A Gutiérrez

Treatment planning and delivery of IMRT using 6 and 18 MV photon beams without flattening filter

In light of the increasing use of intensity modulated radiation therapy (IMRT) in modern radiotherapy practice, the use of a flattening filter may no longer be necessary. Commissioning data have been measured for a Varian 23EX linear accelerator with 6 and 18 MV photon energies without a flattening filter. Measurements collected for the commissioning of the linac included percent depth dose curves and profiles for field sizes ranging from 2 x 2 to 40 x 40 cm(2) as defined by the jaws and multileaf collimator. Machine total scatter factors were measured and calculated. Measurements were used to model the unflattened beams with the Pinnacle(3) treatment planning system. IMRT plans for prostate, lung, brain and head and neck cancer cases were generated using the flattening filter and flattening filter-free beams. From our results, no difference in the quality of the treatment plans between the flat and unflattened photon beams was noted. There was however a significant decrease in the number of monitor units required for unflattened beam treatment plans due to the increase in linac output-approximately two times and four times higher for the 6 and 18 MV, respectively.

On the quantification of the dosimetric accuracy of collapsed cone convolution superposition (CCCS) algorithm for small lung volumes using IMRT

Specialized techniques that make use of small field dosimetry are common practice in todays clinics. These new techniques represent a big challenge to the treatment planning systems due to the lack of lateral electronic equilibrium. Because of this, the necessity of planning systems to overcome such difficulties and provide an accurate representation of the true value is of significant importance. Pinnacle3 is one such planning system. During the IMRT optimization process, Pinnacle3 treatment planning system allows the user to specify a minimum segment size which results in multiple beams composed of several subsets of different widths. In this study, the accuracy of the engine dose calculation, collapsed cone convolution superposition algorithm (CCCS) used by Pinnacle3, was quantified by Monte Carlo simulations, ionization chamber, and Kodak extended dose range film (EDR2) measurements for 11 SBRT lung patients. Lesions were < 3.0 cm in maximal diameter and <27.0 cm3 in volume. The Monte Carlo EGSnrc\BEAMnrc and EGS4\MCSIM were used in the comparison. The minimum segment size allowable during optimization had a direct impact on the number of monitor units calculated for each beam. Plans with the smallest minimum segment size (0.1 cm2 to 2.0 cm2) had the largest number of MUs. Although PTV coverage remained unaffected, the segment size did have an effect on the dose to the organs at risk. Pinnacle3‐calculated PTV mean doses were in agreement with Monte Carlo‐calculated mean doses to within 5.6% for all plans. On average, the mean dose difference between Monte Carlo and Pinnacle3 for all 88 plans was 1.38%. The largest discrepancy in maximum dose was 5.8%, and was noted for one of the plans using a minimum segment size of 0.1 cm2. For minimum dose to the PTV, a maximum discrepancy between Monte Carlo and Pinnacle3 was noted of 12.5% for a plan using a 6.0 cm2 minimum segment size. Agreement between point dose measurements and Pinnacle3‐calculated doses were on average within 0.7% in both phantoms. The profiles show a good agreement between Pinnacle3, Monte Carlo, and EDR2 film. The gamma index and the isodose lines support the result. PACS number: 87.56.bd

Evaluation of a commercially-available block for spatially fractionated radiation therapy

In this paper, we present the dosimetric characteristics of a commercially‐produced universal GRID block for spatially fractioned radiation therapy. The dosimetric properties of the GRID block were evaluated. Ionization chamber and film measurements using both Kodak EDR2 and Gafchromic EBT film were performed in a solid water phantom to determine the relative output of the GRID block as well as its spatial dosimetric characteristics. The surface dose under the block and at the openings was measured using ultra thin TLDs. After introducing the GRID block into the treatment planning system, a treatment plan was created using the GRID block and also by creating a GRID pattern using the multi‐leaf collimator. The percent depth doses measured with film showed that there is a shift of the dmax towards shallower depths for both energies (6 MV and 18 MV) under investigation. It was observed that the skin dose at the GRID openings was higher than the corresponding open field by a factor as high as 50% for both photon energies. The profiles showed the transmission under the block was in the order of 15–20% for 6 MV and 30% for 18 MV. The MUs calculated for a real patient using the block were about 80% less than the corresponding MUs for the same plan using the multileaf collimator to define the GRID. Based on this investigation, this brass GRID compensator is a viable alternative to other solid compensators or MLC‐based fields currently in use. Its ease of creation and use give it decided advantages. Its ability to be created once and used for multiple patients (by varying the collimation of the linear accelerator jaws) makes it attractive from a cost perspective. We believe this compensator can be put to clinical use, and will allow more centers to offer GRID therapy to their patients. PACS number: 87.53.Mr

MU-Tomo: Independent Dose Validation Software for Helical TomoTherapy

A software program, MU-Tomo, has been developed to perform an independent point dose calculation and compare it to the dose calculated from the TomoTherapy (TomoTherapy, Inc., Madison, WI) treatment planning system (TPS). Input parameters required for this software include: archived tomotherapy patient files, QA plan image coordinates, tomotherapy-calculated point dose and machine-specific dosimetric parameters such as the off-axis ratios (OARx and OARy), tissue phantom ratios (TPR) and output functions (Scp). The software was validated on four phantom models and fifty tomotherapy patient plans representing various anatomical sites. Our results indicate that MU-Tomo can perform in a few seconds an independent dose calculation accurately and provide a secondary check for a point dose validation of helical tomotherapy plans.

Evaluation of PTW Seven29 for tomotherapy patient-specific quality assurance and comparison with ScandiDos Delta(4).

For routine quality assurance of helical tomotherapy plans, an alternative method, as opposed to the TomoTherapy suggested cylindrical solid water phantom with film and ionization chamber, is proposed using the PTW Seven29 2D-ARRAY inserted in a dedicated octagonal phantom, called Octavius. First, the sensitivity of the array to pitch was studied by varying the pitch during planning to 0.287, 0.433, 1.0, and 2.0. For each pitch selected, the dependence on field size was investigated by generating plans with field widths (FWs) of 1.06 cm, 2.49 cm, and 5.02 cm, for a total of 12 plans. Secondly, a total of 15 patient QA plans were delivered using helical tomotherapy with the Delta4 and Seven29/Octavius for comparison. Using the clinical gamma criteria, 3% and 3 mm, all FW and pitch plans had a passing percentage of >90%. For patient QA plans, the average gamma pass percentage was 97.0% (94.4–99.8%) for the Delta4 and 97.6% (92.5-100.0%) for the Seven29/Octavius. Both the Seven29/Octavius and Delta4 performed to a high standard of measurement accuracy and had a 90% or greater gamma percent for all plans and were considered clinically acceptable.

Monte Carlo modeling of a Novalis TX Varian 6 MV with HD-120 multileaf collimator

A Monte Carlo model of the Novalis Tx linear accelerator equipped with high‐definition multileaf collimator (HD‐120 HD‐MLC) was commissioned using ionization chamber measurements in water. All measurements in water were performed using a liquid filled ionization chamber. Film measurements were made using EDR2 film in solid water. Open rectangular fields defined by the jaws or the HD‐MLC were used for comparison against measurements. Furthermore, inter‐ and intraleaf leakage calculated by the Monte Carlo model was compared against film measurements. The statistical uncertainty of the Monte Carlo calculations was less than 1% for all simulations. Results for all regular field sizes show an excellent agreement with commissioning data (percent depth‐dose curves and profiles), well within 1% of difference in the relative dose and 1 mm distance to agreement. The computed leakage through HD‐MLCs shows good agreement with film measurements. The Monte Carlo model developed in this study accurately represents the new Novalis Tx Varian linac with HD‐MLC and can be used for reliable patient dose calculations. PACS number: 87.10.Rt

Clinical evaluation of an immbolization system for stereotactic body radiotherapy using helical tomotherapy

In this study, a clinical evaluation of the Body Pro-Lok™ System combined with the TomoTherapy megavoltage computed tomography (MVCT) was performed for lung and liver stereotactic body radiotherapy (SBRT) to reduce interfractional setup uncertainty. Twenty patients treated with 3-5 fractions of SBRT were analyzed retrospectively. The Body Pro-Lok™ system was used in both CT simulation and during patient treatment setup. Patients were immobilized with a vacuum cushion placed posteriorly over the thoracic region, an abdominal compression plate, and a knee and foot sponge. Pretreatment MVCT scans of the TomoTherapy HI-ART II unit were fused with the planning kVCT before delivery of each fraction to determine the interfractional setup error. A total of 84 shifts were analyzed to assess the interfractional setup accuracy. Results showed that the mean interfractional setup errors and standard deviations were -0.9 ± 3.1 mm, 1.2 ± 5.5 mm, and 6.5 ± 2.6 mm for lateral (IEC-X), longitudinal (IEC-Y), and vertical (IEC-Z) variations, respectively. The maximum motion was 17.1 mm in the longitudinal direction. When all 3 translational coordinates were analyzed, a mean composite displacement vector of 8.2 ± 2.0 mm (range 4.1-11.7 mm) was obtained for all patients. Based on the findings, image-guided SBRT using the Body Pro-Lok™ system in conjunction with the MVCT of TomoTherapy is capable of minimizing interfractional setup error and improving treatment accuracy.

Patient Specific Pre-Treatment QA Verification Using an EPID Approach

A software program [MU-EPID], has been developed to perform patient specific pre-treatment quality assurance (QA) verification for intensity modulated radiation therapy (IMRT) using fluence maps measured with an electronic portal imaging device (EPID). The software converts the EPID acquired images of each IMRT beam, to fluence maps that are equivalent to those calculated by the treatment planning system (TPS). The software has the capability to process Varian, Elekta and Siemens EPID DICOM images. In the present investigation, several IMRT plans for different treatment sites were used to validate the software using the Varian a-Si 1000 EPID with the Pinnacle TPS. A total of 20 IMRT plans of different treatment sites were analyzed. Isodose distributions, dose profiles, dose volume histograms (DVHs) and gamma analysis comparisons were performed to evaluate the accuracy of our method. A gamma index analysis of the isocenter coronal plane was done for each plan and showed an average of 97.44% of gamma passing rate using a 3% and 3 mm gamma criterion. Isodose, DVH and dose profile comparisons were conducted between the original calculated plan and the measured reconstructed plan from the EPID images processed through the MU-EPID software. The results suggest that MU-EPID can be used clinically for patient specific IMRT QA, providing a comprehensive 3D dosimetric evaluation through DVH comparison as well as an option for a 2D gamma analysis.

Dosimetric effect of photon beam energy on volumetric modulated arc therapy treatment plan quality due to body habitus in advanced prostate cancer

PURPOSE The purpose of this study was to dosimetrically compare 6- and 10-MV photon beam energies in high-risk prostate cancer patients of various body habitus using a volumetric modulated arc therapy (VMAT) radiation delivery technique. The objectives of the study were to evaluate whether dosimetric differences exist and to investigate whether differences are dependent on patient body habitus. METHODS AND MATERIALS Forty patients with various body habitus who had previously received treatment to the prostate and pelvic lymph nodes with VMAT techniques were chosen. Patients were planned in the Pinnacle(3) treatment planning system with double or triple SmartArc plans with 6- and 10-MV photon energies. All patients were optimized with the same planning objectives and normalized such that 95% of the planning target volume (PTV) received the prescription dose. Patients were evaluated for PTV and organ at risk (OAR) parameters for the bladder, rectum, small bowel, penile bulb, and sigmoid colon. Metrics used for comparison were D2%, D98%, homogeneity, conformity, and dose falloff for the PTV and D(2%), D(mean), V(80%), V(60%), and V(40%) for OARs. Statistical differences were evaluated with a paired-sample Wilcoxon signed rank test with a significance level of .05. RESULTS For the PTV, there were no statistically significant differences in D(mean), D(2cc), conformation number, and homogeneity index values, but the dose falloff parameters, R50 and R25, showed a median improvement of 6.7% (P<.01) and 6.2% (P<.01), respectively, with 10 MV. A correlation between patient anterior-posterior distance (d(AP)) and percentage reduction in R50 of 0.436% per centimeter (P<.01) was determined. For OARs, statistically significant reductions in dose metrics were found in the small bowel and bladder, but increases in the D(2cc) of 3.5% in the penile bulb (P<.01) and 0.2% in the rectum (P=.02) were shown with 10 MV. The use of 10 MV also demonstrated a statistically significant reduction in the total number of monitor units of 15.9% (P<.01) compared with 6 MV. CONCLUSIONS The study showed that 10 MV provides a faster dose falloff than 6 MV for patients whose prostate and pelvic lymph nodes are treated using a VMAT technique irrespective of body habitus; however, the improvement in dose falloff is dependent on body habitus and increases as the patient body habitus increases.

An evaluation of the stability of image-quality parameters of Varian on-board imaging (OBI) and EPID imaging systems

Quality assurance (QA) of the image quality for image-guided localization systems is crucial to ensure accurate visualization and localization of regions of interest within the patient. In this study, the temporal stability of selected image parameters was assessed and evaluated for kV CBCT mode, planar radiographic kV, and MV modes. The motivation of the study was to better characterize the temporal variability in specific image-quality parameters. The CATPHAN, QckV-1, and QC-3 phantoms were used to evaluate the image-quality parameters of the imaging systems on a Varian Novalis Tx linear accelerator. The planar radiographic images were analyzed in PIPSpro with high-contrast spatial resolution (f30, f40,f50 lp/mm) being recorded. For OBI kV CBCT, high-quality head full-fan acquisition and pelvis half-fan acquisition modes were evaluated for uniformity, noise, spatial resolution, HU constancy, and geometric distortion. Dose and X-ray energy for the OBI were recorded using the Unfors RaySafe Xi system with the R/F High Detector for kV planar radiographic and the CT detector for kV CBCT. Dose for the MV EPID was recorded using a PTW975 Semiflex ion chamber, PTW UNIDOS electrometer, and CNMC Plastic Water. For each image-quality parameter, values were normalized to the mean, and the normalized standard deviations were recorded to evaluate the parameters temporal variability. For planar radiographic modes, the normalized standard deviations of the spatial resolution (f30, f40, & f50) were 0.015, 0.008, 0.004 lp/mm and 0.006, 0.009, 0.018 lp/mm for the kV and MV, respectively. The normalized standard deviation of dose for kV and MV were 0.010 mGy and 0.005mGy, respectively. The standard deviations for full- and half-fan kV CBCT modes were averaged together. The following normalized standard deviations for each kV CBCT parameter were: 0.075 HU (uniformity), 0.071 HU (noise), 0.006mm (AP-geometric distortion), 0.005 mm (LAT-geometric distortion), 0.058mm (slice thickness), 0.124 (f50), 0.031 (HU constancy - Lung), 0.063 (HU constancy- Water), 0.020 (HU constancy - Bone), 0.006 mGy (Dose - Center), 0.004 mGy (Dose -Periphery). Using control chart analysis, institutional QA tolerances were reported as warning and action thresholds based on 1σ and 2σ thresholds. A study was performed to characterize the stability of image-quality parameters recommended by AAPM Task Group-142 for the Varian OBI and EPID imaging systems. Both imaging systems show consistent imaging and dosimetric properties over the evaluated time frame.Quality assurance (QA) of the image quality for image‐guided localization systems is crucial to ensure accurate visualization and localization of regions of interest within the patient. In this study, the temporal stability of selected image parameters was assessed and evaluated for kV CBCT mode, planar radiographic k V, and MV modes. The motivation of the study was to better characterize the temporal variability in specific image‐quality parameters. The CATPHAN, QckV‐1, and QC‐3 phantoms were used to evaluate the image‐quality parameters of the imaging systems on a Varian Novalis Tx linear accelerator. The planar radiographic images were analyzed in PIPSpro with high‐contrast spatial resolution (f30,f40,f50lp/mm) being recorded. For OBI kV CBCT, high‐quality head full‐fan acquisition and pelvis half‐fan acquisition modes were evaluated for uniformity, noise, spatial resolution, HU constancy, and geometric distortion. Dose and X‐ray energy for the OBI were recorded using the Unfors RaySafe Xi system with the R/F High Detector for kV planar radiographic and the CT detector for kV CBCT. Dose for the MV EPID was recorded using a PTW975 Semiflex ion chamber, PTW UNIDOS electrometer, and CNMC Plastic Water. For each image‐quality parameter, values were normalized to the mean, and the normalized standard deviations were recorded to evaluate the parameters temporal variability. For planar radiographic modes, the normalized standard deviations of the spatial resolution (f30,f40,& f50) were 0.015, 0.008, 0.004 lp/mm and 0.006, 0.009, 0.018 lp/mm for the kV and MV, respectively. The normalized standard deviation of dose for kV and MV were 0.010 mGy and 0.005 mGy, respectively. The standard deviations for full‐and half‐fan kV CBCT modes were averaged together. The following normalized standard deviations for each kV CBCT parameter were: 0.075 HU (uniformity), 0.071 HU (noise), 0.006 mm (AP‐geometric distortion), 0.005 mm (LAT‐geometric distortion), 0.058 mm (slice thickness), 0.124 (f50), 0.031 (HU constancy – Lung), 0.063 (HU constancy – Water), 0.020 (HU constancy – Bone), 0.006 mGy (Dose – Center), 0.004 mGy (Dose –Periphery). Using control chart analysis, institutional QA tolerances were reported as warning and action thresholds based on 1σ and 2σ thresholds. A study was performed to characterize the stability of image‐quality parameters recommended by AAPM Task Group‐142 for the Varian OBI and EPID imaging systems. Both imaging systems show consistent imaging and dosimetric properties over the evaluated time frame. PACS number: 87.10.‐e