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Title: SU-F-T-629: Effect of Multi-Leaf Collimator (MLC) Width On Plan Quality of Single-Isocenter VMAT Intracranial Stereotactic Radiosurgery for Multiple Metastases

Abstract

Purpose: Single-isocenter VMAT has been shown able to create high quality plans for complex intracranial multiple metastasis SRS cases. Linacs capable of the technique are typically outfitted with an MLC that consists of a combination of 5 mm and 10 mm leaves (standard) or 2.5 mm and 5 mm leaves (high-definition). In this study, we test the hypothesis that thinner collimator leaves are associated with improved plan quality. Methods: Ten multiple metastasis cases were identified and planned for VMAT SRS using a 10 MV flattening filter free beam. Plans were created for a standard (std) and a high-definition (HD) MLC. Published values for leaf transmission factor and dosimetric leaf gap were utilized. All other parameters were invariant. Conformity (plan and individual target), moderate isodose spill (V50%), and low isodose spill (mean brain dose) were selected for analysis. Results: Compared to standard MLC, HD-MLC improved overall plan conformity (median: Paddick CI-HD = 0.83, Paddick CI-std = 0.79; p = 0.004 and median: RTOG CI-HD =1.18, RTOG CI-std =1.24; p = 0.01 ), improved individual lesion conformity (median: Paddick CI-HD,i =0.77, Paddick CI-std,i =0.72; p < 0.001 and median: RTOG CI-HD,i = 1.28, RTOG CI-std,i =1.35; p < 0.001), improved moderate isodosemore » spill (median: V50%-HD = 37.0 cc, V50%-std = 45.7 cc; p = 0.002), and improved low dose spill (median: dmean-HD = 2.90 Gy, dmean-std = 3.19 Gy; p = 0.002). Conclusion: For the single-isocenter VMAT SRS of multiple metastasis plans examined, use of HD-MLC modestly improved conformity, moderate isodose, and low isodose spill compared to standard MLC. However, in all cases we were able to generate clinically acceptable plans with the standard MLC. More work is need to further quantify the difference in cases with higher numbers of small targets and to better understand any potential clinical significance. This research was supported in part by Varian Medical Systems.« less

Authors:
; ; ; ;  [1]
  1. University Alabama Birmingham, Birmingham, AL (United States)
Publication Date:
OSTI Identifier:
22649189
Resource Type:
Journal Article
Resource Relation:
Journal Name: Medical Physics; Journal Volume: 43; Journal Issue: 6; Other Information: (c) 2016 American Association of Physicists in Medicine; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
60 APPLIED LIFE SCIENCES; 61 RADIATION PROTECTION AND DOSIMETRY; COLLIMATORS; LINEAR ACCELERATORS; METASTASES; RADIOTHERAPY

Citation Formats

Kraus, J, Thomas, E, Wu, X, Fiveash, J, and Popple, R. SU-F-T-629: Effect of Multi-Leaf Collimator (MLC) Width On Plan Quality of Single-Isocenter VMAT Intracranial Stereotactic Radiosurgery for Multiple Metastases. United States: N. p., 2016. Web. doi:10.1118/1.4956814.
Kraus, J, Thomas, E, Wu, X, Fiveash, J, & Popple, R. SU-F-T-629: Effect of Multi-Leaf Collimator (MLC) Width On Plan Quality of Single-Isocenter VMAT Intracranial Stereotactic Radiosurgery for Multiple Metastases. United States. doi:10.1118/1.4956814.
Kraus, J, Thomas, E, Wu, X, Fiveash, J, and Popple, R. 2016. "SU-F-T-629: Effect of Multi-Leaf Collimator (MLC) Width On Plan Quality of Single-Isocenter VMAT Intracranial Stereotactic Radiosurgery for Multiple Metastases". United States. doi:10.1118/1.4956814.
@article{osti_22649189,
title = {SU-F-T-629: Effect of Multi-Leaf Collimator (MLC) Width On Plan Quality of Single-Isocenter VMAT Intracranial Stereotactic Radiosurgery for Multiple Metastases},
author = {Kraus, J and Thomas, E and Wu, X and Fiveash, J and Popple, R},
abstractNote = {Purpose: Single-isocenter VMAT has been shown able to create high quality plans for complex intracranial multiple metastasis SRS cases. Linacs capable of the technique are typically outfitted with an MLC that consists of a combination of 5 mm and 10 mm leaves (standard) or 2.5 mm and 5 mm leaves (high-definition). In this study, we test the hypothesis that thinner collimator leaves are associated with improved plan quality. Methods: Ten multiple metastasis cases were identified and planned for VMAT SRS using a 10 MV flattening filter free beam. Plans were created for a standard (std) and a high-definition (HD) MLC. Published values for leaf transmission factor and dosimetric leaf gap were utilized. All other parameters were invariant. Conformity (plan and individual target), moderate isodose spill (V50%), and low isodose spill (mean brain dose) were selected for analysis. Results: Compared to standard MLC, HD-MLC improved overall plan conformity (median: Paddick CI-HD = 0.83, Paddick CI-std = 0.79; p = 0.004 and median: RTOG CI-HD =1.18, RTOG CI-std =1.24; p = 0.01 ), improved individual lesion conformity (median: Paddick CI-HD,i =0.77, Paddick CI-std,i =0.72; p < 0.001 and median: RTOG CI-HD,i = 1.28, RTOG CI-std,i =1.35; p < 0.001), improved moderate isodose spill (median: V50%-HD = 37.0 cc, V50%-std = 45.7 cc; p = 0.002), and improved low dose spill (median: dmean-HD = 2.90 Gy, dmean-std = 3.19 Gy; p = 0.002). Conclusion: For the single-isocenter VMAT SRS of multiple metastasis plans examined, use of HD-MLC modestly improved conformity, moderate isodose, and low isodose spill compared to standard MLC. However, in all cases we were able to generate clinically acceptable plans with the standard MLC. More work is need to further quantify the difference in cases with higher numbers of small targets and to better understand any potential clinical significance. This research was supported in part by Varian Medical Systems.},
doi = {10.1118/1.4956814},
journal = {Medical Physics},
number = 6,
volume = 43,
place = {United States},
year = 2016,
month = 6
}
  • Purpose: To determine the effects of multileaf collimator (MLC) leaf width on normal-brain-tissue doses and dose conformity of SRS RapidArc treatment plans for brain tumors. Methods: Ten patients with 24 intracranial tumors (seven with 1–2 and three with 4–6 lesions) were planned using RapidArc for both Varian Millennium 120 MLC (5 mm leaf width) and high definition (HD) MLC (2.5 mm leaf width). Between 2 and 8 arcs were used with two full coplanar arcs and the rest non-coplanar half arcs. 6 MV beams were used and plans were optimized with a high priority to the Normal Tissue Objective (tomore » achieve dose conformity and sharp dose fall-off) and normal brain tissue. Calculation was done using AAA on a 1 mm grid size. The prescription dose ranged from 14–22 Gy. Plans were normalized such that 99% of the target received the prescription dose. Identical beam geometries, optimizations, calculations, and normalizations were used for both plans. Paddick Conformity Index (PCI), V4, V8 and V12 Gy for normal brain tissue and Integral Dose were used for analysis. Results: In all cases, HD MLC plans performed better in sparing normal brain tissue, achieving a higher PCI with a lower Integral Dose. The average PCI for all 24 targets was 0.75±0.23 and 0.70±0.23 (p ≤0.0015) for HD MLC and Millennium MLC plans, respectively. The average ratio of normal brain doses for Millennium MLC to HD MLC plans was 1.30±0.16, 1.27±0.15, and 1.31±0.18 for the V4, V8, and V12, respectively. The differences in normal brain dose for all criteria were statistically significant with p-value < 0.02. On average Millennium MLC plans had a 16% higher integral dose than HD MLC plans. Conclusion: Significantly better dose conformity with reduced volume of normal brain tissue and integral dose was achieved with HD MLC plans compared to Millennium MLC plans.« less
  • Purpose: To assess the impacts that multileaf collimator (MLC) leaf width has on the dose conformity and normal brain tissue doses of single and multiple isocenter stereotactic IMRT (SRT) plans for multiple intracranial tumors. Methods: Fourteen patients with 2–3 targets were studied retrospectively. Patients treated with multiple isocenter treatment plans using 9 to 12 non-coplanar beams per lesion underwent repeat planning using single isocenter and 10 to 12 non-coplanar beams with 2.5mm, 3mm and 5mm MLC leaf widths. Brainlab iPlan treatment planning system for delivery with the 2.5mm MLC served as reference. Identical contour sets and dose-volume constraints were applied.more » The prescribed dose to each target was 25 Gy to be delivered over 5 fractions with a minimum of 99% dose to cover ≥ 95% of the target volume. Results: The lesions and normal brains ranged in size from 0.11 to 51.67cc (median, 2.75cc) and 1090 to 1641cc (median, 1401cc), respectively. The Paddick conformity index for single and multiple isocenter (2.5mm vs. 3mm and 5mm MLCs) was (0.79±0.08 vs. 0.79±0.07 and 0.77±0.08) and (0.79±0.09 vs. 0.77±0.09 and 0.76±0.08), respectively. The average normal brain volumes receiving 15 Gy for single and multiple isocenter (2.5mm vs. 3mm and 5mm MLCs) were (3.65% vs. 3.95% and 4.09%) and (2.89% vs. 2.91% and 2.92%), respectively. Conclusion: The average dose conformity observed for the different leaf width for single and multiple isocenter plans were similar, throughout. However, the average normal brain volumes receiving 2.5 to 15 Gy were consistently lower for the 2.5mm MLC leaf width, especially for single isocenter plans. The clinical consequences of these integral normal brain tissue doses are still unknown, but employing the use of the 2.5mm MLC option is desirable at sparing normal brain tissue for both single and multiple isocenter cases.« less
  • Purpose: To demonstrate a patient specific, image-guided quality assurance method that tests both dosimetric and geometric accuracy for single-isocenter multiple-target VMAT radiosurgery (SIMT-VMAT-SRS) Method: We used a new film type, EBT-XD (optimal range 0.4–40Gy), and an in-house PMMA phantom having a coronal plane for film and a 0.125 cm3 ionization chamber (IC). The phantom contained fiducial features for kV image guided setup and for accurate film marking. Five patient plans with multiple targets sizes ranging from 3 to 21mm in diameter and prescribed doses from 14 to 18 Gy were selected. Two verification plans were generated for each case withmore » the film plane passing through the center of the largest and smallest targets. For the four largest targets we obtained an IC measurement. For each case, a calibration film was irradiated using a custom designed step pattern. The films were scanned using a flatbed color scanner and converted to dose using the calibration film and the three channel calibration method. Image registration was performed between film and treatment planning system calculations to evaluate the geometric accuracy. Results: The mean registration vector had an average magnitude of 0.47 mm (range from 0.13mm to 0.64 mm). For the four largest targets, the mean ratio of the IC and film measurement to expected dose was 0.990 (range 0.968 to 1.009) and 1.032 (1.021 to 1.046), respectively. The fraction of pixels having gamma index < 1 for criteria of 3%/3mm, 3%/2mm, 3%/1mm was 98.8%, 97.5% and 87.2% before geometric registration and 99.1%, 98.3% and 94.8% after registration. Conclusion: We have demonstrated an image-guided QA method can assess both geometric and dosimetric accuracy. The phantom was positioned with sub-millimeter accuracy. Absolute film dosimetry using EBT-XD film was sufficiently accurate for assessment of dose to multi-targets too small for IC measurement in SRS VMAT plans.« less
  • Purpose: To compare the plan quality of linear accelerator (linac)-based stereotactic radiosurgery (SRS) using single-isocenter volumetric arc therapy (SI-VMAT), restricted single-isocenter dynamic-arc (RSI-DARC), and multi-isocenter DARC (MI-DARC) techniques. Methods: Fifteen SRS cases were randomly selected and re-planned using the SI-VMAT (Pinnacle), RSI-DARC (iPlanNet) and MI-DARC (iPlanNet). The number of planning target volumes (PTVs) for each plan ranged from 1 to 6. For SI-VMAT, a single isocenter and 3-4 VMAT beams are used for all PTVs, while for MI-DARC, each PTV has its own isocetner with 3 DARC beams. RSI-DARC uses one isocnter with 3-6 DARC beams to irradiate all PTVsmore » within 2.5-cm radius. Both SI-DARC and RSI-DARC plans were optimized manually. The prescription dose was 20 Gy to each PTV. The maximal dose was 25 Gy for RSI-DARC and MI-DARC, but could not be controlled for SI-VMAT due to the nature of VMAT planning. Plan quality indexes including PTV coverage, mean dose of PTV (PTVmean) and tissue (Tmean), V12Gy, conformity index (CI), and V10Gy/VPTV were calculated and compared. Results: Full PTV coverage was achieved for all three techniques. Using the MI-DARC plans as the gold standard, the PTVmean of the SI-VMAT plans was 12.5%±8.3% (mean±standard deviation) higher, in comparison to 0.7%±1.4% for the RSI-DARC plans. Similar trend was observed for other indexes including V12Gy (39.4%±27.3% vs. 9.3%±7.8%), Tmean (35.0%±26.8% vs. 2.8%±3.4%), and V10Gy/VPTV (42.2%±31.5% vs. 9.9%±8.2%). CI is comparable (6.2%±14.2% vs. 6.3%±7.2%). Assuming the treatment time is proportional to the number of isocenters, the reduction of the treatment time in comparison to MI-DARC was 70% for SI-VMAT and 42% for RSI-DARC. Conclusion: Although the SI-VMAT can save a considerable amount of treatment time, the plan indexes also significantly deviates from the gold standard, MI-DARC. RSI-DARC, on the other hand, provides a good compromise between the treatment time and plan quality.« less
  • Purpose: To develop an optimization algorithm to reduce normal brain dose by optimizing couch and collimator angles for single isocenter multiple targets treatment of stereotactic radiosurgery. Methods: Three metastatic brain lesions were retrospectively planned using single-isocenter volumetric modulated arc therapy (VMAT). Three matrices were developed to calculate the projection of each lesion on Beam’s Eye View (BEV) by the rotating couch, collimator and gantry respectively. The island blocking problem was addressed by computing the total area of open space between any two lesions with shared MLC leaf pairs. The couch and collimator angles resulting in the smallest open areas weremore » the optimized angles for each treatment arc. Two treatment plans with and without couch and collimator angle optimization were developed using the same objective functions and to achieve 99% of each target volume receiving full prescription dose of 18Gy. Plan quality was evaluated by calculating each target’s Conformity Index (CI), Gradient Index (GI), and Homogeneity index (HI), and absolute volume of normal brain V8Gy, V10Gy, V12Gy, and V14Gy. Results: Using the new couch/collimator optimization strategy, dose to normal brain tissue was reduced substantially. V8, V10, V12, and V14 decreased by 2.3%, 3.6%, 3.5%, and 6%, respectively. There were no significant differences in the conformity index, gradient index, and homogeneity index between two treatment plans with and without the new optimization algorithm. Conclusion: We have developed a solution to the island blocking problem in delivering radiation to multiple brain metastases with shared isocenter. Significant reduction in dose to normal brain was achieved by using optimal couch and collimator angles that minimize total area of open space between any of the two lesions with shared MLC leaf pairs. This technique has been integrated into Eclipse treatment system using scripting API.« less