methods, gating methods, and dynamic tracking methods. Among these, gating methods have gained clinical traction as they limit the volume of normal tissue irradiated relative to motion encompassing irradiation, yet provide a reliable and a technical feasible alternative to continuous tracking irradiation [6–11].
With improvements in dynamic delivery, Volumetric Modulated Arc Therapy (VMAT) has emerged as an efficient alternative to static field Intensity Modulated Radiotherapy (IMRT) for producing highly conformal dose distributions [12–14]. More recently, gated VMAT has been implemented for treating tumors influenced by respiratory motion by restricting the arc delivery to a single stable portion of the respiratory cycle, such as the end-of-exhale (EOE) phase. While gated VMAT is a promising technique, since the delivery is restricted to a narrow segment of the respiration, the treatment time is significantly increased, which depending on the width of the gating window, can result as much as 5.5 times relative elongation of the treatment time relative to non-gated treatments [15, 16]. Prolonged treatment time can result in dosimetric inaccuracy, baseline shift, increased patient discomfort, degradation of radiobiological efficacy, and reduction of clinical throughput. Moreover, improving gated delivery efficiency is of particular importance in Stereotactic Body Radiation Therapy (SBRT), because of the already protracted delivery associated with large dose fractions. In general, gating window selection is a compromise between the residual tumor movement within the gating window and dose delivery time. Because there is generally relatively little residual tumor motion within the end-of-exhale (EOE) phase , the EOE window is often selected for gating. However, the end-of-inhale (EOI) phase is also relatively stable, and can be dosimetrically advantageous in certain cases [8–10]. A delivery scheme that enables beam-on during both EOE and EOI would make it possible to combine the delivery advantages of both phases. It is noted that, while theoretically, continuous tracking delivery has the potential for full duty cycle efficiency , due to stringent technical demands, and more importantly, clinical concerns in accurately predicting and irradiating the more variable portions of the breathing cycle in-between the more stable exhale and inhale, its clinical application on a conventional multi-leaf collimator linac has been limited. For these reasons, a simple and technically practical solution for enhancing gated delivery that irradiates only the clinically reliable portions of the breathing cycle is desirable.
To address the limitations in efficiency of gated VMAT, in this work we introduce and demonstrate Dual-Gated VMAT (DG-VMAT) – a method that alternatively delivers dose at both the inhale and exhale phases during a VMAT delivery. While gating on a single phase is currently implementable on most modern linacs gating on more than a single phase requires the synchronization of the MLCs and gantry motion to the different locations of the target at the two different phases of the respiratory cycle. A practical approach to such an implementation is presented, and the feasibility is demonstrated by experimentally implementing DG-VMAT on linac using custom scripting.
DG-VMAT delivery scheme
The method necessitates two independent plans to be optimized at the EOE and EOI phases using a 4DCT simulation scan. In the feasibility study, an SBRT patient case with a 4D planning CT and approximately 2 cm superior-inferior tumor motion between EOE and EOI was selected to demonstrate the DG-VMAT planning and delivery process. Two independent VMAT treatment plans were generated using Varian’s Eclipse treatment planning system (Version 8.9); one for EOE and the other for EOI, based on the respective exhale and inhale phase images of the 4DCT scans. For each of the two plans, a dose of 15 Gy was prescribed to the 95% of PTV volume, and a full arc plan consisting of 177 segments (control points) was optimized for the same dose prescription and arc optimization criteria. 10 MV Flattering Filter Free (FFF) beams were used for treatment planning. A fractionation scheme 15 Gy in 3 was set for delivery.
Experimental implementation and dosimetric validation
DG-VMAT plan scheme in Figure 1 was experimentally implemented for an SBRT plan using Developer Mode XML scripting in the TrueBeam™ STx platform (Varian Medical Systems, Palo Alto, CA), which enables programmed control of all system parameters. Using control points specifying the MLC leaf sequences and MUs derived from the independent single phase plans, a DG-VMAT delivery was programmed by sequentially interleaving the control points of the EOE and EOI plans, and enabling beam-on at the two corresponding phases of the respiratory surrogate phase.
Dual-gated VMAT plans were delivered and validated by running the formulated XML script, triggered by the RPM infrared camera and reflector system affixed to the motion platform (Varian Medical Systems, Palo Alto, CA). The delivered dose was measured by using a 0.015 cc PTW N31014 pinpoint chamber (PTW, Freiburg, Germany) in 14 cm of solid water and a Delta 4 diode array (ScandiDos, Uppsala, Sweden). Both the pinpoint chamber and Delta 4 phantom were placed on top of a motion platform, which served to simulate breathing motion. A 2 cm motion in the superior-inferior direction with a period of 6 seconds was utilized, with the inhale and exhale gating windows set at 25% of the full period. The measured dual-gated dose distribution was compared to the summed inhale and exhale dose distributions computed using an Eclipse AAA version 8.9.08. The delivery time was measured and compared to conventional single gated delivery, corresponding to the EOE plan scaled to an equivalent total dose.
The delivery time reduction was assessed through comparison of the DG-VMAT delivery time with that of the conventional EOE plan scaled to the same dose. The conventional EOE gated VMAT delivery requires 346 seconds for the studied case, while the proposed DG-VMAT technique is delivered in 177 seconds per fraction. Thus, for this particular case, dual-gated VMAT provides a 95.5% improvement in delivery efficiency compared to the corresponding single-gated delivery.
Implementation of DG-VMAT requires the synchronization of the gantry motion and MLC with two phases of the respiratory cycle. As such, for half the transitions between exhale and inhale phases, the gantry is required to roll back between the phases, as depicted in Figure 1c,d. This is shown to be possible with the TrueBeam™ STx, which has a gantry rotation speed of 6 degrees/second, and a MLC leaf speed of 2.5 cm/second at isocenter. The DG-VMAT delivery required a roll-back of an average of 2.05 degrees, which is achieved in 0.34 seconds. Since the transition time between exhale and inhale gating windows was 1.5 seconds, there was more than sufficient time for the gantry and MLC to move to the planned positions in preparation for the subsequent nodal delivery. Considering an average breathing cycle of 4–6 seconds, such a motion is within the limits of current linacs as demonstrated in this first experimental demonstration.
In this initial work, treatment planning was performed with the inhale and exhale phase optimized independently of each other. 4D treatment planning [20–22] may be adapted for a more cohesive optimization of the two phases through which DVH parameters are simultaneously optimized.
Several observations on the limitations and advantages of DG-VMAT can be made in relation to other respiratory management techniques. DG-VMAT is technically more complex than breath-hold techniques. Deep inspiration breath hold may achieve more advantageous anatomical separation for normal tissue sparing and has become more feasible with the use of high dose-rate flattening-filter-free beams [23, 24]. However, prolonged breath holding, as required to deliver SBRT doses, may not be tolerated by portions of the patient population, specifically those with already compromised lung function. Gating presents an alternative solution for such patients. Gating however, inherently results in significantly higher total treatment times due the fact the beam is conventionally activated for one phase of the breathing cycle. Dual gating, aims to enhance the efficiency of gating. While the technical complexity for such a delivery is higher than conventional gating, it represents significant simplification of alternate dynamic tracking proposals. More importantly, relative to tracking, dual gating only utilizes the stable portions of the respiratory cycle, and thereby eliminates intermediate irradiation between exhale and inhale which is known to be unstable and unpredictable.
To enhance the delivery efficiency of gated VMAT, a technique for dual-gated delivery, leveraging the natural pauses that occur at peak-inspiration and exhalation for irradiation, has been proposed. The technique which necessarily coordinates the gantry rotation and MLC modulation with two different phases of respiratory cycle was experimentally implemented using custom XML programing in TrueBeam™ STx Developer Mode. The results presented herein demonstrate the first successful delivery of DG-VMAT which is shown to result in nearly a doubling of treatment delivery efficiency for ideal sinusoidal respiratory motion. For clinical implementation on patients, audio-visual guidance may be used to coordinate the breathing with the delivery. Dual-gated delivery efficiency can be further improved with additional linac hardware and software modifications to enable implementation in clinical mode. As compared to the existing respiratory-gating VMAT technique, a major advantage of DG-VMAT is that it substantially reduces treatment duration with a modest but practically achievable increase in complexity of the treatment delivery processes. DG-VMAT can potentially provide a compromise between breath-hold, gating, and tracking techniques by increasing the tolerability relative to breath-hold, reducing technical demand and potential inaccuracies associated with irradiation of variable portions of the respiratory cycle relative to tracking techniques, and increasing the efficiency of treatment relative to conventional single window gating.
Benjamin Fahimian and Junqing Wu are co-first author.
The authors would like to acknowledge Michelle Svatos, Thanos Etmektzoglou, and Jianguo Qian for their assistance in this work. This work was supported in part by NCI (1R01 CA133474 and 1R21 408 CA153587), NSF (0854492), and NIH (T32 CA09695).
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