Importance of Beam-Matching between TrueBeam STx and Novalis Tx in Pre-Treatment Quality Assurance Using Portal Dosimetry.

Flexibility and efficiency in a radiotherapy department with different linear accelerators (linacs) can be improved if they are dosimetrically equivalent, and there is no need of plan or patient-specific quality assurance (PSQA) modification. From 2012 to 2017, our institution purchased three Novalis Tx and one TrueBeam STx beam-matched accelerators with the same high-resolution multileaf collimator (MLC). They are matched taking as reference dosimetric data from Novalis Tx SN-5479. We showed the importance of beam-matched dosimetric units by the use of electronic portal image device (EPID) and Delta4 PSQA. It was able to treat patients on a different machine than the machine used for PSQA. Depth dose, beam profiles, output factors, dosimetric leaf gap, and MLC transmission were compared for all energies and linacs. PSQA in all linacs for 30 volumetric-modulated arc therapy plans was also compared. Prostate, breast, and head-and-neck cases were selected to consider low, middle, and high plan complexity, respectively. The comparisons were evaluated using EPID and Delta4 phantom. Dosimetric differences between the three Novalis Tx and TrueBeam STx in all energies were lower than 1%. The only significant difference was observed in Novalis EPID in middle complexity when the criterion was tighter in distance. This result could be related with the nonsymmetric dose delivery of semi arcs. In all other cases, there were no differences in two different EPID evaluations. However, TrueBeam EPID values were slightly higher than Novalis EPID values. This could be associated with the high-resolution novel diode detector TrueBeam EPID. The dosimetric data indicated that the Novalis Tx and TrueBeam STx are equivalent. PSQA using EPID and Delta4 phantom showed that there are no dosimetric differences in any of the linacs. These results revealed the flexibility performance in PSQA by beam-matching.

Dose-response of Fricke- and PAGAT-dosimetry gels in kilovoltage and megavoltage photon beams: Impact of LET on sensitivity.

PURPOSE: Dosimetry of ionizing radiation quantifies the energy deposited by an incident beam to the medium. This study presents the relative response of two types of gel dosimeters describing their differences by estimating radiation chemical yields produced in water radiolysis. METHODS: Two types of gel dosimeter were used, namely an acid ferrous ion solution infused with xylenol orange known as Fricke gel and a polymer gel based on acrylamide and N,N'-methylenebis(acrylamide) known as PAGAT. Samples were irradiated using two photon beam energies, one from a conventional X-ray tube operated at 44 kV and the other one from a LINAC operated at 6 MV. The dosimeters were analyzed by optical absorbance and magnetic resonance imaging. Additionally, the linear energy transfer of each beam was calculated using Monte Carlo simulations for further estimation of the radiation chemical yields produced during water radiolysis. RESULTS: Obtained results for both gel dosimeters indicate that their response at 44 kV and 6 MV are different, regardless of the read-out technique. On average, the sensitivity at 44 kV was found to be 65 % of the response at 6 MV. The calculated radiation chemical yields are in agreement with the observed experimental results. CONCLUSIONS: The main reason for the difference in the response of the dosimeters may be related to the linear energy transfer of each photon beam, which varies the production of primary chemical species during water radiolysis.

Knowledge-Based Volumetric Modulated Arc Therapy Treatment Planning for Breast Cancer.

Purpose: To create and to validate knowledge-based volumetric modulated arc therapy (VMAT) models for breast cancer treatments without lymph node irradiation. Materials and Methods: One hundred VMAT-based breast plans (manual plans [MP]) were selected to create two knowledge-based VMAT models (breast left and breast right) using RapidPlan™. The plans were generated on Eclipse v15.5 (Varian Medical Systems, Palo Alto, CA) with 6 MV of a Novalis Tx equipped with a high-resolution multileaf collimator. The models were verified based on goodness-of-fit statistics using the coefficients of determination (R 2) and Chi-square (χ2), and the goodness-of-estimation statistics through the mean square error (MSE). Geometrical and dosimetrical constraints were identified and removed from the RP models using statistical evaluation metrics and plots. For validation, 20 plans that integrate the models and 20 plans that do not were reoptimized with RP (closed and opened validation). Dosimetrical parameters of interest were used to compare MP versus RP plans for the Heart, Homolateral_Lung, Contralateral_Lung, and Contralateral_Breast. Optimization planning time and user independency were also analyzed. Results: The most unfavorable results of R2 in both models for the organs at risk were as follows: for Contralateral_Lung 0.51 in RP right breast (RP_RB) and for Heart 0.60 in RP left breast (RP_LB). The most unfavorable results of χ2 test were: for Contralateral_Breast 1.02 in RP_RB and for Heart 1.03 in RP_LB. These goodness-of-fit results show that no overfitting occurred in either of the models. There were no unfavorable results of mean square error (MSE, all < 0.05) in any of the two models. These goodness-of-estimation results show that the models have good estimation power. For closed validation, significant differences were found in RP_RB for Homolateral_Lung (all P ≤ 0.001), and in the RP_LB differences were found for the heart (all P ≤ 0.04) and for Homolateral_Lung (all P ≤ 0.022). For open validation, no statistically significant differences were obtained in either of the models. RP models had little impact on reducing optimization planning times for expert planners; nevertheless, the result showed a 30% reduction time for beginner planners. The use of RP models generates high-quality plans, without differences from the planner experience. Conclusion: Two RP models for breast cancer treatment using VMAT were successfully implemented. The use of RP models for breast cancer reduces the optimization planning time and improves the efficiency of the treatment planning process while ensuring high-quality plans.

IAEA methodology for on-site end-to-end IMRT/VMAT audits: an international pilot study.

Background: The IAEA has developed and tested an on-site, end-to-end IMRT/VMAT dosimetry audit methodology for head and neck cases using an anthropomorphic phantom. The audit methodology is described, and the results of the international pilot testing are presented.Material and methods: The audit utilizes a specially designed, commercially available anthropomorphic phantom capable of accommodating a small volume ion chamber (IC) in four locations (three in planning target volumes (PTVs) and one in an organ at risk (OAR)) and a Gafchromic film in a coronal plane for the absorbed dose to water and two-dimensional dose distribution measurements, respectively. The audit consists of a pre-visit and on-site phases. The pre-visit phase is carried out remotely and includes a treatment planning task and a set of computational exercises. The on-site phase aims at comparing the treatment planning system (TPS) calculations with measurements in the anthropomorphic phantom following an end-to-end approach. Two main aspects were tested in the pilot study: feasibility of the planning constraints and the accuracy of IC and film results in comparison with TPS calculations. Treatment plan quality was scored from 0 to 100.Results: Forty-two treatment plans were submitted by 14 institutions from 10 countries, with 79% of them having a plan quality score over 90. Seventeen sets of IC measurement results were collected, and the average measured to calculated dose ratio was 0.988 ± 0.016 for PTVs and 1.020 ± 0.029 for OAR. For 13 film measurement results, the average gamma passing rate was 94.1% using criteria of 3%/3 mm, 20% threshold and global gamma.Conclusions: The audit methodology was proved to be feasible and ready to be adopted by national dosimetry audit networks for local implementation.

A method to enhance 2D ion chamber array patient specific quality assurance for IMRT.

Gamma index comparison has been established as a method for patient specific quality assurance in IMRT. Detector arrays can replace radiographic film systems to record 2D dose distributions and fulfill quality assurance requirements. These electronic devices present spatial resolution disadvantages with respect to films. This handicap can be partially overcome with a multiple acquisition sequence of adjacent 2D dose distributions. The detector spatial response influence can also be taken into account through the convolution of the calculated dose with the detector spatial response. A methodology that employs both approaches could allow for enhancements of the quality assurance procedure. 35 beams from different step and shoot IMRT plans were delivered on a phantom. 2D dose distributions were recorded with a PTW-729 ion chamber array for individual beams, following the multiple acquisition methodology. 2D dose distributions were also recorded on radiographic films. Measured dose distributions with films and with the PTW-729 array were processed with the software RITv5.2 for Gamma index comparison with calculated doses. Calculated dose was also convolved with the ion chamber 2D response and the Gamma index comparisons with the 2D dose distribution measured with the PTW-729 array was repeated. 3.7 ± 2.7% of points surpassed the accepted Gamma index when using radiographic films compared with calculated dose, with a minimum of 0.67 and a maximum of 13.27. With the PTW-729 multiple acquisition methodology compared with calculated dose, 4.1 ± 1.3% of points surpassed the accepted Gamma index, with a minimum of 1.44 and a maximum of 11.26. With the PTW- multiple acquisition methodology compared with convolved calculated dose, 2.7 ± 1.3% of points surpassed the accepted Gamma index, with a minimum of 0.42 and a maximum of 5.75. The results obtained in this work suggest that the comparison of merged adjacent dose distributions with convolved calculated dose represents an enhancement in the methodology for IMRT patient specific quality assurance with the PTW-729 ion chamber array.

Measurements of the dose delivered during CT exams using AAPM Task Group Report No. 111.

The computed tomography dose index (CTDI) measured with a 10 cm long pencil ionization chamber placed in a 14 cm long PMMA phantom is typically used to evaluate the doses delivered during CT procedure. For the new generation of CT scanners, the efficiency of this methodology is low because it excludes the contribution of radiation scattered beyond the 100 mm range of integration along z. The AAPM TG111 Report proposes a new measurement modality using a small volume ionization chamber positioned in a phantom long enough to establish dose equilibrium at the location of the chamber. In this work, the AAPM report was implemented. The minimum scanning length needed to obtain cumulative dose equilibrium was evaluated. The equilibrium dose was determined and compared to CTDI values informed by the CT scanner, and the dose values were confirmed with TLD measurements. The difference between doses measured with TLD and with the ionization chamber (IC) was below 1% and the repeatability of the measurements' setup was 0.4%. The measurements showed that the scanning lengths needed to reach the cumulated dose equilibrium were 450 mm and 380 mm for the central and peripheral axes, respectively, which justifies the phantom length. For the studied clinical protocols, the doses measured were about 30% higher than those informed by the CT scanner. For the new generation of CT systems with wider longitudinal detector size or cone-beam technology, the current CTDI measurements may no longer be adequate, and the informed CTDI tends to undervalue the dose delivered. It is therefore important to evaluate CT radiation doses following the AAPM TG111 methodology.

A method to enhance spatial resolution of a 2D ion chamber array for quality control of MLC.

This work introduces a new method for verifying MLC leaf positions with enough spatial resolution to replace film-based methods in performing QA tests. It is implemented on a 2D ion chamber array, and it is based on the principle of varying signal response of a volumetric detector to partial irradiation. A PTW 2D-ARRAY seven29 (PTW-729 2D) array was used to assess a Siemens OPTIFOCUS MLC. Partial volume response curves for chambers in the array were obtained by irradiating them with the leaves of the MLC, progressively covering varying portions of the chambers correlated with the leaf positions. The readings from the array's chambers are processed with an in-house program; it generates a reference response that translates readings into leaf positions. This principle allows discriminating errors in pairs of opposing leaves that could combine to cancel their detection with other tools. Patterns of leaf positions, similar to the Bayouth test but with different, purposefully introduced errors, were generated and used to test the effectiveness of the method. The same patterns were exposed on radiographic film and analyzed with the RIT software for validation. For four test patterns with a total of 100 errors of ± 1 mm, ± 2 mm and ± 3 mm, all were correctly determined with the proposed method. The analysis of the same pattern with film using the Bayouth routine in the RIT software resulted in either somewhat low true positives combined with a large fraction of false positives, or a low true positive rate with a low false positive ratio, the results being significantly affected by the threshold selected for the analysis. This method provides an effective, easy to use tool for quantitative MLC QA assessment, with excellent spatial resolution. It can be easily applied to other 2D arrays, as long as they exhibit a partial volume detector response.

Cáncer de cuello uterino: revisión de la técnica de irradiación de la pelvis por cuatro campos valor de la RM en su planificación / Cervix carcinoma: four-field pelvic radiation tecnique with MRI treatment planning

El objetivo de este trabajo es revisar la técnica de radioterapia por cuatro campos (Técnica de Box) para el tratamiento del cáncer de cuello uterino, a fin de determinar si los límites anatómicos estándar envuelven el volumen tumoral y su extensión microscópica y evaluar el valor de la RM para la planificación de dicha tácnica. Se revisaron retrospectivamente 35 RM de pacientes con cáncer de cuello uterino (Estadio Ib-IVa) y 10 linfografías pelvianas realizadas en el mismo grupo de pacientes, entre 1993 y 1996. La RM permitió determinar las relaciones existentes entre el útero con los límites AP y laterales, en el 100 por ciento de los casos. Se sugiere que no es posible establecer límites estándar para los campos laterales de la técnica de Box debido a las variaciones anatómicas producidas por el tumor y/o enfermedades asociadas. Si se decide esta técnica debería diseñarse en base a RM

Cáncer de cuello uterino: revisión de la técnica de irradiación de la pelvis por cuatro campos valor de la RM en su planificación / Cervix carcinoma: four-field pelvic radiation tecnique with MRI treatment planning

El objetivo de este trabajo es revisar la técnica de radioterapia por cuatro campos (Técnica de Box) para el tratamiento del cáncer de cuello uterino, a fin de determinar si los límites anatómicos estándar envuelven el volumen tumoral y su extensión microscópica y evaluar el valor de la RM para la planificación de dicha tácnica. Se revisaron retrospectivamente 35 RM de pacientes con cáncer de cuello uterino (Estadio Ib-IVa) y 10 linfografías pelvianas realizadas en el mismo grupo de pacientes, entre 1993 y 1996. La RM permitió determinar las relaciones existentes entre el útero con los límites AP y laterales, en el 100 por ciento de los casos. Se sugiere que no es posible establecer límites estándar para los campos laterales de la técnica de Box debido a las variaciones anatómicas producidas por el tumor y/o enfermedades asociadas. Si se decide esta técnica debería diseñarse en base a RM (AU)