Dosimetric Validation of Digital Megavolt Imager for Flattening Filter Free Beams in the Pre-Treatment Quality Assurance of Stereotactic Body Radiation Therapy for Liver Metastases
01 Jun 2020-Asian Pacific Journal of Cancer Prevention (West Asia Organization for Cancer Prevention (WAOCP), APOCP's West Asia Chapter)-Vol. 21, Iss: 6, pp 1659-1665
TL;DR: The characteristics of aS1200 evaluated at FFF beams have shown its potential ability as QA tool and can be used in SBRT QA for liver metastases with greater confidence.
Abstract: Aim: The aim is to evaluate the use of digital megavolt imager (DMI) aS1200 in portal dosimetry with flattening filter free (FFF) beams. Materials and Methods: Dosimetric properties of DMI is characterized at 6MV FFF beams for signal saturation, dose linearity, dependency on dose-rate and source-detector distance (SDD), signal lag (ghosting), and back scatter. Portal dosimetry is done for twenty volumetric modulated arc therapy (VMAT) based stereotactic body radiotherapy (SBRT) plans for the treatment of liver metastases and the results are compared with repeated measurements of Octavius 4D. Results: The detector signal to monitor unit (MU) ratio drops drastically below 25MU. The detector linearity with dose is within 1% and no evidence of signal saturation as such. The aS1200 response variation across various dose rates and SDD is <0.4% and <0.2% respectively. The effect of ghosting increased distinctly at higher dose rate but however it is negligible (0.1%). The impact of back scatter is <0.3% because of additional shielding provided at the back of the detector. The portal dosimetry results of SBRT QA plans evaluated at the gamma criteria of 2mm/2% (DTA/DD) both under global and local mode analysis has shown an average gamma passing rate of area gamma (<1) 97.9±0.8% and 96.4±0.9%. The SBRT QA results observed in aS1200 are inline and consistent with Octavius 4D measured results. Conclusion: The characteristics of aS1200 evaluated at FFF beams have shown its potential ability as QA tool and can be used in SBRT QA for liver metastases with greater confidence.
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TL;DR: A composite distribution has recently been developed that presents the dose difference in regions that fail both dose-difference and DTA comparison criteria, and a technique is developed to unify dose distribution comparisons using the acceptance criteria.
Abstract: The commissioning of a three-dimensional treatment planning system requires comparisons of measured and calculated dose distributions. Techniques have been developed to facilitate quantitative comparisons, including superimposed isodoses, dose-difference, and distance-to-agreement (DTA) distributions. The criterion for acceptable calculation performance is generally defined as a tolerance of the dose and DTA in regions of low and high dose gradients, respectively. The dose difference and DTA distributions complement each other in their useful regions. A composite distribution has recently been developed that presents the dose difference in regions that fail both dose-difference and DTA comparison criteria. Although the composite distribution identifies locations where the calculation fails the preselected criteria, no numerical quality measure is provided for display or analysis. A technique is developed to unify dose distribution comparisons using the acceptance criteria. The measure of acceptability is the multidimensional distance between the measurement and calculation points in both the dose and the physical distance, scaled as a fraction of the acceptance criteria. In a space composed of dose and spatial coordinates, the acceptance criteria form an ellipsoid surface, the major axis scales of which are determined by individual acceptance criteria and the center of which is located at the measurement point in question. When the calculated dose distribution surface passes through the ellipsoid, the calculation passes the acceptance test for the measurement point. The minimum radial distance between the measurement point and the calculation points (expressed as a surface in the dose–distance space) is termed the γ index. Regions where γ>1 correspond to locations where the calculation does not meet the acceptance criteria. The determination of γ throughout the measured dose distribution provides a presentation that quantitatively indicates the calculation accuracy. Examples of a 6 MV beam penumbra are used to illustrate the γ index.
2,584 citations
"Dosimetric Validation of Digital Me..." refers methods in this paper
...The change in detector response with SDD was studied by acquiring images at various SDD ranging from 100cm Asian Pacific Journal of Cancer Prevention, Vol 21 1661 DOI:10.31557/APJCP.2020.21.6.1659 SBRT QA with DMI for FFF Beams and compared against the predicted images using a method of Gamma Evaluation (Low et al., 1998) by defining Distance to Agreement (DTA) and Dose Difference (DD) values in the portal dosimetry workspace in Eclipse....
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...…Vol 21 1661 DOI:10.31557/APJCP.2020.21.6.1659 SBRT QA with DMI for FFF Beams and compared against the predicted images using a method of Gamma Evaluation (Low et al., 1998) by defining Distance to Agreement (DTA) and Dose Difference (DD) values in the portal dosimetry workspace in Eclipse....
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...and compared against the predicted images using a method of Gamma Evaluation (Low et al., 1998) by defining Distance to Agreement (DTA) and Dose Difference (DD) values in the portal dosimetry workspace in Eclipse....
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TL;DR: In this article, the authors used Stereotactic Body Radiation Therapy (SBRT) to deliver a potent ablative dose to deep-seated tumors in the lung, liver, spine, pancreas, kidney, and prostate.
Abstract: Introduction Stereotactic body radiation therapy (SBRT) uses advanced technology to deliver a potent ablative dose to deep-seated tumors in the lung, liver, spine, pancreas, kidney, and prostate. Methods SBRT involves constructing very compact high-dose volumes in and about the tumor. Tumor position must be accurately assessed throughout treatment, especially for tumors that move with respiration. Sophisticated image guidance and related treatment delivery technologies have developed to account for such motion and efficiently deliver high daily dose. All this serves to allow the delivery of ablative dose fractionation to the target capable of both disrupting tumor mitosis and cellular function. Results Prospective phase I dose-escalation trials have been carried out to reach potent tumoricidal dose levels capable of eradicating tumors with high likelihood. These studies indicate a clear dose-response relationship for tumor control with escalating dose of SBRT. Prospective phase II studies have been report...
401 citations
"Dosimetric Validation of Digital Me..." refers background in this paper
...Stereotactic Body Radiation Therapy (SBRT) has proved its efficacy in the treatment management of primary and metastatic liver tumors and leads to local control rates higher than 70% - 80% resulting in better survival and quality of life (Timmerman et al., 2007; Schefter et al., 2005; Scorsetti et al., 2014)....
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...…Body Radiation Therapy (SBRT) has proved its efficacy in the treatment management of primary and metastatic liver tumors and leads to local control rates higher than 70% - 80% resulting in better survival and quality of life (Timmerman et al., 2007; Schefter et al., 2005; Scorsetti et al., 2014)....
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TL;DR: Biologically potent doses of SBRT are well tolerated in patients with limited liver metastases, and form the basis for an ongoing Phase II S BRT study of 60 Gy over three fractions for liver metastased patients.
Abstract: Purpose: To determine the maximum tolerated dose (MTD) of stereotactic body radiation therapy (SBRT) for liver metastases. Methods and Materials: A multicenter Phase I clinical trial was conducted. Eligible patients had one to three liver metastases, tumor diameter Results: Eighteen patients were enrolled (10 male, 8 female): median age, 55 years (range, 26–83 years); most common primary site, colorectal (6 patients); median aggregate gross tumor volume, 18 ml (range, 3–98 ml). Four patients had multiple tumors. No patient experienced a DLT, and dose was escalated to 60 Gy/3F without reaching MTD. Conclusions: Biologically potent doses of SBRT are well tolerated in patients with limited liver metastases. Results of this study form the basis for an ongoing Phase II SBRT study of 60 Gy over three fractions for liver metastases.
389 citations
TL;DR: The basic dosimetric characteristics of an aSi portal imager investigated are reproducible and self-consistent, lending itself to quality assurance measurements, and the acquisition mode is accurate in integrating all PD over a wide range of monitor units, provided detector saturation is avoided.
370 citations
TL;DR: The EPID shows promise as a device for verification of IMRT, the major limitation currently being due to dead-time in frame acquisition.
Abstract: Dosimetric properties of an amorphous silicon electronic portal imaging device (EPID) for verification of dynamic intensity modulated radiation therapy (IMRT) delivery were investigated. The EPID was utilized with continuous frame-averaging during the beam delivery. Properties studied included effect of buildup, dose linearity, field size response, sampling of rapid multileaf collimator (MLC) leaf speeds, response to dose-rate fluctuations, memory effect, and reproducibility. The dependence of response on EPID calibration and a dead time in image frame acquisition occurring every 64 frames were measured. EPID measurements were also compared to ion chamber and film for open and wedged static fields and IMRT fields. The EPID was linear with dose and dose rate, and response to MLC leaf speeds up to 2.5 cm s(-1) was found to be linear. A field size dependent response of up to 5% relative to dmax ion-chamber measurement was found. Reproducibility was within 0.8% (1 standard deviation) for an IMRT delivery recorded at intervals over a period of one month. The dead time in frame acquisition resulted in errors in the EPID that increased with leaf speed and were over 20% for a 1 cm leaf gap moving at 1.0 cm s(-1). The EPID measurements were also found to depend on the input beam profile utilized for EPID flood-field calibration. The EPID shows promise as a device for verification of IMRT, the major limitation currently being due to dead-time in frame acquisition.
256 citations