Abstract
“Burst‐mode” modulated arc therapy (hereafter referred to as “mARC”) is a form of volumetric‐modulated arc therapy characterized by variable gantry rotation speed, static MLCs while the radiation beam is on, and MLC repositioning while the beam is off. We present our clinical experience with the planning techniques and plan quality assurance measurements of mARC delivery. Clinical mARC plans for five representative cases (prostate, low‐dose‐rate brain, brain with partial‐arc vertex fields, pancreas, and liver SBRT) were generated using a Monte Carlo–based treatment planning system. A conventional‐dose‐rate flat 6 MV and a high‐dose‐rate non‐flat 7 MV beam are available for planning and delivery. mARC plans for intact‐prostate cases can typically be created using one 360° arc, and treatment times per fraction seldom exceed 6 min using the flat beam; using the nonflat beam results in slightly higher MU per fraction, but also in delivery times less than 4 min and with reduced mean dose to distal organs at risk. mARC also has utility in low‐dose‐rate brain irradiation; mARC fields can be designed which deliver a uniform 20 cGy dose to the PTV in approximately 3‐minute intervals, making it a viable alternative to conventional 3D CRT. For brain cases using noncoplanar arcs, delivery time is approximately six min using the nonflat beam. For pancreas cases using the nonflat beam, two overlapping 360° arcs are required, and delivery times are approximately 10 min. For liver SBRT, the time to deliver 800 cGy per fraction is at least 12 min. Plan QA measurements indicate that the mARC delivery is consistent with the plan calculation for all cases. mARC has been incorporated into routine practice within our clinic; currently, on average approximately 15 patients per day are treated using mARC; and with the exception of LDR brain cases, all are treated using the nonflat beam.PACS number(s): 87.55.D‐, 87.55.K‐, 87.53.Ay. 87.56.N‐
Highlights
Kainz et al.: Burst-mode modulated arc therapy beam is off when the MLC leaves are in transition between each optimization point (OP), and the angular velocity of the gantry rotation is variable within each OP
This leads to less blocking of the beam than either tomotherapy or volumetric-modulated arc therapy (VMAT); this may explain why a comparison of mARC and VMAT plans demonstrated that mARC resulted in fewer monitor units per fraction than did VMAT.[3]. Second, mARC delivery more closely resembles the static-beam approximations made within the planning system, which makes delivery verification with mARC more intuitive compared to dynamic-MLC techniques such as VMAT
A comparison of mARC, tomotherapy, and VMAT plans demonstrated that all three techniques provided comparable PTV dose coverage, the PTV uniformity was best with tomotherapy
Summary
Kainz et al.: Burst-mode modulated arc therapy beam is off when the MLC leaves are in transition between each optimization point (OP), and the angular velocity of the gantry rotation is variable within each OP. When the linac is equipped with a high-dose-rate beam, mARC is similar to step-and-shoot delivery; the planning and verification of such an arc therapy treatment is simplified. During an mARC delivery the beam is turned off while the MLCs are in motion This leads to less blocking of the beam than either tomotherapy or VMAT; this may explain why a comparison of mARC and VMAT plans demonstrated that mARC resulted in fewer monitor units per fraction than did VMAT.[3] Second, mARC delivery more closely resembles the static-beam approximations made within the planning system, which makes delivery verification with mARC more intuitive compared to dynamic-MLC techniques such as VMAT. A comparison of mARC, tomotherapy, and VMAT plans demonstrated that all three techniques provided comparable PTV dose coverage, the PTV uniformity was best with tomotherapy. We present our recent experience in the planning techniques, quality assurance (QA) measurements, and delivery of mARC plans that were generated using a Monte Carlo-based planning system with a two-step optimization algorithm, applying both flattened and unflat beams
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