Study of the dosimetric differences between (192)Ir and (60)Co sources of high dose rate brachytherapy for breast interstitial implant.

AIM: The study intends to compare (192)Ir source against the (60)Co source for interstitial breast metal implant in high dose rate brachytherapy. BACKGROUND: Few studies have been reported to compare (60)Co and (192)Ir on HDR brachytherapy in gynaecology and prostate cancer and very few with reference to breast cancer. MATERIALS AND METHODS: Twenty patients who had undergone interstitial template guided breast implant were treated in HDR (192)Ir brachytherapy unit. Plans were generated substituting (60)Co source without changing the dwell positions and optimization. Cumulative dose volume histograms were compared. RESULTS: The reference isodose line enclosing CTV (CTVref) and the 2.34% difference seen in the volume enclosed by the reference isodose line (V ref) between the two isotopes show small but statistically significant difference (p < 0.05). In DHI, no difference was observed in the relative dose between the two sources (p = 0.823). The over dose volume index showed 11% difference. The conformity index showed 2.32% difference compared to (192)Ir (p < 0.05). D mean (%) and D max (%) for the heart, ipsilateral lung, ipsilateral ribs, skin presented very small difference. V 5% and V 10% of the heart shows 25% and 32% difference in dose. D 2cc (%) and D 0.1cc (%) for the contralateral breast, contralateral lung and D 2cc (%) of the skin displayed significant difference (p < 0.05). However, D 0.1cc (%) of the skin indicated no noteworthy difference with p = 0.343. CONCLUSION: Based on the 3D dosimetric analysis of patient plans considered in this study, most of the DVH parameters showed statistically significant differences which can be reduced by treatment planning optimization techniques. (60)Co isotope can be used as a viable alternative because of its long half-life, logistic advantages in procurement, infrequent need of source replacement and disposal of used source.

Validation and clinical implementation of commercial secondary check software with heterogeneity corrections.

AIM: To validate and implement PTW diamond secondary check software (SCS) in a routine clinical use. BACKGROUND: The secondary independent monitor unit or dose calculation verifications have led to a significant increase in the workflow associated with QA treatments. Modelling, validation and commissioning are necessary steps thereby making it a useful tool for QA. MATERIALS AND METHODS: PTW Diamond SCS is capable of calculating VMAT fields, based on modified Clarkson integration, accounting for multi-leaf collimators (MLC) transmission and measured collimator scatter factors. Validation for heterogeneity corrections is made using circular phantom with inserts of various density materials. 150 VMAT plans were compared using (i) plans calculated in homogeneous cylindrical phantom and (ii) VMAT plans calculated with heterogeneity corrections using electron density values for each organ. RESULTS: Diamond SCS calculated dose for homogeneous cylindrical phantom resulted in average deviation of (0.1 ± 2.14%) with Eclipse TPS calculated dose and (-2.0 ± 1.66%) with absolute measured dose. PTW's OCTAVIUS-4D phantom with 729 ion chamber detector array measurements agreed well with Eclipse TPS calculated dose showing an average deviation of (-1.69 ± 1.56%). Diamond SCS dose calculations were performed with heterogeneity corrections for 124 VMAT plans with isocentre at a region above -350 HU. The overall MU variations between Diamond SCS and TPS Acuros-XB algorithms were within ±5%. CONCLUSION: Hence, the Diamond SCS can be used as an additional tool along with phantom measurements for patient specific quality assurance of VMAT plans with heterogeneity corrections having isocentre at a region above -350 HU.

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 Clarkson's integration technique for 6 MV photons beams. In-house Excel Spreadsheet based monitor unit verification calculation (MUVC) program and PTW's 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.

Dosimetric comparison of Acuros™ BV with AAPM TG43 dose calculation formalism in breast interstitial high-dose-rate brachytherapy with the use of metal catheters.

PURPOSE: Radiotherapy for breast cancer includes different techniques and methods. The purpose of this study is to compare dosimetric calculations using TG-43 dose formalism and Varian Acuros™ BV (GBBS) dose calculation algorithm for interstitial implant of breast using metal catheters in high-dose-rate (HDR) brachytherapy, using (192)Ir. MATERIAL AND METHODS: Twenty patients who were considered for breast conservative surgery (BCS), underwent lumpectomy and axillary dissection. These patients received perioperative interstitial HDR brachytherapy as upfront boost using rigid metal implants. Whole breast irradiation was delivered TG-43 after a gap of two weeks. Standard brachytherapy dose calculation was done by dosimetry. This does not take into account tissue heterogeneity, attenuation and scatter in the metal applicator, and effects of patient boundary. Acuros™ BV is a Grid Based Boltzmann Solver code (GBBS), which takes into consideration all the above, was used to compute dosimetry and the two systems were compared. RESULTS: Comparison of GBBS and TG-43 formalism on interstitial metal catheters shows difference in dose prescribed to CTV and other OARs. While the estimated dose to CTV was only marginally different with the two systems, there is a significant difference in estimated doses of starting from 4 to 53% in the mean value of all parameters analyzed. CONCLUSIONS: TG-43 algorithm seems to significantly overestimate the dose to various volumes of interest; GBBS based dose calculation algorithm has impact on CTV, heart, ipsilateral lung, heart, contralateral breast, skin, and ribs of the ipsilateral breast side; the prescription changes occurred due to effect of metal catheters, inhomogeneities, and scatter conditions.

A study on the correlation between plan complexity and gamma index analysis in patient specific quality assurance of volumetric modulated arc therapy.

AIM: To evaluate the new Octavius 4D system for patient specific quality assurance and to study the correlation between plan complexity and gamma index analysis in patient specific quality assurance of VMAT using the Octavius 4D system. BACKGROUND: McNiven (2010) proposed a study to evaluate the utility of a complexity metric, the Modulation Complexity Score, to evaluate the relationship of the metric with deliverability in IMRT. MATERIALS AND METHODS: Evaluation of the Octavius 4D system was carried out by gamma evaluation of user defined MLC created patterns and AAPM TG 119 benchmark plans. The relationship between plan complexity expressed as Modulation Complexity Score (MCS) and the gamma index analysis was established by a planar and volumetric gamma analysis of 106 clinically approved VMAT patient plans of different sites. RESULTS: Average volumetric 3D global gamma evaluation (3 mm/3%) results for the evaluation plans was 97.41% for 6 MV X-rays and 98.30% for 15 MV X-rays. Average MCS values for the head and neck, pelvic and thoracic plans were 0.2224, 0.3615 and 0.1874. Average volumetric 3D global gamma analysis (3 mm/3%) results for the head and neck, pelvic and thoracic VMAT plans were 95.45%, 97.51% and 96.98%, respectively. Out of 90 correlation analyses between the MCS and gamma passing rate, only 3 had the r value greater than 0.5. CONCLUSIONS: The Octavius 4D system is a suitable device for patient specific pretreatment QA. Global and local gamma analysis results showed a weak correlation with the MCS.

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.

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 Clarkson's 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.

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.

Exit fluence analysis using portal dosimetry in volumetric modulated arc therapy.

AIM: In measuring exit fluences, there are several sources of deviations which include the changes in the entrance fluence, changes in the detector response and patient orientation or geometry. The purpose of this work is to quantify these sources of errors. BACKGROUND: The use of the volumetric modulated arc therapy treatment with the help of image guidance in radiotherapy results in high accuracy of delivering complex dose distributions while sparing critical organs. The transit dosimetry has the potential of Verifying dose delivery by the linac, Multileaf collimator positional accuracy and the calculation of dose to a patient or phantom. MATERIALS AND METHODS: The quantification of errors caused by a machine delivery is done by comparing static and arc picket fence test for 30 days. A RapidArc plan, created for the pelvis site was delivered without and with Rando phantom and exit portal images were acquired. The day to day dose variation were analysed by comparing the daily exit dose images during the course of treatment. The gamma criterion used for analysis is 3% dose difference and 3 mm distance to agreement with a threshold of 10% of maximum dose. RESULTS: The maximum standard deviation for the static and arc picket fence test fields were 0.19 CU and 1.3 CU, respectively. The delivery of the RapidArc plans without a phantom shows the maximum standard deviation of 1.85 CU and the maximum gamma value of 0.59. The maximum gamma value for the RapidArc plan delivered with the phantom was found to be 1.2. The largest observed fluence deviation during the delivery to patient was 5.7% and the maximum standard deviation was 4.1 CU. CONCLUSION: It is found from this study that the variation due to patient anatomy and interfraction organ motion is significant.

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 phantom study on the effects of target motion in non-gated kV-CBCT imaging.

Kilo-voltage cone beam computed tomography (kV-CBCT) integrated with a linac can produce online volumetric and anatomical images for patient set-up and dosimetric analysis in adaptive radiotherapy. However CBCT is prone to motion artifacts. This study investigates the impact of target motion in CBCT imaging. To simulate respiratory movement, a dynamic phantom was moved in three-dimensions with a period of 4 s and two different amplitudes (PA1 and PA2). The targets of well defined geometries were made using wax. A reference image of the static target was achieved with fan beam CT. Using CBCT, the targets in static and dynamic modes were imaged under full-fan beam conditions. The length of average HU spread was reduced in range from 19.35 to 44.44% along the cranio-caudal direction of targets. The percentage volume loss of dynamic targets imaged using CBCT (for Hounsfield Units with window width -500 to 0) ranged from 14.35 to 30.95% for PA1 and 21.29 to 43.80% for PA2 in comparison with static targets imaged with fan beam CT. A significant loss of volumetric information may result for non-gated CBCT imaging of moving targets and may result in a systematic error in re-contouring when CBCT images are used for radiotherapy re-planning.

The effects of target motion in kV-CBCT imaging.

BACKGROUND: To study the impact of target motion in kV-CBCT imaging. MATERIAL/METHODS: To simulate the respiratory movement, dynamic phantom was programmed to move in three-dimension with a period of four seconds and of two different amplitudes (PA1 and PA2). The targets of well defined geometries (cylinder, sphere, solid triangle, U-shaped and dumbbell) were made using wax. The static targets were CT imaged (reference image). Using CBCT, the targets in static and dynamic modes were imaged under full-fan beam. The line profiles along cranio-caudal direction, influence of target's initial moving phase and volume estimation using auto-contouring tool were used to analyze the effects of target motion on CBCT imaging. RESULTS: Comparing the line profiles of targets in CBCT with CT, the length of average HU spread was reduced by 42.54±1.85%, except the cylindrical target which is by 19.35% for PA1. The percentage difference in reconstructed volume of static targets imaged using CBCT and CT (HU WW -500 to 0) ranges from -1.32% to -5.94%. The volume losses for targets imaged in dynamic mode PA1 ranges from 14.35% to 30.95% and for PA2 it was 21.29% to 43.80%. The solid triangle and cylindrical targets suffered the maximum and minimum volume losses respectively. CONCLUSIONS: Non-gated CBCT imaging of the moving targets encounters significant loss of volumetric information, due to scatter artifacts. These may result in a systematic error in re-contouring when CBCT images are used for the re-planning work.