Improvements in dose accuracy delivered with static‐MLC IMRT on an integrated linear accelerator control system

Li, Ji; Wiersma, R; Stepaniak, Christopher; Farrey, Karl; Al‐Hallaq, Hania

doi:10.1118/1.3701778

Cited by 14 publications

(21 citation statements)

References 12 publications

(21 reference statements)

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“…These differences may be a consequence of the retrospective and prospective communication utilized within the TrueBeam, rather than the retrospective communication used in the previous C‐Series linacs. This is in agreement with previously reported studies that demonstrated improvements in delivery accuracy between C‐Series linacs and TrueBeam linacs for step‐and‐shoot IMRT 27 , 38 …”

Section: Discussionsupporting

confidence: 93%

Correlation of phantom‐based and log file patient‐specific QA with complexity scores for VMAT

Agnew

Irvine

McGarry

2014

J Applied Clin Med Phys

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The motivation for this study was to reduce physics workload relating to patient‐specific quality assurance (QA). VMAT plan delivery accuracy was determined from analysis of pre‐ and on‐treatment trajectory log files and phantom‐based ionization chamber array measurements. The correlation in this combination of measurements for patient‐specific QA was investigated. The relationship between delivery errors and plan complexity was investigated as a potential method to further reduce patient‐specific QA workload. Thirty VMAT plans from three treatment sites — prostate only, prostate and pelvic node (PPN), and head and neck (H&N) — were retrospectively analyzed in this work. The 2D fluence delivery reconstructed from pretreatment and on‐treatment trajectory log files was compared with the planned fluence using gamma analysis. Pretreatment dose delivery verification was also carried out using gamma analysis of ionization chamber array measurements compared with calculated doses. Pearson correlations were used to explore any relationship between trajectory log file (pretreatment and on‐treatment) and ionization chamber array gamma results (pretreatment). Plan complexity was assessed using the MU/ arc and the modulation complexity score (MCS), with Pearson correlations used to examine any relationships between complexity metrics and plan delivery accuracy. Trajectory log files were also used to further explore the accuracy of MLC and gantry positions. Pretreatment 1%/1 mm gamma passing rates for trajectory log file analysis were 99.1% (98.7%–99.2%), 99.3% (99.1%–99.5%), and 98.4% (97.3%–98.8%) (median (IQR)) for prostate, PPN, and H&N, respectively, and were significantly correlated to on‐treatment trajectory log file gamma results (normalR=0.989,p<0.001). Pretreatment ionization chamber array (2%/2 mm) gamma results were also significantly correlated with on‐treatment trajectory log file gamma results (normalR=0.623,p<0.001). Furthermore, all gamma results displayed a significant correlation with MCS (normalR>0.57,p<0.001), but not with MU/arc. Average MLC position and gantry angle errors were 0.001±0.002.15emmm and 0.025°±0.008° over all treatment sites and were not found to affect delivery accuracy. However, variability in MLC speed was found to be directly related to MLC position accuracy. The accuracy of VMAT plan delivery assessed using pretreatment trajectory log file fluence delivery and ionization chamber array measurements were strongly correlated with on‐treatment trajectory log file fluence delivery. The strong correlation between trajectory log file and phantom‐based gamma results demonstrates potential to reduce our current patient‐specific QA. Additionally, insight into MLC and gantry position accuracy through trajectory log file analysis and the strong correlation between gamma analysis results and the MCS could also provide further methodologies to both optimize the VMAT planning and QA process.PACS number: 87.53.Bn, 87.55.Kh, 87.55.Qr

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Section: Discussionsupporting

confidence: 93%

Correlation of phantom‐based and log file patient‐specific QA with complexity scores for VMAT

Agnew

Irvine

McGarry

2014

J Applied Clin Med Phys

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“…According to Li et al, SMLC-IMRT affords accurate dose delivery in low MU segments of the recent Varian TrueBeam (Varian Medical Systems Inc., Palo Alto, CA). They reported an accuracy of ±0.2% for all combinations of low MU per segments (1 -10) and high dose rates (200 -600 MU/min) [13]. In our study, the Vero4DRT system achieved dose errors within 2% above 5 MU at all dose rates.…”

Section: Discussionmentioning

confidence: 52%

Beam Characteristics at Low Dose Monitor Unit Settings for Vero4DRT

Miura¹,

Ozawa²,

Tsuda³

et al. 2015

IJMPCERO

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The stability of delivery of low monitor unit (MU) setting is important especially for step-andshoot intensity-modulated radiotherapy (IMRT), because the nature of the technique is inherent to repeat beam on/off according to the number of the segments. This study evaluates the dose linearity and profile flatness/symmetry under low MU settings for Vero4DRT, a new linear-accelerator based irradiation system that currently implements step-and-shoot IMRT. To evaluate the dose linearity and flatness/symmetry, the point doses and beam profiles were measured as functions of MU and dose rates. The accuracy of dose delivery depended on the dose rate. Under all dose rates, the dose was linear within 1% above 5 MU and within 2% above 3 MU. The beam symmetry was degraded in-line compared with crossline, although both profiles were symmetric within 2% at all dose settings. The profile flatness was also within 2% above 5 MU at any dose rate and showed no significant variation among the low MU settings. To ensure stable beam delivery without increasing the treatment time of Vero4DRT, we recommend a delivery of 5 MU per segment at a dose rate of 500 MU/min.

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“…The association between the delivery accuracy and number of small MU segments and high dose rates stems from the communication delay between the MLC console and MU console. For the older models of Varian linear accelerators (prior to TrueBeam models), this communication delay is about 50 ms. With more segments with small MU, this delay may result in some of these segments being skipped (7) . The effect will be amplified when a high dose rate is applied because 50 ms can deliver more MUs when a high dose rate is used.…”

Section: Discussionmentioning

confidence: 99%

“…The dosimetric effect of such a communication delay between the MU console and MLC console on an IMRT plan depended on the delivery dose rate and the percentage of MU segments included in the IMRT plan. For the later Varian TrueBeam linac models, the communication delay has been decreased from 50 ms to 10 ms, mitigating the dosimetric effect of skipping segments 6 , 7 . Using the Pinnacle treatment planning system (Philips Healthcare, Andover, MA), the default number of segments for a full arc in an VMAT is typically 90 if calculated in every 4° and increases to 180 if calculated in every 2°.…”

Section: Introductionmentioning

confidence: 99%

Impact of small MU/segment and dose rate on delivery accuracy of volumetric‐modulated arc therapy (VMAT)

Huang

Zhuang²,

Mastroianni³

et al. 2016

J Applied Clin Med Phys

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Volumetric‐modulated arc therapy (VMAT) plans may require more control points (or segments) than some of fixed‐beam IMRT plans that are created with a limited number of segments. Increasing number of control points in a VMAT plan for a given prescription dose could create a large portion of the total number of segments with small number monitor units (MUs) per segment. The purpose of this study is to investigate the impact of the small number MU/segment on the delivery accuracy of VMAT delivered with various dose rates. Ten patient datasets were planned for hippocampus sparing for whole brain irradiation. For each dataset, two VMAT plans were created with maximum dose rates of 600 MU/min (the maximum field size of 21×40 cm2) and 1000 MU/min (the maximum field size of 15×15 cm2) for a daily dose of 3 Gy. Without reoptimization, the daily dose of these plans was purposely reduced to 1.5 Gy and 1.0 Gy while keeping the same total dose. Using the two dose rates and three different daily doses, six VMAT plans for each dataset were delivered to a physical phantom to investigate how the changes of dose rate and daily doses impact on delivery accuracy. Using the gamma index, we directly compared the delivered planar dose profiles with the reduced daily doses (1.5 Gy and 1.0 Gy) to the delivered planar dose at 3 Gy daily dose, delivered at dose rate of 600 MU/min and 1000 MU/min, respectively. The average numbers of segments with MU/segment≤1 were 35±8, 87±6 for VMAT‐600 1.5 Gy, VMAT‐600 1 Gy plans, and 30±7 and 42±6 for VMAT‐1000 1.5 Gy and VMAT‐1000 1 Gy plans, respectively. When delivered at 600 MU/min dose rate, the average gamma index passing rates (1%/1 mm criteria) of comparing delivered 1.5 Gy VMAT planar dose profiles to 3.0 Gy VMAT delivered planar dose profiles was 98.28%±1.66%, and the average gamma index passing rate of comparing delivered 1.0 Gy VMAT planar dose to 3.0 Gy VMAT delivered planar dose was 83.75%±4.86%. If using 2%/2 mm and 3%/3 mm criteria, the gamma index passing rates were greater than 97% for both 1.5 Gy VMAT and 1.0 Gy VMAT delivered planar doses. At 1000 MU/min dose rate, the average gamma index passing rates were 96.59%±2.70% for 1.5 Gy VMAT planar dose profiles and 79.37%±9.96% for 1.0 Gy VMAT planar dose profiles when compared to the 3.0 Gy VMAT planar delivered dose profile. When using 2%/2 mm and 3%/3 mm criteria, the gamma index passing rates were greater than 93% for both 1.5 Gy VMAT and 1.0 Gy VMAT planar delivered dose. Under a stricter gamma index criterion (1%/1 mm), significant differences in delivered planar dose profiles at different daily doses were detected, indicating that the known communication delay between the MU console and MLC console may affect VMAT delivery accuracy.PACS number(s): 87.56.bd, 87.55.‐x

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Improvements in dose accuracy delivered with static‐MLC IMRT on an integrated linear accelerator control system

Cited by 14 publications

References 12 publications

Correlation of phantom‐based and log file patient‐specific QA with complexity scores for VMAT

Correlation of phantom‐based and log file patient‐specific QA with complexity scores for VMAT

Beam Characteristics at Low Dose Monitor Unit Settings for Vero4DRT

Impact of small MU/segment and dose rate on delivery accuracy of volumetric‐modulated arc therapy (VMAT)

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