Identification of a potential source of error for 6FFF beams delivered on an Agility^TM multileaf collimator

Abstract

Purpose: The performance of the Agility TM multileaf collimator was investigated with a focus on dynamic, small fields for flattening filter free (FFF) beams. Methods: In this study we have developed a simple tool to test the robustness of the control mechanisms during dynamic beam delivery for Elekta's VersaHD linear accelerator with Integrity 4.0.4 control software. We have programed the planning system to calculate dose for delivery of sweeping gaps. These sweeping gaps have a constant speed, constant size, and are delivered at a constant dose rate. Therefore they specifically identify delivery problems in dynamic mode. Results: The Elekta Agility TM control mechanism fails to maintain accurate delivery for small, dynamic sweeping gaps. For small gap sizes, the Agility TM control mechanism delivers a field that is more than four times the size of the planned field width without generating an interlock. This has dosimetric implications: The discrepancy between calculated and measured doses increases with decreasing gap size and exceeds 10% and 60% at isocenter for a 3.5 mm and 1 mm gap size, respectively. Conclusion: A deficiency of the Agility TM control system was identified in this study. This deficiency is a potential source of error for volumetric modulated arc therapy fields and could therefore contribute to relatively high failure rates in quality assurance measurements, especially for FFF beams.

Response: Potential dose variability for small‐field plans delivered with Elekta Agility collimators

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Dear Editor, We read with interest the letter to the editor written by Foy et al 1 regarding a recently published article on 6 FFF beams delivered on an Agility multileaf collimator (MLC). 2 We appreciate the interest in this publication by the authors; however, after careful review we have identified several apparent misunderstandings in the letter to the editor by Foy et al The assertions presented by Foy et al do not specifically pertain to our article as the data discussed in their letter do not relate directly to the purpose of the cited article. 2 Specifically, we would like to correct the following misinterpretations by Foy et al:

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“…1 It was shown by Lorenz and Paris that Monaco's sequencer can generate a relatively high proportion of 3.5 mm leaf apertures in complex VMAT plans. 2 The extent to which small gaps exist and affect a facility's assemblage of plans will depend upon the TPS and plan complexity. The assumption by Foy et al that "3.5 mm will not necessarily be found in most clinical plans" 1 may not be universally valid.…”

Potential dose variability for small‐field plans delivered with Elekta Agility collimators

“…Additionally, the Agility TM MLC requires a gap between corresponding leaves of 5 mm when projected at isocenter. Therefore, the dose errors reported by Lorenz and Paris 1 for field sizes smaller than 3.5 mm will not necessarily be found in most clinical plans. This limitation in leaf gap width may also be a source of discrepancy.…”

“…A recent publication by Lorenz and Paris 1 “Identification of a potential source of error for 6 FFF beams delivered on an Agility TM multileaf collimator” reported on discrepancy between calculated and measured doses to be 10% and 60% at isocenter for a 3.5‐mm and 1‐mm gap field size, respectively. The investigators attempted to determine the cause of the relatively high failure rates during their patient‐specific quality assurance (PSQA) when using flattening filter‐free (FFF) photon beams.…”

“…In the context of the publication by Lorenz and Paris, 1 we retrospectively analyzed the PSQA results from 20 VMAT treatments planned with FFF and non‐FFF (referred to as FF) photon beams for prostate, brain, liver, lung, and head and neck tumors with varied complexity in modulation. Each treatment was planned on an Elekta Versa TM (Agility TM MLC) machine with Pinnacle TM TPS and measured with ArcCheck TM .…”