On the MLC leaves alignment in the direction orthogonal to movement

Abstract The main focus of the recommended spatial accuracy tests for the multi‐leaf collimators (MLC) is calibration of the leaf position along the movement direction and overall alignment to the radiation isocenter. No explicit attention was typically paid to the alignment of the leaves from the opposing banks in the direction orthogonal to movement. This paper is a case study demonstrating that verification of such alignment at the time of acceptance testing is prudent. The original standard MLC (SMLC) on an MRIdian MRI‐guided linac (ViewRay Inc., Mountain View, CA, USA) was upgraded to a high‐speed MLC (HSMLC), which is supposed to be mechanically identical to the SMLC except for the higher drive screw pitch. The results of the end‐to‐end IMRT tests demonstrated unacceptable dosimetric results exemplified by an average and maximum ion chamber (IC) point dose error in the high‐dose low‐gradient region of 2.5 ± 1.4% and 4.6%, respectively. Before the upgrade, those values were 0.3 ± 0.7% and 0.9%, respectively. An exhaustive analysis of possible failure modes eventually zeroed in on the average misalignment of about 1 mm in the Y (along the couch) direction between the right and left upper MLC banks. The MLC was replaced, reducing the Y‐direction misalignment to 0.4 mm. As a result, the average and maximum IC dose‐errors became acceptable 1.0 ± 0.7% and 1.6%, respectively. Simple film and/or chamber array tests during acceptance testing can easily detect Y‐direction misalignments between opposing leaves banks measuring a fraction of a mm at isocenter. Left undetected, such misalignment can cause nontrivial dosimetric consequences.

possible misalignment in the direction orthogonal to movement between the leaves from the opposing MLC banks has not been explicitly addressed before. This paper is a case study demonstrating that during acceptance testing this alignment should be verified.
The MRIdian MRI-guided linac radiotherapy system (ViewRay Inc, Mountain View, CA, USA) has been in service at our institution for over a year. It underwent rigorous dosimetric commissioning including a successful independent end-to-end audit with an MRI-visible head and neck mail-in dosimetry phantom from IROC Houston. 4  2 | ME TH ODS

2.A | The MLC
The MLC design on a single-head linac-based system is different from the three-head 60 Co unit. 5 The defining feature is the doublestack double-focused configuration with an individual leaf width of 8.3 mm as projected at the isocenter (Fig. 1). The leaf sides are flat, without a tongue-and-groove arrangement. The leaves are chamfered at the tip to facilitate smoother interdigitation. They can travel across the entire field width. The MLC is the only variable beamshaping device in MRIdian. There is no field light and the MLC does not rotate.

2.B | MLC tests
The MLC tests can be divided into two groups by the timeline: the standard tests included in the acceptance test of the new MLC and the additional tests introduced after the initial failure of the IMRT dosimetric evaluation with HSMLC#1.
The standard tests included picket fence films at three cardinal gantry angles for each MLC layer, combined MLC transmission and intraleaf leakage measurements with a Farmer chamber at isocenter for each layer, and a radiochromic film leakage measurement with both layers closed, to locate any potential unintended hot spots.
Also included in this group of MLC-related checks were the gantry star shot with radiochromic film and a complementary gantry isocentricity test with an ion chamber array. 6 The latter two are used to define the radiation isocenter position at the time of installation.
As an additional test, a simple MLC check was designed to show any potential misalignment between the leaves in the left and right banks in the Y-direction (IEC Standard 61217 Y, along the couch).

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The relative signal was plotted against the table shift and the slope of the linear fit indicated the signal gradient in the Y direction.
The signal was defined as an average of the readings of the two chambers ±1 cm from the central axis. Their long axes were oriented in the Y direction. The MRIdian table position readout resolution is 0.1 mm. It has been shown previously that the Y-position readout correlated tightly with the radiation beam center measurement. 6

2.C | Dosimetric IMRT tests
MRIdian allows only static gantry step-and-shoot IMRT. The test plans were developed at the initial commissioning of the system and consisted of the 4-plan test suite from TG119 7 and three cases from MPPG 5a 8head and neck, prostate bed, and abdomen. The  Table 1 for the SMLC and two replace-  Fig. 4(b)].
The precision of the ICP measurements can be estimated from the goodness-of-fit parameters for the ΔY vs. relative dose regression line in the insert on Fig. 4(a). The correlation coefficient

3.B | Dosimetric IMRT Tests
The ion chamber measurement results for three MLCs are summarized in the first part of  The standard acceptance tests at the time were not well geared towards detecting a potential Y-direction misalignment between the leaves from the opposing banks. Such misalignment can be detected easily with film or ion chamber measurements described in this report. If a site has access to an ICP, the slope value from Fig. 4 can be used to directly convert the signal difference into the geometrical distance between the edges of the left and right bank leaves. If using another ion chamber, the slope has to be determined first as it depends on the detector size.
The vendor failure mode analysis determined the cause of the misalignment to be a shift in one of the mechanical tension guides confining the leaf bank Y position. The damage has likely occurred during shipping, as the shock sensor had been tripped. The MLC undergoes rigorous testing at the factory and such a misalignment would be unlikely to pass the inspection.

| CONCLUSION S
The alignment between the opposing leaf banks in the direction orthogonal to movement has not received much attention in the literature, perhaps because of the tacit assumption that in the absence of movement the mechanical specifications of the MLC assembly were sufficiently tight. However, with a certain MLC design such misalignment can be systematic across the radiation field and lead to R. Loteyexperimental design, execution, and manuscript approval.
V. Feygelmanexperimental design, execution, and manuscript drafting and approval.

D A T A A V A I L A B I L I T Y S T A T E M E N T
Data available upon reasonable request from the authors.