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Title: SU-F-T-574: MLC Based SRS Beam Commissioning - Minimum Target Size Investigation

Abstract

Purpose: To implement a MLC accelerator based SRS program using small fields down to 1 cm × 1 cm and to determine the smallest target size safe for clinical treatment. Methods: Computerized beam scanning was performed in water using a diode detector and a linac-head attached transmission ion chamber to characterize the small field dosimetric aspects of a 6 MV photon beam (Trilogy-Varian Medical Systems, Inc.). The output factors, PDD and profiles of field sizes 1, 2, 3, 4, and 10 cm{sup 2} were measured and utilized to create a new treatment planning system (TPS) model (AAA ver 11021). Static MLC SRS treatment plans were created and delivered to a homogeneous phantom (Cube 20, CIRS, Inc.) for a 1.0 cm and 1.5 cm “PTV” target. A 12 field DMLC plan was created for a 2.1 cm target. Radiochromic film (EBT3, Ashland Inc.) was used to measure the planar dose in the axial, coronal and sagittal planes. A micro ion chamber (0.007 cc) was used to measure the dose at isocenter for each treatment delivery. Results: The new TPS model was validated by using a tolerance criteria of 2% dose and 2 mm distance to agreement. For fields ≤ 3 cm{supmore » 2}, the max PDD, Profile and OF difference was 0.9%, 2%/2mm and 1.4% respectively. The measured radiochromic film planar dose distributions had gamma scores of 95.3% or higher using a 3%/2mm criteria. Ion chamber measurements for all 3 test plans effectively met our goal of delivering the dose accurately to within 5% when compared to the expected dose reported by the TPS (1 cm plan Δ= −5.2%, 1.5 cm plan Δ= −2.0%, 2 cm plan Δ= 1.5%). Conclusion: End to end testing confirmed that MLC defined SRS for target sizes ≥ 1.0 cm can be safely planned and delivered.« less

Authors:
 [1];  [2]
  1. Florida Cancer Specialists - Largo, Largo, FL (United States)
  2. Florida Cancer Specialists - New Port Richey, New Port Richey, FL (United States)
Publication Date:
OSTI Identifier:
22649149
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:
07 ISOTOPES AND RADIATION SOURCES; 60 APPLIED LIFE SCIENCES; IONIZATION CHAMBERS; PHOTON BEAMS; PLANNING; RADIATION DOSE DISTRIBUTIONS

Citation Formats

Zakikhani, R, and Able, C. SU-F-T-574: MLC Based SRS Beam Commissioning - Minimum Target Size Investigation. United States: N. p., 2016. Web. doi:10.1118/1.4956759.
Zakikhani, R, & Able, C. SU-F-T-574: MLC Based SRS Beam Commissioning - Minimum Target Size Investigation. United States. doi:10.1118/1.4956759.
Zakikhani, R, and Able, C. Wed . "SU-F-T-574: MLC Based SRS Beam Commissioning - Minimum Target Size Investigation". United States. doi:10.1118/1.4956759.
@article{osti_22649149,
title = {SU-F-T-574: MLC Based SRS Beam Commissioning - Minimum Target Size Investigation},
author = {Zakikhani, R and Able, C},
abstractNote = {Purpose: To implement a MLC accelerator based SRS program using small fields down to 1 cm × 1 cm and to determine the smallest target size safe for clinical treatment. Methods: Computerized beam scanning was performed in water using a diode detector and a linac-head attached transmission ion chamber to characterize the small field dosimetric aspects of a 6 MV photon beam (Trilogy-Varian Medical Systems, Inc.). The output factors, PDD and profiles of field sizes 1, 2, 3, 4, and 10 cm{sup 2} were measured and utilized to create a new treatment planning system (TPS) model (AAA ver 11021). Static MLC SRS treatment plans were created and delivered to a homogeneous phantom (Cube 20, CIRS, Inc.) for a 1.0 cm and 1.5 cm “PTV” target. A 12 field DMLC plan was created for a 2.1 cm target. Radiochromic film (EBT3, Ashland Inc.) was used to measure the planar dose in the axial, coronal and sagittal planes. A micro ion chamber (0.007 cc) was used to measure the dose at isocenter for each treatment delivery. Results: The new TPS model was validated by using a tolerance criteria of 2% dose and 2 mm distance to agreement. For fields ≤ 3 cm{sup 2}, the max PDD, Profile and OF difference was 0.9%, 2%/2mm and 1.4% respectively. The measured radiochromic film planar dose distributions had gamma scores of 95.3% or higher using a 3%/2mm criteria. Ion chamber measurements for all 3 test plans effectively met our goal of delivering the dose accurately to within 5% when compared to the expected dose reported by the TPS (1 cm plan Δ= −5.2%, 1.5 cm plan Δ= −2.0%, 2 cm plan Δ= 1.5%). Conclusion: End to end testing confirmed that MLC defined SRS for target sizes ≥ 1.0 cm can be safely planned and delivered.},
doi = {10.1118/1.4956759},
journal = {Medical Physics},
number = 6,
volume = 43,
place = {United States},
year = {Wed Jun 15 00:00:00 EDT 2016},
month = {Wed Jun 15 00:00:00 EDT 2016}
}
  • Purpose: Finite size pencil beam (FSPB) algorithms are commonly used to pre-calculate the beamlet dose distribution for IMRT treatment planning. FSPB commissioning, which usually requires fine tuning of the FSPB kernel parameters, is crucial to the dose calculation accuracy and hence the plan quality. Yet due to the large number of beamlets, FSPB commissioning could be very tedious. This abstract reports an optimization-based FSPB commissioning tool we have developed in MatLab to facilitate the commissioning. Methods: A FSPB dose kernel generally contains two types of parameters: the profile parameters determining the dose kernel shape, and a 2D scaling factors accountingmore » for the longitudinal and off-axis corrections. The former were fitted using the penumbra of a reference broad beam’s dose profile with Levenberg-Marquardt algorithm. Since the dose distribution of a broad beam is simply a linear superposition of the dose kernel of each beamlet calculated with the fitted profile parameters and scaled using the scaling factors, these factors could be determined by solving an optimization problem which minimizes the discrepancies between the calculated dose of broad beams and the reference dose. Results: We have commissioned a FSPB algorithm for three linac photon beams (6MV, 15MV and 6MVFFF). Dose of four field sizes (6*6cm2, 10*10cm2, 15*15cm2 and 20*20cm2) were calculated and compared with the reference dose exported from Eclipse TPS system. For depth dose curves, the differences are less than 1% of maximum dose after maximum dose depth for most cases. For lateral dose profiles, the differences are less than 2% of central dose at inner-beam regions. The differences of the output factors are within 1% for all the three beams. Conclusion: We have developed an optimization-based commissioning tool for FSPB algorithms to facilitate the commissioning, providing sufficient accuracy of beamlet dose calculation for IMRT optimization.« less
  • Purpose: To commission the Monaco Treatment Planning System for the Novalis Tx machine. Methods: The commissioning of Monte-Carlo (MC), Collapsed Cone (CC) and electron Monte-Carlo (eMC) beam models was performed through a series of measurements and calculations in medium and in water. In medium measurements relied Octavius 4D QA system with the 1000 SRS detector array for field sizes less than 4 cm × 4 cm and the 1500 detector array for larger field sizes. Heterogeneity corrections were validated using a custom built phantom. Prior to clinical implementation, an end to end testing of a Prostate and H&N VMAT plansmore » was performed. Results: Using a 0.5% uncertainty and 2 mm grid sizes, Tables I and II summarize the MC validation at 6 MV and 18 MV in both medium and water. Tables III and IV show similar comparisons for CC. Using the custom heterogeneity phantom setup of Figure 1 and IGRT guidance summarized in Figure 2, Table V lists the percent pass rate for a 2%, 2 mm gamma criteria at 6 and 18 MV for both MC and CC. The relationship between MC calculations settings of uncertainty and grid size and the gamma passing rate for a prostate and H&N case is shown in Table VI. Table VII lists the results of the eMC calculations compared to measured data for clinically available applicators and Table VIII for small field cutouts. Conclusion: MU calculations using MC are highly sensitive to uncertainty and grid size settings. The difference can be of the order of several per cents. MC is superior to CC for small fields and when using heterogeneity corrections, regardless of field size, making it more suitable for SRS, SBRT and VMAT deliveries. eMC showed good agreement with measurements down to 2 cm − 2 cm field size.« less
  • Purpose: To present a clinical case which challenges the base assumption of log-file based QA, by showing that the actual position of a MLC leaf can suddenly deviate from its programmed and logged position by >1 mm as observed with real-time imaging. Methods: An EPID-based exit-fluence dosimetry system designed to prevent gross delivery errors was used in cine mode to capture portal images during treatment. Visual monitoring identified an anomalous MLC leaf pair gap not otherwise detected by the automatic position verification. The position of the erred leaf was measured on EPID images and log files were analyzed for themore » treatment in question, the prior day’s treatment, and for daily MLC test patterns acquired on those treatment days. Additional standard test patterns were used to quantify the leaf position. Results: Whereas the log file reported no difference between planned and recorded positions, image-based measurements showed the leaf to be 1.3±0.1 mm medial from the planned position. This offset was confirmed with the test pattern irradiations. Conclusion: It has been clinically observed that log-file derived leaf positions can differ from their actual positions by >1 mm, and therefore cannot be considered to be the actual leaf positions. This cautions the use of log-based methods for MLC or patient quality assurance without independent confirmation of log integrity. Frequent verification of MLC positions through independent means is a necessary precondition to trusting log file records. Intra-treatment EPID imaging provides a method to capture departures from MLC planned positions. Work was supported in part by Varian Medical Systems.« less
  • Purpose: To determine the proton output factors for an SRS cone set using standardized apertures and varied range compensators (bolus blanks); specifically, to determine the best method for modeling the bolus gap factor (BGF) and eliminate the need for patient specific calibrations. Methods: A Standard Imaging A-16 chamber was placed in a Plastic Water phantom to measure the change in dose/MU with different treatment combinations for a proton SRS cone, using standardized apertures and range compensators. Measurements were made with all apertures in the SRS cone set, with four different range compensator thicknesses and five different air gaps between themore » end of the SRS cone and the surface of the phantom. The chamber was located at iso-center and maintained at a constant depth at the center of modulation for all measurements. Each aperture was placed in the cone to measure the change in MU needed to maintain constant dose at the chamber, as the air gap was increased with different thicknesses of bolus. Results: The dose/MU varied significantly with decreasing aperture size, increasing bolus thickness, or increasing air gap. The measured data was fitted with the lowest order polynomials that accurately described the data, to create a model for determining the change in output for any potential combination of devices used to treat a patient. For a given standardized aperture, the BGF could be described by its constituent factors: the bolus thickness factor (BTF) and the nozzle extension factor (NEF). Conclusion: The methods used to model the dose at the calibration point could be used to accurately predict the change in output for SRS proton beams due to the BGF, eliminating the need for patient specific calibrations. This method for modeling SRS treatments could also be applied to model other treatments using passively scattered proton beams.« less
  • Purpose: SRS is an effective non-invasive alternative treatment modality with minimal-toxicity used to treat patients with medically/surgically refractory trigeminal neuralgia root(TNR) or those who may not tolerate surgical intervention. We present our linac-based SRS procedure for TNR treatment and simultaneously report our clinical outcomes. Methods: Twenty-eight TNR-patients treated with frame-based SRS at our institution (2009–2015) with a single-fraction point-dose of 60-80Gy to TNR were included in this IRB-approved study. Experienced neurosurgeon and radiation oncologist delineated the TNR on 1.0mm thin 3D-FIESTA-MRI that was co-registered with 0.7mm thin planning-CT. Treatment plans were generated in iPlan (BrainLAB) with a 4-mm diameter conemore » using 79 arcs with differential-weighting for Novalis-TX 6MV-SRS(1000MU/min) beam and optimized to minimize brainstem dose. Winston-Lutz test was performed before each treatment delivery with sub-millimeter isocenter accuracy. Quality assurance of frame placement was maintained by helmet-bobble-measurement before simulation-CT and before patient setup at treatment couch. OBI-CBCT scan was performed for patient setup verification without applying shifts. On clinical follow up, treatment response was assessed using Barrow Neurological Institute Pain Intensity Score(BNI-score:I–V). Results: 26/28 TNR-patients (16-males/10-females) who were treated with following single-fraction point-dose to isocenter: 80Gy(n=22),75Gy(n=1),70Gy(n=2) and 60Gy(n=1, re-treatment) were followed up. Median follow-up interval was 8.5-months (ranged:1–48.5months). Median age was 70-yr (ranged:43–93-yr). Right/left TNR ratio was 15/11. Delivered total # of average MUs was 19034±1204. Average beam-on-time: 19.0±1.3min. Brainstem max-dose and dose to 0.5cc were 13.3±2.4Gy (ranged:8.1–16.5Gy) and 3.6±0.4Gy (ranged:3.0–4.9Gy). On average, max-dose to optic-apparatus was ≤1.2Gy. Mean value of max-dose to eyes/lens was 0.26Gy/0.11Gy. Overall, 20-patients (77%) responded to treatment: 5(19%) achieved complete pain relief without medication (BNI score: I); 5(19%) had no-pain, decreased medication (BNI-score:II); 2(7.7%) had no-pain, but, continued medication (BNI-score:IIIA), and 8(30.8%) had pain that was well controlled by medication (BNI-score: IIIB). Six-patients (23.0%) did not respond to treatment (BNI-score:IV–V). Neither cranial nerve deficit nor radio-necrosis of temporal lobe was clinically observed. Conclusion: Linac-based SRS for medically/surgically refractory TNR provided an effective treatment option for pain resolution/control with very minimal if any normal tissue toxicity. Longer follow up of these patients is anticipated/needed to confirm our observations.« less