Prabakar Sukumar

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.

A patient-specific quality assurance study on absolute dose verification using ionization chambers of different volumes in RapidArc treatments

The recalculation of 1 fraction from a patient treatment plan on a phantom and subsequent measurements have become the norms for measurement-based verification, which combines the quality assurance recommendations that deal with the treatment planning system and the beam delivery system. This type of evaluation has prompted attention to measurement equipment and techniques. Ionization chambers are considered the gold standard because of their precision, availability, and relative ease of use. This study evaluates and compares 5 different ionization chambers: phantom combinations for verification in routine patient-specific quality assurance of RapidArc treatments. Fifteen different RapidArc plans conforming to the clinical standards were selected for the study. Verification plans were then created for each treatment plan with different chamber-phantom combinations scanned by computed tomography. This includes Medtec intensity modulated radiation therapy (IMRT) phantom with micro-ionization chamber (0.007 cm(3)) and pinpoint chamber (0.015 cm(3)), PTW-Octavius phantom with semiflex chamber (0.125 cm(3)) and 2D array (0.125 cm(3)), and indigenously made Circular wax phantom with 0.6 cm(3) chamber. The measured isocenter absolute dose was compared with the treatment planning system (TPS) plan. The micro-ionization chamber shows more deviations when compared with semiflex and 0.6 cm(3) with a maximum variation of -4.76%, -1.49%, and 2.23% for micro-ionization, semiflex, and farmer chambers, respectively. The positive variations indicate that the chamber with larger volume overestimates. Farmer chamber shows higher deviation when compared with 0.125 cm(3). In general the deviation was found to be <1% with the semiflex and farmer chambers. A maximum variation of 2% was observed for the 0.007 cm(3) ionization chamber, except in a few cases. Pinpoint chamber underestimates the calculated isocenter dose by a maximum of 4.8%. Absolute dose measurements using the semiflex ionization chamber with intermediate volume (0.125 cm(3)) shows good agreement with the TPS calculated among the detectors used in this study. Positioning is very important when using smaller volume chambers because they are more sensitive to geometrical errors within the treatment fields. It is also suggested to average the dose over the sensitive volume for larger-volume chambers. The ionization chamber-phantom combinations used in this study can be used interchangeably for routine RapidArc patient-specific quality assurance with a satisfactory accuracy for clinical practice.

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.

Characterisation of a two-dimensional liquid-filled ion chamber detector array using flattened and unflattened beams for small fields, small MUs and high dose-rates

In this study, the PTW 1000SRS array with Octavius 4D phantom was characterised for FF and FFF beams. MU linearity, field size, dose rate, dose per pulse (DPP) response and dynamic conformal arc treatment accuracy of the 1000SRS array were assessed for 6MV, 6FFF and 10FFF beams using a Varian TrueBeam STx linac. The measurements were compared with a pinpoint IC, microdiamond IC and EBT3 Gafchromic film. Measured dose profiles and FWHMs were compared with film measurements. Verification of FFF volumetric modulated arc therapy (VMAT) clinical plans were assessed using gamma analysis with 3%/3 mm and 2%/2 mm tolerances (10% threshold). To assess the effect of cross calibration dose rate, clinical plans with different dose rates were delivered and analysed. Output factors agreed with film measurements to within 4.5% for fields between 0.5 and 1 cm and within 2.7% for field sizes between 1.5 and 10 cm and were highly correlated with the microdiamond IC detector. Field sizes measured with the 1000SRS array were within 0.5 mm of film measurements. A drop in response of up to 1.8%, 2.4% and 5.2% for 6MV, 6FFF and 10FFF beams respectively was observed with increasing nominal dose rate. With an increase in DPP, a drop of up to 1.7%, 2.4% and 4.2% was observed in 6MV, 6FFF and 10FFF respectively. The differences in dose following dynamic conformal arc deliveries were less than 1% (all energies) from calculated. Delivered VMAT plans showed an average pass percentage of 99.5(±0.8)% and 98.4(±3.4)% with 2%/2 mm criteria for 6FFF and 10FFF respectively. A drop to 97.7(±2.2)% and 88.4(±9.6)% were observed for 6FFF and 10FFF respectively when plans were delivered at the minimum dose rate and calibrated at the maximum dose rate. Calibration using a beam with the average dose rate of the plan may be an efficient method to overcome the dose rate effects observed by the 1000SRS array.

Study of dosimetric variation due to interfraction organ movement in High Dose Rate Interstital (MUPIT) brachytherapy for gynecologic malignancies

Study of dosimetric variation due to interfraction organ movement in High Dose Rate Interstital (MUPIT) brachytherapy for gynecologic malignancies Ten patients with cancer of uterine cervix who underwent interstitial brachytherapy using MUPIT templates were CT scanned (CT1) using which bladder, rectum and CTV were delineated. The treatment plan PCT1 was generated and optimized geometrically on the volume. CT scan (CT2) was repeated before the second fraction of the treatment CTV and critical organs were delineated. The plan (PCT2) was created by reproducing the Plan PCT1 in the CT2 images and compared with PCT1. Bladder, Rectum and CTV percentage volume variation ranges from +28.6% to -34.3%, 38.4% to -14.9% and 8.5% to -15.2% respectively. Maximum dose variation in bladder was +17.1%, in rectum was up to +410% and in CTV was -13.0%. The dose to these structures varies independently with no strong correlation with the volume variation. Hence it is suggested that repeat CT and re-planning is mandatory before second fraction execution.