Evaluation of prototype of improved electron collimation system for Elekta linear accelerators

Abstract Purpose This study evaluated a new electron collimation system design for Elekta 6–20 MeV beams, which should reduce applicator weights by 25%–30%. Such reductions, as great as 3.9 kg for the largest applicator, should result in considerably easier handling by members of the radiotherapy team. Methods Prototype 10 × 10 and 20 × 20‐cm2 applicators, used to measure weight, in‐field flatness, and out‐of‐field leakage dose, were constructed according to the previously published design with two minor modifications: (a) rather than tungsten, lead was used for trimmer material; and (b) continuous trimmer outer‐edge bevel was approximated by three steps. Because of lead plate softness, a 0.32‐cm aluminum plate replaced the equivalent lead thickness on the trimmer's downstream surface for structural support. Models of all applicators (6 × 6–25 × 25 cm2) with these modifications were inserted into a Monte Carlo (MC) model for dose calculations using 7, 13, and 20 MeV beams. Planar dose distributions were measured and calculated at 1‐ and 2‐cm water depths to evaluate in‐field beam flatness and out‐of‐field leakage dose. Results Prototype 10 × 10 and 20 × 20‐cm2 applicator measurements agreed with calculated weights, in‐field flatness, and out‐of‐field leakage doses for 7, 13, and 20 MeV beams. Also, MC dose calculations showed that all applicators (6 × 6–25 × 25 cm2) and 7, 13, and 20 MeV beams met our stringent in‐field flatness specifications (±3% major axes; ±4% diagonals) and IEC out‐of‐field leakage dose specifications. Conclusions Our results validated the new electron collimating system design for Elekta 6–20 MeV electron beams, which could serve as basis for a new clinical electron collimating system with significantly reduced applicator weights.


| INTRODUCTION
Our cancer clinic has seven Elekta Infinity radiotherapy accelerators (MLCi2 treatment head) with matched, custom electron beams spanning 7-20 MeV (R 90 values of 2.0, 2.5, 3.0, 3.5, 4.0, 5.0, and 6.0 AE 0.1 cm) and slightly modified scattering foils. 1,2 Although they have exceptional in-field flatness laterally (AE3% along major axes; AE4% along diagonals 3 ) and out-of-field leakage doses well below IEC standards 4 for all applicators (6 9 6-25 9 25 cm 2 ), there is opportunity to improve the delivery technology by reducing the weight of the electron applicators, particularly those for larger fields. 5,6 To that end, an optimization procedure utilizing analytical pencil beam and Monte Carlo (MC) calculations was developed to design significantly lighter applicators with similar in-field flatness and out-of-field leakage dose as the current ones. Results showed significantly reduced applicator trimmer weights, which should translate to applicator weights for 6 9 6, 10 9 10, 14 9 14, 20 9 20, and 25 9 25-cm 2 applicators for 6-20 MeV beams being reduced by 7.0?5.1, 7.7? 5.8, 9.1?6.7, 10.9?7.6, and 13.4?9.5 kg, respectively. 5 Such reductions should result in considerably easier handling by members of the radiotherapy team.
The purpose of this study was to validate these designs. First, prototype 10 9 10 and 20 9 20-cm 2 applicators were constructed and used to measure weight, in-field flatness, and out-of-field leakage dose. The two applicators were constructed according to previously published design 5 with two minor modifications: (a) lead was substituted for tungsten for trimmer material, which required a 0.32cm aluminum plate replacing the equivalent lead thickness on the trimmer's downstream surface for structural support, and (b) trimmer outer-edge bevel was approximated by three steps. Second, all five applicators were modeled with the two minor modifications in a MC code for calculation of in-field flatness and out-of-field leakage doses. Results will show that both measurements and calculations exhibited expected weights, in-field flatness, and out-of-field leakage doses.

| ME TH ODS
Methods used to evaluate the new, lighter electron applicators designed and previously reported by Pitcher et al. 5 are described.

2.A | Designs for new, lighter applicators
Pitcher et al. 5 outlined a design procedure for a full set of applicators (6 9 6-25 9 25 cm 2 ) for 6-20 MeV beams. Its 10 9 10 and 20 9 20-cm 2 applicator designs (trimmers and x-ray jaw settings) provided the basis for a new Elekta electron collimation system with lighter applicators, including 6 9 6, 14 9 14, and 25 9 25-cm 2 applicators. All new applicator designs retained the design parameters from the 10 9 10 and 20 9 20-cm 2 applicators (i.e., trimmer positions, materials, inner-edge divergence angles, outer-edge bevel forming fluence matching off-axis ratios (OARs), and trimmer thicknesses) except for OARs at each trimmers inner edge, as calculated using a pencil beam algorithm. The 6 9 6 and 25 9 25-cm 2 applicators used the same inner-edge OARs as the 10 9 10 and 20 9 20cm 2 applicators, respectively, for a 6-MeV beam. The 14 9 14-cm 2 applicator inner-edge OARs were linearly interpolated between the 10 9 10 and 20 9 20-cm 2 OARs for a 6-MeV beam. x-ray jaw positions for each energy were determined by linearly interpolating the OARs at the upper trimmer inner edge between the 6-MeV value and 55% at 20 MeV for all applicators (c.f. Table 1).
2.B | Fabrication of 10 3 10 and 20 3 20-cm 2 prototype applicators Prototype 10 9 10 and 20 9 20-cm 2 applicators were fabricated by the Louisiana State University (LSU) Physics and Astronomy machine shop according to the new design specifications summarized above with two minor modifications. First, the prototype trimmers were milled from lead rather than tungsten because of the difficulty in milling tungsten in our shop. Second, the smooth, beveled shape of the trimmer outer edge was approximated by three steps due to the shop not possessing the equipment required to mill the curved shape.
Previous MC studies of electron transmission in lead and tungsten showed that the thicknesses to stop 99% of the electrons in a 20 MeV beam were close, 9.08 and 9.56 g cm À2 , respectively. 6 Although lead is more effective at stopping electrons, it requires a greater thickness due to their densities of 11.3 and 19.0 g cm À3 , respectively. Because of the softness of lead plate the trimmers were backed with a 0.32-cm aluminum plate (2.7 g cm À3 ) for T A B L E 1 Trimmer inner-edge fluence matching OARs for the design of each prototype applicator.  although previous investigators have shown the benefit of placing aluminum on the upstream surface, 7 our goal was to best simulate the tungsten design. The lead plate's thickness (g cm À2 ) was reduced by 9%, the added thickness (g cm À2 ) of the aluminum plate for the middle and lower trimmers. This same reduction was not made for the upper trimmer, as the thickness of this trimmer had already been reduced, 5 and any further reduction in its thickness would cause difficulties in machining the lead. To fabricate using the available machining equipment, the outeredge bevel was approximated by a series of steps, rather than a smooth curve. Four discrete energies were selected to match the width and thickness of each step: 6, 9, 13, and 20 MeV. The outer edge of each step matched the off-axis position of the specified fluence OAR for the associated beam energy. The thickness of each step was calculated with the same electron energy using the range energy curve developed from the MC 1% threshold analysis. 6 The effect of this modification to the outer-edge bevel can be seen in  symmetrization was performed for evaluating the out-of-field results.

2.D | MC lateral leakage analysis
Relative dose was assumed equal to relative ionization, i.e., conversion factors from ionization to dose were assumed identical for central-axis and off-axis positions at equal depths. These measurements were used to compare the leakage dose of the prototype collimation system with those of the current clinical Elekta applicators.
In-field flatness was evaluated according to the criteria described by Hogstrom, 3 which states that off-axis dose vary from the centralaxis dose by no more than AE3% along the major axes (in-plane and cross-plane) and AE4% along the diagonal axes. These specifications are assessed at 1 cm depth in water for E p,0 ≤ 9 MeV and 2 cm for E p,0 > 9 MeV within a region 2 cm inside the field edge for the major axes and 2 ffiffiffi 2 p cm for the diagonal axes. 1,3 The out-of-field leakage dose was evaluated according to criteria threshold method equation is slightly less for lead than tungsten. 6 Second, by approximating the outer edge as a series of steps, a small amount of material was removed from the designed bevel shape (c.f. Fig. 1).
Once assembled, the full weight (trimmers plus all structural materials, such as the attachment plate, aluminum spacer tubes, and threaded rods) of the prototype 10 9 10 and 20 9 20-cm 2 applicators was measured to be 5.5 and 6.8 kg, respectively. These values do not include the weight of the various components (e.g., the locking mechanism for the field defining inserts and the electronic components associated with the collisional interlocking system), which are present on the current Elekta applicators but not on the fabricated prototype. The full applicator weights of the new design were estimated by adding the difference in the full applicator and trimmer weights of the current Elekta applicators to the trimmer weights of the prototype design. This calculation estimated the full weight of the new 10 9 10 and 20 9 20-cm 2 applicator designs to be 5.8 and 7.6 kg, respectively, compared with 7.7 and 10.9 kg for the current Elekta applicators.
These weight results are listed in Table 2 Table 3.

3.B | In-field beam flatness at 100-cm SSD
T A B L E 2 Comparison of applicator weights in kg of both trimmers only and full applicator for current Elekta, current Varian, and prototype Elekta electron collimating systems for each applicator size (6 9 6-25 9 25 cm 2 ). Varian trimmer weights were estimated as full applicator weight minus 1.5 kg.
c Prototype Elekta full applicator weights were estimated as the prototype trimmer weight, plus the difference in the current Elekta full applicator and trimmer weights. Both measured and MC-calculated data in Table 3 show that the prototype applicators produced mean percent leakage doses well below IEC specifications for each beam energy and applicator combination. For example, the measured mean leakage dose at 7 MeV with the 20 9 20-cm 2 applicator had the closest value, being 0.24% below the 1.00% IEC specified maximum. For the MC-calculated mean leakage doses, the 20 MeV beam with the 14 9 14-cm 2 applicator had the closest value, being 0.18% below the 1.34% IEC specified maximum. Similarly, data in Table 3 show that the prototype applicators produced maximum percent leakage dose well below the F I G . 4. Comparison of measured and MC-calculated cross-plane profiles of relative dose versus off-axis position in water (100-cm SSD) with the same comparisons and measurement conditions as Fig. 3. The calculated profiles all have three common characteristics: (a) there is a general trend of the leakage dose to decrease with T A B L E 3 IEC-specified mean (left columns) and maximum (right columns) leakage doses (percent of D max ) determined from MC-calculated and measured doses for the prototype collimation system (6 9 6-25 9 25 cm 2 ) and 7, 13, and 20 MeV beams. Measured doses are shown only for the fabricated prototype applicators, 10 9 10 and 20 9 20 cm 2 . Bottom two rows show E p,0 values and maximum allowed leakage doses specified by the IEC for each nominal beam energy. increasing distance from the applicator; (b) exceptions to (a) are characterized by peaks, which are believed based on electrons scattered from inner edges of the electron collimating system (trimmers and x-ray collimators), 1 (c) in-plane is less than cross-plane leakage; and