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Purpose The positional accuracy of MLC is an important element in establishing the exact dosimetry in VMAT. We comprehensively analyzed factors that may affect MLC positional accuracy in VMAT, and constructed a model to predict MLC positional deviation and estimate planning delivery quality according to the VMAT plans before delivery. Methods A total of 744 “dynalog” files for 23 VMAT plans were extracted randomly from treatment database. Multi‐correlation was used to analyzed the potential influences on MLC positional accuracy, including the spatial characteristics and temporal variability of VMAT fluence, and the mechanical wear parameters of MLC. We developed a model to forecast the accuracy of MLC moving position utilizing the random forest (RF) ensemble learning method. Spearman correlation was used to further investigate the associations between MLC positional deviation and dosage deviations as well as gamma passing rates. Results The MLC positional deviation and effective impact factors show a strong multi‐correlation (R = 0.701, p‐value < 0.05). This leads to the development of a highly accurate prediction model with average variables explained of 95.03% and average MSE of 0.059 in the 5‐fold cross‐validation, and MSE of 0.074 for the test data was obtained. The absolute dose deviations caused by MLC positional deviation ranging from 12.948 to 210.235 cGy, while the relative volume deviation remained small at 0.470%–5.161%. The average MLC positional deviation correlated substantially with gamma passing rates (with correlation coefficient of −0.506 to −0.720 and p‐value < 0.05) but marginally with dosage deviations (with correlation coefficient < 0.498 and p‐value > 0.05). Conclusions The RF predictive model provides a prior tool for VMAT quality assurance.

Background With the introduction of a new multi‐leaf collimator (MLC) enhanced leaf model (ELM) in the Varian Eclipse™ treatment planning system, there is currently limited data regarding the dosimetric sensitivity to real‐world variation in the ELM parameters, and its clinical relevance. Purpose To characterize the variation in ELM parameters across a large department with ten linear accelerators and investigate the feasibility of using a single machine‐averaged ELM for treatment planning. This could achieve time and resource savings from reduced quality assurance, while allowing easy transfer of patients between machines. Methods Clinical plans of a range of sites (head and neck, prostate, breast, lung, and brain), techniques (VMAT, IMRT, SBRT, and SRS), and energies (6 MV, 6 MV FFF, 10 MV, and 10 MV FFF) were recalculated on Varian TrueBeam™ (120 MLC) and Varian EDGE™ (HD120 MLC), with machine‐specific ELM beam models, an averaged machine and an outlier machine model. A range of clinically relevant metrics relating to target coverage (e.g. PTV D98%, D50%, D2%) and OAR doses (dosimetric, volumetric, conformity, and gradient indices) were evaluated. Results For the target metrics, the maximum percentage deviation from the mean was 0.422%, 0.157%, and 1.956% for the cases of the individual machines, the averaged machine and the outlier machine correspondingly, while the maximum absolute dose differences were 0.28 Gy, 0.07 Gy, and 0.38 Gy. For the OAR metrics, the maximum deviation from the mean was 1.833%, 0.204%, and 5.722% for the individual, averaged, and outlier machines, while the maximum absolute dose differences were 0.41 Gy, 0.10 Gy, and 0.97 Gy. Conclusions For machines that are well matched in terms of dosimetry for transmission and sweeping gap fields, the use of an averaged machine model is unlikely to introduce clinically significant dosimetric differences to treatment plans.

This study aimed to evaluate the clinical beam commissioning results and lateral penumbra characteristics of our new pencil beam scanning (PBS) proton therapy using a multi‐leaf collimator (MLC) calculated by use of a commercial Monte Carlo dose engine. Eighteen collimated uniform dose plans for cubic targets were optimized by the RayStation 9A treatment planning system (TPS), varying scan area, modulation widths, measurement depths, and collimator angles. To test the patient‐specific measurements, we also created and verified five clinically realistic PBS plans with the MLC, such as the liver, prostate, base‐of‐skull, C‐shape, and head‐and‐neck. The verification measurements consist of the depth dose (DD), lateral profile (LP), and absolute dose (AD). We compared the LPs and ADs between the calculation and measurements. For the cubic plans, the gamma index pass rates (γ‐passing) were on average 96.5% ± 4.0% at 3%/3 mm for the DD and 95.2% ± 7.6% at 2%/2 mm for the LP. In several LP measurements less than 75 mm depths, the γ‐passing deteriorated (increased the measured doses) by less than 90% with the scattering such as the MLC edge and range shifter. The deteriorated γ‐passing was satisfied by more than 90% at 2%/2 mm using uncollimated beams instead of collimated beams except for three planes. The AD differences and the lateral penumbra width (80%–20% distance) were within ±1.9% and ± 1.1 mm, respectively. For the clinical plan measurements, the γ‐passing of LP at 2%/2 mm and the AD differences were 97.7% ± 4.2% on average and within ±1.8%, respectively. The measurements were in good agreement with the calculations of both the cubic and clinical plans inserted in the MLC except for LPs less than 75 mm regions of some cubic and clinical plans. The calculation errors in collimated beams can be mitigated by substituting uncollimated beams.

Background The Elekta Unity MR‐Linac system integrates magnetic resonance imaging (MRI) with a linear accelerator (Linac) for adaptive radiation therapy. Traditional quality assurance (QA) methods for multi‐leaf collimators (MLCs) face challenges in this system due to the magnetic field and limited field size of electronic portal imaging devices (EPID). Purpose This study aims to develop a ‘virtual picket fence’ test using machine log files to evaluate MLC performance in the Elekta Unity MR‐Linac system, providing a more efficient and comprehensive QA method that overcomes the limitations of traditional approaches. Methods A picket fence test plan with 11 segments was delivered on the Elekta Unity system. Maximum absolute error and root mean square (RMS) error for each leaf were calculated by comparing log file data with nominal values. A deliberate 1 mm error was introduced in selected MLCs to test the sensitivity of the virtual test. The results from the log file were further compared with measurements from radiochromic films. Results The maximum deviation between log file data and nominal values was within 1 mm for all leaves. The virtual picket fence test successfully identified MLCs with deviations beyond the 0.5 mm warning threshold in the error‐introduced test. Comparisons with film‐based measurements showed good agreement, with deviations between film and log file data also within 1 mm. Conclusions The virtual picket fence test provides an efficient and comprehensive method for MLC QA in the Elekta Unity MR‐Linac system. This method can be integrated into weekly QA workflows alongside traditional film‐based methods for thorough quality control.

Purpose The purpose of this study was to evaluate the performance of our quality assurance (QA) automation system and to evaluate the machine performance of a new type linear accelerator uRT‐linac 506c within 6 months using this system. Methods This QA automation system consists of a hollow cylindrical phantom with 18 steel balls in the phantom surface and an analysis software to process electronic portal imaging device (EPID) measurement image data and report the results. The performance of the QA automation system was evaluated by the tests of repeatability, archivable precision, detectability of introduced errors, and the impact of set‐up errors on QA results. The performance of this linac was evaluated by 31 items using this QA system over 6 months. Results This QA system was able to automatically deliver QA plan, EPID image acquisition, and automatic analysis. All images acquiring and analysis took approximately 4.6 min per energy. The preset error of 0.1 mm in multi‐leaf collimator (MLC) leaf were detected as 0.12 ± 0.01 mm for Bank A and 0.10 ± 0.01 mm in Bank B. The 2 mm setup error was detected as −1.95 ± 0.01 mm, −2.02 ± 0.01 mm, 2.01 ± 0.01 mm for X, Y, Z directions, respectively. And data from the tests of repeatability and detectability of introduced errors showed the standard deviation were all within 0.1 mm and 0.1°. and data of the machine performance were all within the tolerance specified by AAPM TG‐142. Conclusions The QA automation system has high precision and good performance, and it can improve the QA efficiency. The performance of the new accelerator has also performed very well during the testing period.

Background The advent of volumetric modulated arc therapy (VMAT) in radiotherapy has made it one of the most commonly used techniques in clinical practice. VMAT is the delivery of intensity modulated radiation therapy (IMRT) while the gantry is in motion, and existing literature has shown it has decreased treatment delivery times and the number of monitor units without sacrificing coverage. It has previously been shown that for IMRT, multi‐leaf collimators (MLC) with narrower leaf widths produce demonstrably higher treatment plan quality. However, as VMAT is rapidly becoming the global standard, this needs to be re‐evaluated, especially in a global context. This study assesses the impact of MLC leaf width on VMAT treatment plans and asks whether reducing the number of leaves‐ and thus increasing leaf width‐ provides clinically acceptable treatment plans using VMAT delivery. Material & methods Using Varian Eclipse, 51 anonymised patients with prostate, lung, liver, colorectal, or cervical cancer had VMAT treatment plans generated. Treatment plans were generated for MLC leaf widths of 2.5, 5 and 10 mm. Plans were compared using D2[%], D50[%], and D98[%], homogeneity index (HI), conformity index (CI), average leaf pair opening (ALPO), modulation factor (MF) and Estimated Treatment Delivery Time. Results The dose to the target structures showed little difference between 2.5 and 5 mm MLC leaves, however 10 mm MLC provided 5% more median dose than the narrower leaf widths for D2[%] (p < 0.05) and D50[%] (p < 0.05). The average HI per leaf width was 0.0777 for 2.5 mm, 0.0752 for 5 mm, and 0.0890 for 10 mm. Organs At Risk (OAR) sparing was consistent between all leaf widths except at low dose percentages, where 10 mm MLC delivered an extra dose to the bladder (p < 0.05) and the heart (p < 0.05). The average ALPO was 38.0 mm for 2.5 mm, 34.1 mm for 5 mm, and 32.7 mm for 10 mm leaf width. 10 mm MLC leaves traveled a shorter distance from the center (p < 0.05). The median MF was 336 MU/Gy for 2.5 mm, 344 MU/Gy for 5 mm, and 384 MU/Gy for 10 mm. There were no differences in estimated treatment delivery time between MLC leaf width. Conclusion There is little difference in treatment quality between any of the investigated MLC leaf widths. This work demonstrates that for VMAT treatments, wider MLC leaf widths can still deliver acceptable treatment plans. This finding has potential implications for radiotherapy in low‐ and middle‐income countries and low socio‐economic or rural areas where a focus on MLC robustness and LINAC up‐time is paramount.

Background and purpose This study aimed to compare the dosimetric attributes of two multi‐leaf collimator based techniques, HyperArc and Incise CyberKnife, in the treatment of brain metastases. Material and methods 17 cases of brain metastases were selected including 6 patients of single lesion and 11 patients of multiple lesions. Treatment plans of HyperArc and CyberKnife were designed in Eclipse 15.5 and Precision 1.0, respectively, and transferred to Velocity 3.2 for comparison. Results HyperArc plans provided superior Conformity Index (0.91 ± 0.06 vs. 0.77 ± 0.07, p < 0.01) with reduced dose distribution in organs at risk (Dmax, p < 0.05) and lower normal tissue exposure (V4Gy–V20Gy, p < 0.05) in contrast to CyberKnife plans, although the Gradient Indexes were similar. CyberKnife plans showed higher Homogeneity Index (1.54 ± 0.17 vs. 1.39 ± 0.09, p < 0.05) and increased D2% and D50% in the target (p < 0.05). Additionally, HyperArc plans had significantly fewer Monitor Units (MUs) and beam‐on time (p < 0.01). Conclusion HyperArc plans demonstrated superior performance compared with MLC‐based CyberKnife plans in terms of conformity and the sparing of critical organs and normal tissues, although no significant difference in GI outcomes was noted. Conversely, CyberKnife plans achieved a higher target dose and HI. The study suggests that HyperArc is more efficient and particularly suitable for treating larger lesions in brain metastases.

The purpose is to reduce normal tissue radiation toxicity for electron therapy through the creation of a surface‐conforming electron multileaf collimator (SCEM). The SCEM combines the benefits of skin collimation, electron conformal radiotherapy, and modulated electron radiotherapy. An early concept for the SCEM was constructed. It consists of leaves that protrude towards the patient, allowing the leaves to conform closely to irregular patient surfaces. The leaves are made of acrylic to decrease bremsstrahlung, thereby decreasing the out‐of‐field dose. Water tank scans were performed with the SCEM in place for various field sizes for all available electron energies (6, 9, 12, and 15 MeV) with a 0.5 cm air gap to the water surface at 100 cm source‐to‐surface distance (SSD). These measurements were compared with Cerrobend cutouts with the field size‐matched at 100 and 110 cm SSD. Output factor measurements were taken in solid water for each energy at dmax for both the cerrobend cutouts and SCEM at 100 cm SSD. Percent depth dose (PDD) curves for the SCEM shifted shallower for all energies and field sizes. The SCEM also produced a higher surface dose relative to Cerrobend cutouts, with the maximum being a 9.8% increase for the 3 cm × 9 cm field at 9 MeV. When compared to the Cerrobend cutouts at 110 cm SSD, the SCEM showed a significant decrease in the penumbra, particularly for lower energies (i.e., 6 and 9 MeV). The SCEM also showed reduced out‐of‐field dose and lower bremsstrahlung production than the Cerrobend cutouts. The SCEM provides significant improvement in the penumbra and out‐of‐field dose by allowing collimation close to the skin surface compared to Cerrobend cutouts. However, the added scatter from the SCEM increases shallow PDD values. Future work will focus on reducing this scatter while maintaining the penumbra and out‐of‐field benefits the SCEM has over conventional collimation.

Background The Radixact treatment system is equipped with a delivery analysis feature. This feature enables dose reconstruction using the patient's treatment‐planning computed tomography scans and allows verification of the multileaf collimator (MLC) performance before and during treatment. In the Radixact system, the opening time of the MLC leaves is determined based on the treatment plan. Purpose This study aimed to evaluate MLC driving accuracy by assessing the MLC leaf open time (LOT) during treatment. Methods Using Delivery Analysis version 2.3, we compared the treatment plan LOT with the LOT measured during treatment to determine the average and one standard deviation (%) of the LOT attainment rate. The analysis included comparisons of treated sites across 39 cases: nine prostate, eight pelvic, seven head, six chest, five head and neck (H&N), and four stereotactic body radiation therapy (SBRT) treatment plans. Results The average and one standard deviation (%) of the LOT attainment rate for all patients on treatment was 94.56 ± 2.37. The values of each site were as follows: prostate, 95.93 ± 0.68; pelvis, 93.37 ± 2.16; head, 95.05 ± 1.99; chest, 97.61 ± 0.78; H&N, 92.44 ± 1.32; and SBRT, 98.39 ± 0.57. The treatment plans with the lowest attainment rates for each site were as follows: prostate, 95.19 ± 0.39; pelvis, 90.59 ± 0.16; head, 92.20 ± 0.15; chest, 95.76 ± 0.04; H&N, 90.55 ± 0.30; and SBRT, 97.32 ± 0.07. The plans with the largest one standard deviation (%) per site were as follows: prostate, ± 0.97; pelvis, ± 0.26; head, ± 0.57; chest, ± 0.23; H&N, ± 0.30; and SBRT, ± 0.07. Conclusions We proposed a simple method for quantitatively analyzing the LOT of an MLC. The average LOT attainment rate and its standard deviation varied by treatment site. Since the standard deviation differed by plan, the LOT attainment rate during treatment should be carefully monitored.

Background and purpose The lack of equitable access to radiotherapy (RA) linear accelerators (LINACs) is a substantial barrier to cancer care in low‐ and middle‐income countries (LMICs). These nations are expected to bear up to 75% of cancer‐related deaths globally by 2030. State‐of‐the‐art LINACs in LMICs experience major issues in terms of robustness, with mechanical and electrical breakdowns resulting in downtimes ranging from days to months. While existing research has identified the higher failure frequency and downtimes between LMICs (Nigeria, Botswana) compared to high‐income countries (HICs, the UK), there has been a need for additional data and study particularly relating to multileaf collimators (MLCs). Materials and methods This study presents for the first time the analysis of data gathered through a dedicated survey and workshop including participants from 14 Indonesian hospitals, representing a total of 19 LINACs. We show the pathways to failure of radiotherapy LINACs and frequency of breakdowns with a focus on the MLC subsystem. Results This dataset shows that LINACs throughout Indonesia are out of operation for seven times longer than HICs, and the mean time between failures of a LINAC in Indonesia is 341.58 h or about 14 days. Furthermore, of the LINACs with an MLC fitted, 59.02−1.61+1.98 $59.02_{ - 1.61}^{ + 1.98}$ % of all mechanical faults are due to the MLC, and 57.14−1.27+0.78 $57.14_{ - 1.27}^{ + 0.78}$% of cases requiring a replacement component are related to the MLC. Conclusion These results highlight the pressing need to improve robustness of RT technology for use in LMICs, highlighting the MLC as a particularly problematic component. This work motivates a reassessment of the current generation of RT LINACs and demonstrates the need for dedicated efforts toward a future where cancer treatment technology is robust for use in all environments where it is needed.