Assessing the dependency of the uncertainties in the Elekta Agility MLC calibration procedure on the focal spot position

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.

Section: Methodsmentioning

confidence: 69%

Section: Methodsmentioning

confidence: 99%

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

Lorenz

Paris

2021

“…Because an assessment between the most congruent linacs (FS1, TC1) evidenced a significantly reduced range of OPF and effective field size for the smallest field (Table 7), we investigated the position of the radiation focal spot, with respect to the axis of rotation of the collimator, across the four linacs. Chojnowski et al 42 reported that a focal spot offset ≥0.4 mm can affect dosimetric and geometric properties of the beam. We measured the focal spot position using four square fields of length 10 cm at gantry zero, exposing the MV iView EPID (Elekta iView GT) panel; data were analyzed using a MATLAB script 43 .…”

Section: Discussionmentioning

confidence: 99%

Consistency of small‐field dosimetry, on and off axis, in beam‐matched linacs used for stereotactic radiosurgery

Muñoz

Kron

Petasecca

et al. 2021

Purpose Stereotactic radiosurgery (SRS) can be delivered with a standard linear accelerator (linac). At institutions having more than one linac, beam matching is common practice. In the literature, there are indications that machine central axis (CAX) matching for broad fields does not guarantee matching of small fields with side ≤2 cm. There is no indication on how matching for broad fields on axis translates to matching small fields off axis. These are of interest to multitarget single‐isocenter (MTSI) SRS planning and the present work addresses that gap in the literature. Methods We used 6 MV flattening filter free (FFF) beams from four Elekta VersaHD® linacs equipped with an Agility™ multileaf collimator (MLC). The linacs were strictly matched for broad fields on CAX. We compared output factors (OPFs) and effective field size, measured concurrently using a novel 2D solid‐state dosimeter “Duo” with a spatial resolution of 0.2 mm, in square and rectangular static fields with sides from 0.5 to 2 cm, either on axis or away from it by 5 to 15 cm. Results Among the four linacs, OPF for fields ≥1 × 1 cm2 ranged 1.3% on CAX, whereas off axis a maximum range of 1.9% was observed at 15 cm. A larger variability in OPF was noted for the 0.5 × 0.5 cm2 field, with a range of 5.9% on CAX, which improved to a maximum of 2.3% moving off axis. Two linacs showed greater consistency with a range of 1.4% on CAX and 2.2% at 15 cm off axis. Between linacs, the effective field size varied by <0.04 cm in most cases, both on and off axis. Tighter matching was observed for linacs with a similar focal spot position. Conclusions Verification of small‐field consistency for matched linacs used for SRS is an important task for dosimetric validation. A significant benefit of concurrent measurement of field size and OPF allowed for a comprehensive assessment using a novel diode array. Our study showed the four linacs, strictly matched for broad fields on CAX, were still matched down to a field size of 1 x 1 cm2 on and off axis.

“…In practice, this means that the FSO effect causes the difference between radiation field centers to vary depending on the FSO value, the position of the BLD forming the field aperture and the position of the measurement plane (i.e., EPID). Chojnowski et al 13 1 FSO for an optimized beam on Elekta linacs is about 0.2-0.3 mm, 13,14 this would result in about 0.4-0.5 mm shift of the radiation isocenter longitudinally 1 from the mechanical isocenter. Therefore, nowadays, the radiation isocenter is often used as the treatment isocenter, not the mechanical isocenter as in the past, and the two major linac manufacturers (Elekta and Varian) use it in their proprietary procedures to calibrate (align) the imaging systems' isocenters.…”

Section: Introductionmentioning

confidence: 96%

“…In practice, this means that the FSO effect causes the difference between radiation field centers to vary depending on the FSO value, the position of the BLD forming the field aperture and the position of the measurement plane (i.e., EPID). Chojnowski et al 13 explored the FSO effect for a clinically relevant example of the Elekta Agility MLC calibration procedure, where the reference calibration position is determined using two BLDs (i.e., the MLC and diaphragms) that are physically at different distances from the radiation source. They reported a correlation factor of 0.7 between the MLC calibration error and the FSO scaled back from the EPID level to the isocenter level, i.e., if the FSO is 0.2 mm, it results in an MLC miscalibration of 0.14 mm.…”

Section: Introductionmentioning

confidence: 99%

A novel method to determine linac mechanical isocenter position and size and examples of specific QA applications

Chojnowski

Sykes

Thwaites

2021

Self Cite

The most important geometric characteristic of stereotactic treatment is the accuracy of positioning the target at the treatment isocenter and the accuracy of directing the radiation beam at the treatment isocenter. Commonly, the radiation isocenter is used as the reference for the treatment isocenter, but its method of localization is not strictly defined, and it depends on the linac-specific beam steering parameters. A novel method is presented for determining the linac mechanical isocenter position and size based on the localization of the collimator axis of rotation at arbitrary gantry angle. The collimator axis of rotation position is determined from the radiation beam center position corrected for the focal spot offset. The focal spot offset is determined using the image center shift method with a customdesign rigid phantom with two sets of ball-bearings. Three specific quality assurance (QA) applications and assessment methods are also presented to demonstrate the functionality of linac mechanical isocenter position and size determination in clinical practice. The first is a mechanical and radiation isocenters coincidence test suitable for quick congruence assessment of these two isocenters for a selected energy, usually required after a nonroutine linac repair and/or energy adjustment. The second is a stereotactic beam isocentricity assessment suitable for pretreatment stereotactic QA. The third is a comprehensive linac geometrical performance test suitable for routine linac QA. The uncertainties of the method for determining mechanical isocenter position and size were measured to be 0.05 mm and 0.04 mm, respectively, using four available photon energies, and were significantly smaller than those of determining the radiation isocenter position and size, which were 0.36 mm and 0.12 mm respectively. It is therefore recommended that the mechanical isocenter position and size be used as the reference linac treatment isocenter and a linac mechanical characteristic parameter respectively.