Vivekanandan Nagarajan

Characterization of responses of 2d array seven29 detector and its combined use with octavius phantom for the patient-specific quality assurance in rapidarc treatment delivery.

A commercial 2D array seven29 detector has been characterized and its performance has been evaluated. 2D array ionization chamber equipped with 729 ionization chambers uniformly arranged in a 27 × 27 matrix with an active area of 27 × 27 cm² was used for the study. An octagon-shaped phantom (Octavius Phantom) with a central cavity is used to insert the 2D ion chamber array. All measurements were done with a linear accelerator. The detector dose linearity, reproducibility, output factors, dose rate, source to surface distance (SSD), and directional dependency has been studied. The performance of the 2D array, when measuring clinical dose maps, was also investigated. For pretreatment quality assurance, 10 different RapidArc plans conforming to the clinical standards were selected. The 2D array demonstrates an excellent short-term output reproducibility. The long-term reproducibility was found to be within ±1% over a period of 5 months. Output factor measurements for the central chamber of the array showed no considerable deviation from ion chamber measurements. We found that the 2D array exhibits directional dependency for static fields. Measurement of beam profiles and wedge-modulated fields with the 2D array matched very well with the ion chamber measurements in the water phantom. The study shows that 2D array seven29 is a reliable and accurate dosimeter and a useful tool for quality assurance. The combination of the 2D array with the Octavius phantom proved to be a fast and reliable method for pretreatment verification of rotational treatments.

A study on dosimetric properties of electronic portal imaging device and its use as a quality assurance tool in Volumetric Modulated Arc Therapy

AIM In this study, the dosimetric properties of the electronic portal imaging device were examined and the quality assurance testing of Volumetric Modulated Arc Therapy was performed. BACKGROUND RapidArc involves the variable dose rate, leaf speed and the gantry rotation. The imager was studied for the effects like dose, dose rate, field size, leaf speed and sag during gantry rotation. MATERIALS AND METHODS A Varian RapidArc machine equipped with 120 multileaf collimator and amorphous silicon detector was used for the study. The characteristics that are variable in RapidArc treatment were studied for the portal imager. The accuracy of a dynamic multileaf collimator position at different gantry angles and during gantry rotation was examined using the picket fence test. The control of the dose rate and gantry speed was verified using a test field irradiating seven strips of the same dose with different dose rate and gantry speeds. The control over leaf speed during arc was verified by irradiating four strips of different leaf speeds with the same dose in each strip. To verify the results, the RapidArc test procedure was compared with the X-Omat film and verified for a period of 6 weeks using EPID. RESULTS The effect of gantry rotation on leaf accuracy was minimal. The dose in segments showed good agreement with mean deviation of 0.8% for dose rate control and 1.09% for leaf speed control over different gantry speeds. CONCLUSION The results provided a precise control of gantry speed, dose rate and leaf speeds during RapidArc delivery and were consistent over 6 weeks.

In-house spread sheet based monitor unit verification program for volumetric modulated arc therapy

Independent monitor unit verification calculation (MUVC) has been recommended by several authors for intensity modulated radiotherapy (IMRT) as a patient specific quality assurance tool. Aim of the present work is to develop an in-house excel spread sheet based MUVC program for volumetric modulated arc therapy (VMAT) using Clarksons integration technique. Total scatter factor (S(c,p)) and tissue maximum ratio (TMR) for circular fields obtained from Treatment planning system (TPS) were used for the calculation. Multileaf collimator (MLC) interleaf leakage, MLC round edge transmission and tongue and groove effect were accounted. MUVC calculation was performed for 58 patients both for patient anatomy and for homogenous cylindrical phantom. Radiological path lengths were used as water equivalent depths (WED) for calculations using patient anatomy. Monitor unit (MU) discrepancies between -2.60% and 0.28% with mean deviation of -0.92% ± 0.75% were obtained for homogenous cylindrical phantom calculations. MUVC for patient anatomy resulted in large variations between -19.02% and 0.67% for 14 plans where isocenter was at a region below -350 HU. But For 44 plans where the isocenter was at a region above -350 HU, variations between -3.44% and 0.48% were obtained with mean deviation of -1.73% ± 1.12%. For VMAT patient specific quality assurance, the independent MUVC algorithm can be used as an easy and quick auxiliary to measurement based verification for plans with isocenter at a region above -350 HU.

A study on correlation between 2D and 3D gamma evaluation metrics in patient-specific quality assurance for VMAT

In this study, we investigated the correlation between 2-dimensional (2D) and 3D gamma analysis using the new PTW OCTAVIUS 4D system for various parameters. For this study, we selected 150 clinically approved volumetric-modulated arc therapy (VMAT) plans of head and neck (50), thoracic (esophagus) (50), and pelvic (cervix) (50) sites. Individual verification plans were created and delivered to the OCTAVIUS 4D phantom. Measured and calculated dose distributions were compared using the 2D and 3D gamma analysis by global (maximum), local and selected (isocenter) dose methods. The average gamma passing rate for 2D global gamma analysis in coronal and sagittal plane was 94.81% ± 2.12% and 95.19% ± 1.76%, respectively, for commonly used 3-mm/3% criteria with 10% low-dose threshold. Correspondingly, for the same criteria, the average gamma passing rate for 3D planar global gamma analysis was 95.90% ± 1.57% and 95.61% ± 1.65%. The volumetric 3D gamma passing rate for 3-mm/3% (10% low-dose threshold) global gamma was 96.49% ± 1.49%. Applying stringent gamma criteria resulted in higher differences between 2D planar and 3D planar gamma analysis across all the global, local, and selected dose gamma evaluation methods. The average gamma passing rate for volumetric 3D gamma analysis was 1.49%, 1.36%, and 2.16% higher when compared with 2D planar analyses (coronal and sagittal combined average) for 3mm/3% global, local, and selected dose gamma analysis, respectively. On the basis of the wide range of analysis and correlation study, we conclude that there is no assured correlation or notable pattern that could provide relation between planar 2D and volumetric 3D gamma analysis. Owing to higher passing rates, higher action limits can be set while performing 3D quality assurance. Site-wise action limits may be considered for patient-specific QA in VMAT.

Development of hybrid composite radar wave absorbing structure for stealth applications

The ideally invisible stealth radomes are usually sandwiched constructions composed of E-glass/epoxy composite, polyvinyl chloride foam and frequency selective surfaces (FSS). Nylon 6/6 and balsa wood are well known for their low dielectric properties. In this work the electromagnetic (EM) wave transmission characteristics of the existing stealth radomes were improved by employing nylon 6/6 fibre and balsa wood, along with E-glass/epoxy composite without compromising the mechanical properties. The free space measurement technique was performed to measure the EM wave transmission characteristics in the X-band frequency range (8.2–12.4 GHz) for a specific FSS. The flexural strength of the sandwiched constructions were investigated with three-point bending test.

Comparison between In-house developed and Diamond commercial software for patient specific independent monitor unit calculation and verification with heterogeneity corrections.

The study was aimed to compare two different monitor unit (MU) or dose verification software in volumetric modulated arc therapy (VMAT) using modified Clarksons integration technique for 6 MV photons beams. In-house Excel Spreadsheet based monitor unit verification calculation (MUVC) program and PTWs DIAMOND secondary check software (SCS), version-6 were used as a secondary check to verify the monitor unit (MU) or dose calculated by treatment planning system (TPS). In this study 180 patients were grouped into 61 head and neck, 39 thorax and 80 pelvic sites. Verification plans are created using PTW OCTAVIUS-4D phantom and also measured using 729 detector chamber and array with isocentre as the suitable point of measurement for each field. In the analysis of 154 clinically approved VMAT plans with isocentre at a region above -350 HU, using heterogeneity corrections, In-house Spreadsheet based MUVC program and Diamond SCS showed good agreement TPS. The overall percentage average deviations for all sites were (-0.93% + 1.59%) and (1.37% + 2.72%) for In-house Excel Spreadsheet based MUVC program and Diamond SCS respectively. For 26 clinically approved VMAT plans with isocentre at a region below -350 HU showed higher variations for both In-house Spreadsheet based MUVC program and Diamond SCS. It can be concluded that for patient specific quality assurance (QA), the In-house Excel Spreadsheet based MUVC program and Diamond SCS can be used as a simple and fast accompanying to measurement based verification for plans with isocentre at a region above -350 HU.