Comparison of RTP dose distributions in heterogeneous phantoms with the BEAM Monte Carlo simulation system

Ethylene-vinyl acetate foam as a new lung substitute in radiotherapy

PURPOSE

The purpose of this study was to evaluate ethylene-vinyl acetate (EVA) foam as a new lung substitute in radiotherapy and to study its physical and dosimetric characteristics.

METHODS

We calculated the ideal vinyl acetate (VA) content of EVA foam sheets to mimic the physical and dosimetric characteristics of the ICRU lung tissue. We also computed the water-to-medium mass collision stopping power ratios, mass attenuation coefficients, CT numbers, effective atomic numbers and electron densities for: ICRU lung tissue, the RANDO commercial phantom, scaled WATER and EVA foam sheets with varying VA contents in a range between the minimum and maximum values supplied by the manufacturer. For all these substitutes, we simulated percent depth-dose curves with EGSnrc Monte Carlo (MC PDDs) in a water-lung substitute-water slab phantom expressed as dose-to-medium and dose-to-water for 3 × 3- and 10 × 10-cm² field sizes. PDD for the 10 × 10-cm² field size was also calculated with the MultiGrid Superposition algorithm (MGS PDD) for a relative electron density to water ratio of 0.26. The latter was compared with the MC PDDs in dose-to-water for scaled WATER and EVA foam sheets with the VA content that was most similar to the calculated ideal content that is physically achievable in practice.

RESULTS

We calculated an ideal VA content of 55%; however, the maximum physically achievable content with current manufacturing techniques is 40%. The physical characteristics of the EVA foam sheets with a VA content of 40% (EVA40) are very close to those of the ICRU lung reference. The physical densities of the EVA40 foam sheets ranged from 0.030 to 0.965 g/cm³ , almost covering the entire physical density range of the inflated/deflated lung (0.260-1.050 g/cm³ ). Its mass attenuation coefficient at the effective energy of a 6-MV photon beam agrees within 0.8% of the ICRU reference value, and its CT number agrees within 6 HU. The effective atomic number for EVA40 varies by less than 0.42 of the ICRU value, and its effective electron density is within 0.9%. PDDs expressed in dose-to-medium and dose-to-water agree with the ICRU curve within 2% in all regions. PDDs calculated with both MC and MGS were within 1.5%.

CONCLUSIONS

The EVA40 is an excellent cork-like lung substitute for radiotherapy applications. From a sole material used in footwear, it is possible to obtain a lung substitute that mimics the physical and dosimetric characteristics of ICRU lung tissue even better than the RANDO commercial phantom.

A measurement-based generalized source model for Monte Carlo dose simulations of CT scans

The goal of this study is to develop a generalized source model for accurate Monte Carlo dose simulations of CT scans based solely on the measurement data without a priori knowledge of scanner specifications. The proposed generalized source model consists of an extended circular source located at x-ray target level with its energy spectrum, source distribution and fluence distribution derived from a set of measurement data conveniently available in the clinic. Specifically, the central axis percent depth dose (PDD) curves measured in water and the cone output factors measured in air were used to derive the energy spectrum and the source distribution respectively with a Levenberg-Marquardt algorithm. The in-air film measurement of fan-beam dose profiles at fixed gantry was back-projected to generate the fluence distribution of the source model. A benchmarked Monte Carlo user code was used to simulate the dose distributions in water with the developed source model as beam input. The feasibility and accuracy of the proposed source model was tested on a GE LightSpeed and a Philips Brilliance Big Bore multi-detector CT (MDCT) scanners available in our clinic. In general, the Monte Carlo simulations of the PDDs in water and dose profiles along lateral and longitudinal directions agreed with the measurements within 4%/1 mm for both CT scanners. The absolute dose comparison using two CTDI phantoms (16 cm and 32 cm in diameters) indicated a better than 5% agreement between the Monte Carlo-simulated and the ion chamber-measured doses at a variety of locations for the two scanners. Overall, this study demonstrated that a generalized source model can be constructed based only on a set of measurement data and used for accurate Monte Carlo dose simulations of patients' CT scans, which would facilitate patient-specific CT organ dose estimation and cancer risk management in the diagnostic and therapeutic radiology.

Monte Carlo systems used for treatment planning and dose verification

General-purpose radiation transport Monte Carlo codes have been used for estimation of the absorbed dose distribution in external photon and electron beam radiotherapy patients since several decades. Results obtained with these codes are usually more accurate than those provided by treatment planning systems based on non-stochastic methods. Traditionally, absorbed dose computations based on general-purpose Monte Carlo codes have been used only for research, owing to the difficulties associated with setting up a simulation and the long computation time required. To take advantage of radiation transport Monte Carlo codes applied to routine clinical practice, researchers and private companies have developed treatment planning and dose verification systems that are partly or fully based on fast Monte Carlo algorithms. This review presents a comprehensive list of the currently existing Monte Carlo systems that can be used to calculate or verify an external photon and electron beam radiotherapy treatment plan. Particular attention is given to those systems that are distributed, either freely or commercially, and that do not require programming tasks from the end user. These systems are compared in terms of features and the simulation time required to compute a set of benchmark calculations.

On the use of a convolution-superposition algorithm for plan checking in lung stereotactic body radiation therapy

Stereotactic body radiation therapy (SBRT) aims to deliver a highly conformal ablative dose to a small target. Dosimetric verification of SBRT for lung tumors presents a challenge due to heterogeneities, moving targets, and small fields. Recent software (M3D) designed for dosimetric verification of lung SBRT treatment plans using an advanced convolution–superposition algorithm was evaluated. Ten lung SBRT patients covering a range of tumor volumes were selected. 3D CRT plans were created using the XiO treatment planning system (TPS) with the superposition algorithm. Dose was recalculated in the Eclipse TPS using the AAA algorithm, M3D verification software using the collapsed‐cone‐convolution algorithm, and in‐house Monte Carlo (MC). Target point doses were calculated with RadCalc software. Near‐maximum, median, and near‐minimum target doses, conformity indices, and lung doses were compared with MC as the reference calculation. M3D 3D gamma passing rates were compared with the XiO and Eclipse. Wilcoxon signed‐rank test was used to compare each calculation method with XiO with a threshold of significance of p<0.05. M3D and RadCalc point dose calculations were greater than MC by up to 7.7% and 13.1%, respectively, with M3D being statistically significant (s.s.). AAA and XiO calculated point doses were less than MC by 11.3% and 5.2%, respectively (AAA s.s.). Median and near‐minimum and near‐maximum target doses were less than MC when calculated with AAA and XiO (all s.s.). Near‐maximum and median target doses were higher with M3D compared with MC (s.s.), but there was no difference in near‐minimum M3D doses compared with MC. M3D‐calculated ipsilateral lung V20 Gy and V5 Gy were greater than that calculated with MC (s.s.); AAA‐ and XiO‐calculated V20 Gy was lower than that calculated with MC, but not statistically different to MC for V5 Gy. Nine of the 10 plans achieved M3D gamma passing rates greater than 95% and 80%for 5%/1 mm and 3%/1 mm criteria, respectively. M3D typically calculated a higher target and lung dose than MC for lung SBRT plans. The results show a range of calculated doses with different algorithms and suggest that M3D is in closer agreement with Monte Carlo, thus discrepancies between the TPS and M3D software will be observed for lung SBRT plans. M3D provides a useful supplement to verification of lung SBRT plans by direct measurement, which typically excludes patient specific heterogeneities.

PACS number(s): 87.55.D‐, 87.55.Qr, 87.55.K‐

Dose calculation accuracies in whole breast radiotherapy treatment planning: a multi-institutional study

Our objective in this study was to evaluate the variation in the doses delivered among institutions due to dose calculation inaccuracies in whole breast radiotherapy. We have developed practical procedures for quality assurance (QA) of radiation treatment planning systems. These QA procedures are designed to be performed easily at any institution and to permit comparisons of results across institutions. The dose calculation accuracy was evaluated across seven institutions using various irradiation conditions. In some conditions, there was a >3 % difference between the calculated dose and the measured dose. The dose calculation accuracy differs among institutions because it is dependent on both the dose calculation algorithm and beam modeling. The QA procedures in this study are useful for verifying the accuracy of the dose calculation algorithm and of the beam model before clinical use for whole breast radiotherapy.

An institutional experience of quality assurance of a treatment planning system on photon beam

AIM

The purpose of the present study is to show the application of the IAEA TRS-430 QA procedures of Eclipse™v7.5 TPS for photon energies. In addition, the trends of the deviations found in the conducted tests were determined.

BACKGROUND

In the past, the lack of complete TPS QA procedures led to some serious accidents. So, QA in the radiotherapy treatment planning process is essential for determination of accuracy in the radiotherapy process and avoidance of treatment errors.

MATERIALS AND METHODS

The calculations of TPS and measurements of irradiations of the treatment device were compared in the study. As a result, the local dose deviation values (δ 1: central beam axis, δ 2: penumbra and build up region, δ 3: inside field, δ 4: outside beam edges, δ 50-90: beam fringe, RW50: radiological width) and their confidence limit values (including systematic and random errors) were obtained.

RESULTS

The confidence limit values of δ 4 were detected to increase with expanding field size. The values of δ 1 and δ 3 of hard wedge were larger than open fields. The values of δ 2 and δ 50-90 of the inhomogeneity effect test were larger, especially than other tests of this study. The average deviation was showed to increase with the rise of the wedge angle. The values of δ 3 and δ 4 of lung irradiation were outside tolerance.

CONCLUSIONS

The QA of TPS was done and it was found that there were no reservations in its use in patient treatment. The trend of the deviations is shown.

Dosimetric comparison of different inhomogeneity correction algorithms for external photon beam dose calculations

Dose calculation algorithm is one of the main sources of uncertainty in the radiotherapy sequences. The aim of this study was to compare the accuracy of different inhomogeneity correction algorithms for external photon beam dose calculations. The methodology was based on International Atomic Energy Agency TEC-DOC 1583. The phantom was scanned in every center, using computed tomography and seven tests were planned on three-dimensional treatment planning systems (TPSs). The doses were measured with ion chambers and the deviation between measured and TPS calculated dose was reported. This methodology was tested in five different hospitals which were using six different algorithms/inhomogeneity correction methods implemented in different TPSs. The algorithms in this study were divided into two groups: Measurement-based algorithms (type (a)) and model-based algorithms (type (b)). In type (a) algorithms, we saw 7.6% and 11.3% deviations out of agreement criteria for low- and high-energy photons, respectively. While in type (b) algorithms, these values were 4.3% and 5.1%, respectively. As a general trend, the numbers of measurements with results outside the agreement criteria increase with the beam energy and decrease with advancement of TPS algorithms. More advanced algorithm would be preferable and therefore should be implanted in clinical practice, especially for calculation in inhomogeneous medias like lung and bone and for high-energy beams calculation at low depth points.

Towards accurate dose accumulation for Step-&-Shoot IMRT: Impact of weighting schemes and temporal image resolution on the estimation of dosimetric motion effects

PURPOSE

Breathing-induced motion effects on dose distributions in radiotherapy can be analyzed using 4D CT image sequences and registration-based dose accumulation techniques. Often simplifying assumptions are made during accumulation. In this paper, we study the dosimetric impact of two aspects which may be especially critical for IMRT treatment: the weighting scheme for the dose contributions of IMRT segments at different breathing phases and the temporal resolution of 4D CT images applied for dose accumulation.

METHODS

Based on a continuous problem formulation a patient- and plan-specific scheme for weighting segment dose contributions at different breathing phases is derived for use in step-&-shoot IMRT dose accumulation. Using 4D CT data sets and treatment plans for 5 lung tumor patients, dosimetric motion effects as estimated by the derived scheme are compared to effects resulting from a common equal weighting approach. Effects of reducing the temporal image resolution are evaluated for the same patients and both weighting schemes.

RESULTS

The equal weighting approach underestimates dosimetric motion effects when considering single treatment fractions. Especially interplay effects (relative misplacement of segments due to respiratory tumor motion) for IMRT segments with only a few monitor units are insufficiently represented (local point differences >25% of the prescribed dose for larger tumor motion). The effects, however, tend to be averaged out over the entire treatment course. Regarding temporal image resolution, estimated motion effects in terms of measures of the CTV dose coverage are barely affected (in comparison to the full resolution) when using only half of the original resolution and equal weighting. In contrast, occurence and impact of interplay effects are poorly captured for some cases (large tumor motion, undersized PTV margin) for a resolution of 10/14 phases and the more accurate patient- and plan-specific dose accumulation scheme.

CONCLUSIONS

Radiobiological consequences of reported single fraction local point differences >25% of the prescribed dose are widely unclear and should be subject to future investigation. Meanwhile, if aiming at accurate and reliable estimation of dosimetric motion effects, precise weighting schemes such as the presented patient- and plan-specific scheme for step-&-shoot IMRT and full available temporal 4D CT image resolution should be applied for IMRT dose accumulation.

Evaluation of the usefulness of a MOSFET detector in an anthropomorphic phantom for 6-MV photon beam

In order to evaluate the usefulness of a metal oxide-silicon field-effect transistor (MOSFET) detector as a in vivo dosimeter, we performed in vivo dosimetry using the MOSFET detector with an anthropomorphic phantom. We used the RANDO phantom as an anthropomorphic phantom, and dose measurements were carried out in the abdominal, thoracic, and head and neck regions for simple square field sizes of 10 x 10, 5 x 5, and 3 x 3 cm(2) with a 6-MV photon beam. The dose measured by the MOSFET detector was verified by the dose calculations of the superposition (SP) algorithm in the XiO radiotherapy treatment-planning system. In most cases, the measured doses agreed with the results of the SP algorithm within +/-3%. Our results demonstrated the utility of the MOSFET detector for in vivo dosimetry even in the presence of clinical tissue inhomogeneities.

Monte Carlo dose verification of prostate patients treated with simultaneous integrated boost intensity modulated radiation therapy

BACKGROUND

To evaluate the dosimetric differences between Superposition/Convolution (SC) and Monte Carlo (MC) calculated dose distributions for simultaneous integrated boost (SIB) prostate cancer intensity modulated radiotherapy (IMRT) compared to experimental (film) measurements and the implications for clinical treatments.

METHODS

Twenty-two prostate patients treated with an in-house SIB-IMRT protocol were selected. SC-based plans used for treatment were re-evaluated with EGS4-based MC calculations for treatment verification. Accuracy was evaluated with-respect-to film-based dosimetry. Comparisons used gamma (gamma)-index, distance-to-agreement (DTA), and superimposed dose distributions. The treatment plans were also compared based on dose-volume indices and 3-D gamma index for targets and critical structures.

RESULTS

Flat-phantom comparisons demonstrated that the MC algorithm predicted measurements better than the SC algorithm. The average PTVprostate D98 agreement between SC and MC was 1.2% +/- 1.1. For rectum, the average differences in SC and MC calculated D50 ranged from -3.6% to 3.4%. For small bowel, there were up to 30.2% +/- 40.7 (range: 0.2%, 115%) differences between SC and MC calculated average D50 index. For femurs, the differences in average D50 reached up to 8.6% +/- 3.6 (range: 1.2%, 14.5%). For PTVprostate and PTVnodes, the average gamma scores were >95.0%.

CONCLUSION

MC agrees better with film measurements than SC. Although, on average, SC-calculated doses agreed with MC calculations within the targets within 2%, there were deviations up to 5% for some patient's treatment plans. For some patients, the magnitude of such deviations might decrease the intended target dose levels that are required for the treatment protocol, placing the patients in different dose levels that do not satisfy the protocol dose requirements.

Dosimetric evaluation of heterogeneity corrections for RTOG 0236: stereotactic body radiotherapy of inoperable stage I-II non-small-cell lung cancer

PURPOSE

Using a retrospective analysis of treatment plans submitted from multiple institutions accruing patients to the Radiation Therapy Oncology Group (RTOG) 0236 non-small-cell stereotactic body radiotherapy protocol, the present study determined the dose prescription and critical structure constraints for future stereotactic body radiotherapy lung protocols that mandate density-corrected dose calculations.

METHOD AND MATERIALS

A subset of 20 patients from four institutions participating in the RTOG 0236 protocol and using superposition/convolution algorithms were compared. The RTOG 0236 protocol required a prescription dose of 60 Gy delivered in three fractions to cover 95% of the planning target volume. Additional requirements were specified for target dose heterogeneity and the dose to normal tissue/structures. The protocol required each site to plan the patient's treatment using unit density, and another plan with the same monitor units and applying density corrections was also submitted. These plans were compared to determine the dose differences. Two-sided, paired Student's t tests were used to evaluate these differences.

RESULTS

With heterogeneity corrections applied, the planning target volume receiving >/=60 Gy decreased, on average, 10.1% (standard error, 2.7%) from 95% (p = .001). The maximal dose to any point >/=2 cm away from the planning target volume increased from 35.2 Gy (standard error, 1.7) to 38.5 Gy (standard error, 2.2).

CONCLUSION

Statistically significant dose differences were found with the heterogeneity corrections. The information provided in the present study is being used to design future heterogeneity-corrected RTOG stereotactic body radiotherapy lung protocols to match the true dose delivered for RTOG 0236.

Evaluation of the analytical anisotropic algorithm in an extreme water-lung interface phantom using Monte Carlo dose calculations

Our study compares the performance of the analytical anisotropic algorithm (AAA), a new superposition–convolution algorithm recently implemented in the Eclipse (Varian Medical Systems, Palo Alto, CA) Integrated Treatment Planning System (TPS), to that of the pencil beam convolution (PBC) algorithm in an extreme (C‐shaped, horizontal and vertical boundaries) water–lung interface phantom. Monte Carlo (MC) calculated dose distributions for a variety of clinical beam configurations at nominal energies of 6‐MV and 18‐MV are used as benchmarks in the comparison. Dose profiles extracted at three depths (4, 10, and 16 cm), two‐dimensional (2D) maps of the dose differences, and dose difference statistics are used to quantify the accuracy of both photon‐dose calculation algorithms. Results show that the AAA is considerably more accurate than the PBC, with the standard deviation of the dose differences within a region encompassing the lung block reduced by a factor of 2 and more. Confidence limits with the AAA were 4% or less for all beam configurations investigated; with the PBC, confidence limits ranged from 3.5% to 11.2%. Finally, AAA calculations for the small 4×4 18‐MV beam, which is poorly modeled by PBC (dose differences as high as 16.1%), provided the same accuracy as the PBC model of the 6‐MV beams commonly acceptable in clinical situations.

PACS number: 87.53.Bn

Effectiveness of compensating filters in the presence of tissue inhomogeneities

CT based 3D treatment planning systems (3DTPS) can be used to design compensating filters that, in addition to missing tissue compensation, can account for tissue inhomogeneities. The use of computer-driven systems provides a practical, convenient, and accurate method of fabricating compensating filters. In this work, we have evaluated a commercially available PAR Scientific DIGIMILL milling machine linked with FOCUS 3DTPS. Compensating filters were fabricated using refined gypsum material with no additives. Thus, filters were of manageable dimensions and were not sensitive to common machining errors. Compensating filters were evaluated using a homogeneous step phantom and step phantoms containing various internal inhomogeneities (air, cork, and bone). The accuracy of two planning algorithms used to design filters was experimentally evaluated. The superposition algorithm was found to produce better agreement with measurements than the Clarkson algorithm. Phantom measurements have demonstrated that compensating filters were able to produce a uniform dose distribution along the compensation plane in the presence of tissue inhomogeneity. However, the dose variation was greatly amplified in planes located beyond the inhomogeneity when a single compensated beam was used. The use of parallel-opposed compensated beams eliminated this problem. Both lateral and depth-dose uniformity was achieved throughout the target volume.