Relationship between dosimetric leaf gap and dose calculation errors for high definition multi-leaf collimators in radiotherapy

Kim, Jinkoo; Han, James; Hsia, A; Li, Shidong; Xu, Zhigang; Ryu, Samuel

doi:10.1016/j.phro.2018.01.003

Cited by 27 publications

(45 citation statements)

References 19 publications

Supporting

Mentioning

Contrasting

Unclassified

Order By: Relevance

“…Increasing the DLG in the TPS increased the calculated doses, consequently improving the agreement between calculations and measurements. This increase in the DLG parameter agrees with findings from other investigators: Kielar et al increased the value from 0.5 to 1.7 mm and Kim et al adjusted it from 0.39 to 1.1 mm. Nevertheless, the cause of the discrepancies and the reason to tune the DLG remained unknown to date.…”

Section: Discussionsupporting

confidence: 91%

“…However, this configuration produces underdosage between adjacent leaf pairs in asynchronous MLC movements due to the additional shielding by the tongue of opposing leaf sides during treatment delivery . This underdosage is known as the T&G effect and it can significantly change the dose distribution . In arc treatments, T&G effects are typically smoothed out due to the gantry rotation, but they can produce a reduction in average doses of up to 5%–7% .…”

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

On the need for tuning the dosimetric leaf gap for stereotactic treatment plans in the Eclipse treatment planning system

Vieillevigne

Khamphan

Saez

et al. 2019

J Applied Clin Med Phys

View full text Add to dashboard Cite

The dosimetric leaf gap (DLG) and tongue‐and‐groove (T&G) effects are critical aspects in the modeling of multileaf collimators (MLC) in the treatment planning system (TPS). In this study, we investigated the dosimetric impact of limitations associated with the T&G modeling in stereotactic plans and its relationship with the need for tuning the DLG in the Eclipse TPS. Measurements were carried out using Varian TrueBeam STx systems from two different institutions. Test fields presenting MLC patterns with several MLC gap sizes (meanGap) and different amounts of T&G effect (TGi) were first evaluated. Secondly, dynamic conformal arc (DCA) and volumetric modulated arc therapy (VMAT) deliveries of stereotactic cases were analyzed in terms of meanGap and TGi. Two DLG values were used in the TPS: the measured DLG (DLGmeas) and an optimal DLG (DLGopt). Measured and calculated doses were compared according to dose differences and gamma passing rates (GPR) with strict local gamma criteria of 2%/2 mm. The discrepancies were analyzed for DLGmeas and DLGopt, and their relationships with both TGi and meanGap were investigated. DCA arcs involved significantly lower TGi and larger meanGap than VMAT arcs (P < 0.0001). By using DLGmeas in the TPS, the dose discrepancies increased as TGi increased and meanGap decreased for both test fields and clinical plans. Dose discrepancies dramatically increased with the ratio TGi/meanGap. Adjusting the DLG value was then required to achieve acceptable calculations and configuring the TPS with DLGopt led to an excellent agreement with median GPRs (2%/2 mm) > 99% for both institutions. We also showed that DLGopt could be obtained from the results of the test fields. We demonstrated that the need for tuning the DLG is due to the limitations of the T&G modeling in the Eclipse TPS. A set of sweeping gap tests modified to incorporate T&G effects can be used to determine the optimal DLG value.

show abstract

Section: Discussionsupporting

confidence: 91%

Section: Introductionmentioning

confidence: 99%

On the need for tuning the dosimetric leaf gap for stereotactic treatment plans in the Eclipse treatment planning system

Vieillevigne

Khamphan

Saez

et al. 2019

J Applied Clin Med Phys

View full text Add to dashboard Cite

show abstract

“…More importantly, these differences had no major influence on the resulting dose accuracy as the TG‐119 measurements (Section 3.C) and E2E measurements (Section 3.D) performed in this study showed good agreement between measured and calculated dose and were within the clinically acceptable tolerances. Additionally, several studies have shown that the DLG value which results in the highest dose calculation accuracy depends on the plan characteristics and does not necessarily coincide with the measured DLG value . Hence, measuring the DLG allows to validate whether the MLC performance of the system is within the expected specifications (0.10 mm to 0.30 mm in this case) but does not necessarily suffice to validate the dosimetric performance of the modeled DLG in clinical practice.…”

Section: Discussionmentioning

confidence: 95%

Validation and IMRT/VMAT delivery quality of a preconfigured fast‐rotating O‐ring linac system

et al. 2018

View full text Add to dashboard Cite

Purpose: A fast-rotating O-ring dedicated intensity modulated radiotherapy (IMRT)/volumetric modulated arc therapy (VMAT) delivery system, the Halcyon, is delivered by default with a fully preconfigured photon beam model in the treatment planning system (TPS). This work reports on the validation and achieved IMRT/VMAT delivery quality on the system. Methods: Acceptance testing followed the vendor's installation product acceptance and was supplemented with mechanical QA. The dosimetric calibration was performed according to the IAEA TRS-398 code-of-practice, delivering 600 cGy/min at 10 cm depth, a 90 cm source-surface distance, and a 10 9 10 cm² field size. The output factors, multileaf collimator (MLC) transmission and dosimetric leaf gap (DLG) were validated by comparing measurements with the modeled values in the TPS. Validation of IMRT/VMAT was conducted following AAPM reports (MPPG 5.a, TG-119). Next, dose measurements were performed for end-to-end (E2E) checks in heterogeneous anthropomorphic phantoms using radiochromic film in multiple planes and using ionization chambers (IC) point measurements. E2E checks were performed for VMAT (cranial, rectum, spine, and head and neck) and IMRT (lung). Additionally, IROC Houston mailed dosimetry audits were performed for the beam calibration and E2E measurements using a thorax phantom (IMRT) and a head and neck phantom (VMAT). Lastly, extensive patient-specific QA was performed for the first patients of each new indication, 26 in total (n rectum = 2, n spine = 5, n lung = 5, n esophagus = 2, n head and neck = 7, n cranial = 5), treated on the fast-rotating O-ring linac. The patient-specific QA followed the AAPM TG-218 guidelines and comprised of portal dosimetry, ArcCHECK diode array, radiochromic film dosimetry in a MultiCube phantom, and IC point measurements. Results: The measured output factors showed an agreement <1% for fields ≥3 9 3 cm². Field sizes ≤2 9 2 cm² had a difference of <2%. The measured single-layer MLC transmission was 0.42 AE 0.01% and the measured DLG was 0.27 AE 0.22 mm. The AAPM MPPG 5.a measurements were fully compliant with the guideline criteria. Dose differences larger than 2% were found for the PDD at large depths (>25 cm). TG-119's confidence limits were achieved for the VMAT point dose measurements and for both the IMRT and VMAT radiochromic film measurements. The TG-119 confidence limits were not achieved for IMRT point dose measurements in both the target (5.9%) and the avoidance structure (6.4%). All E2E tests had point differences below 2.3% and gamma agreement scores above 90.6%. The IROC beam calibration audit showed agreement of <1%. The IROC lung IMRT audit and head and neck VMAT audit had results compliant with the IROC Houston's credentialing criteria. All IMRT and VMAT plans selected for patient-specific QA were within the action limits suggested by TG-218. Conclusions:The fast-rotating O-ring linac and its preconfigured TPS are compliant with the international commissioning criteria of AAPM MPPG 5.a and AAPM TG-119. E2E measur...

show abstract

“…At first glance, the results from this work may seem unsurprising; multiple previous studies have likewise noted a strong dependence of the dose calculation for modern treatments (IMRT and VMAT) with the MLC leaf position. 7,13,[18][19][20][21][22] However, the current work is unique in that the dose differences calculated herein are based upon the actual variations in beam modeling adopted by the radiotherapy community, making the magnitude of the dose deviations particularly relevant to clinical practice. Moreover, these differences were evaluated in a reference geometry (the IROC-H H&N phantom) where the radiation oncology community is known to struggle in the clinical trial credentialing process.…”

Section: Discussionmentioning

confidence: 99%

“…Previous studies have investigated the relative errors that several modeling factors related to the multileaf collimator (MLC) can contribute to the overall accuracy, as well as the detectability of these errors. [11][12][13] While generally informative, such works have been relative to single clinical systems, and thus cannot provide wide-ranging context into other clinical scenarios. More problematically, the magnitude of change in each parameter (i.e., how much error is introduced into the MLC offset) and associated effect size, have not been based on clinically realistic values.…”

Section: Introductionmentioning

confidence: 99%

Sensitivity of IROC phantom performance to radiotherapy treatment planning system beam modeling parameters based on community‐driven data

et al. 2020

View full text Add to dashboard Cite

Purpose: Treatment planning system (TPS) dose calculations have previously been shown to be sensitive to modeling errors, especially when treating with complex strategies like intensity-modulated radiation therapy (IMRT). This work investigates the dosimetric impact of several dosimetric and nondosimetric beam modeling parameters, based on their distribution in the radiotherapy community, in two commercial TPSs in order to understand the realistic potential for dose deviations and their clinical effects. Methods and materials: Beam models representing standard 120-leaf Varian Clinac-type machines were developed in Eclipse 13.5 (AAA algorithm) and RayStation 9A (v8.99, collapsed-cone algorithm) based upon median values of dosimetric measurements from Imaging and Radiation Oncology Core (IROC) Houston site visit data and community beam modeling parameter survey data in order to represent a baseline linear accelerator. Five clinically acceptable treatment plans (three IMRT, two VMAT) were developed for the IROC head and neck phantom. Dose distributions for each plan were recalculated after individually modifying parameters of interest (e.g., MLC transmission, percent depth doses [PDDs], and output factors) according to the 2.5th to 97.5 th percentiles of community survey and machine performance data to encompass the realistic extent of variance in the radiotherapy community. The resultant dose distributions were evaluated by examining relative changes in average dose for thermoluminescent dosimeter (TLD) locations across the two target volumes and organ at risk (OAR). Interplay was also examined for parameters generating changes in target dose greater than 1%. Results: For Eclipse, dose calculations were sensitive to changes in the dosimetric leaf gap (DLG), which resulted in differences from À5% to +3% to the targets relative to the baseline beam model. Modifying the MLC transmission factor introduced differences up to AE 1%. For RayStation, parameters determining MLC behaviors likewise contributed substantially; the MLC offset introduced changes in dose from À4% to +7%, and the MLC transmission caused changes of À4% to +2%. Among the dosimetric qualities examined, changes in PDD implementation resulted in the most substantial changes, but these were only up to AE1%. Other dosimetric factors had <1% impact on dose accuracy. Interplay between impactful parameters was found to be minimal. Conclusion: Factors related to the modeling of the MLC, particularly relating to the leaf offset, can cause clinically significant changes in the calculated dose for IMRT and VMAT plans. This should be of concern to the radiotherapy community because the clinical effects of poor TPS commissioning were based on reported data from clinically implemented beam models. These results further reinforce that dose errors caused by poor TPS calculations are often involved in IROC phantom failures.

show abstract

Relationship between dosimetric leaf gap and dose calculation errors for high definition multi-leaf collimators in radiotherapy

Cited by 27 publications

References 19 publications

On the need for tuning the dosimetric leaf gap for stereotactic treatment plans in the Eclipse treatment planning system

On the need for tuning the dosimetric leaf gap for stereotactic treatment plans in the Eclipse treatment planning system

Validation and IMRT/VMAT delivery quality of a preconfigured fast‐rotating O‐ring linac system

Sensitivity of IROC phantom performance to radiotherapy treatment planning system beam modeling parameters based on community‐driven data

Contact Info

Product

Resources

About