Evaluation of a 2D diode array for IMRT quality assurance.

BACKGROUND AND PURPOSE: The QA of intensity modulated radiotherapy (IMRT) dosimetry is a laborious task. The goal of this work is to evaluate the dosimetric characteristics of a new 2D diode array (MapCheck from Sun Nuclear Corporation, Melbourne, Florida) and assess the role it can play in routine IMRT QA. MATERIAL AND METHODS: Fundamental properties of the MapCheck such as reproducibility, linearity and temperature dependence are studied for high-energy photon beams. The accuracy of the correction for difference of diode sensitivity is also assessed. The diode array is benchmarked against film and ion chambers for conventional and IMRT treatments. The MapCheck sensitivity to multileaf collimator position errors is determined. RESULTS: The diode array response is linear with dose up to 295 cGy. All diodes are calibrated to within +/-1% of each other, and mostly within +/-0.5%. The MapCheck readings are reproducible to within a maximum SD of +/-0.15%. A temperature dependence of 0.57%/ degrees C was noted and should be taken into account for absolute dosimetric measurement. Clinical performance of the MapCheck for relative and absolute dosimetry is demonstrated with seven beam (6 MV) head and neck IMRT plans, and compares well with film and ion chamber measurements. Comparison to calculated dose maps demonstrates that the planning system model underestimates the dose gradients in the penumbra region. CONCLUSIONS: The MapCheck offers the dosimetric characteristics required for performing both relative and absolute dose measurements. Its use in the clinic can simplify and reduce the IMRT QA workload.     

Radiation therapy with unflattened photon beams: Dosimetric accuracy of advanced dose calculation algorithms

Purpose

To compare the dosimetric accuracy of advanced dose calculation algorithms for flattened (FF) and unflattened (FFF) photon beams.

Material and methods

We compared the enhanced collapsed cone (eCC) algorithm implemented in OncentraMasterplan and the XVMC (MC) code in Monaco. Test plans were created for 10 MV FF and FFF beams. Single beam tests were delivered to radiochromic films positioned within a solid water phantom and evaluated with 1D γ-index analysis. Conformal plans were verified with ion chambers in an anthropomorphic thorax phantom. IMRT plans were applied to the Delta4 system and evaluated with γ-criteria of 3% and 3 mm.

Results

1D γ-index evaluation revealed significantly lower (p < 0.05) average γ_mean-values of 0.46 ± 0.22 for MC calculated FFF profiles compared to average values of 0.53 ± 0.27 detected for FF beams. Respective values for eCC were 0.42 ± 0.27/0.38 ± 0.26 (FF/FFF). When considering off-axis profiles separately, we found significantly reduced average γ_mean-values for FFF and both algorithms (MC: 0.55 ± 24 vs. 0.45 ± 0.21, eCC: 0.41 ± 0.24 vs. 0.35 ± 0.22). No significant differences were detected on-axis. Absolute dosimetry in the anthropomorphic phantom revealed superior results for MC based dose calculation, with mean deviations of 0.8 ± 0.8/0.0 ± 1.0% compared to −0.1 ± 1.7/−0.5 ± 0.1.7% (FF/FFF) for the eCC algorithm. IMRT plans showed similar results for both linac modes.

Conclusions

The dose calculation accuracy for unflattened beams was found to be at least as high as for flattened beams. The slightly improved dose calculation accuracy observed for off-axis profiles for single FFF beams did not directly translate into better verification results for composite IMRT plans.     

利用0.13cc电离室对头颈部肿瘤调强放疗计划的剂量学验证

目的:利用0.13cc电离室对头颈部肿瘤调强适形放射治疗(IMRT)计划进行剂量学验证.方法:将20例头颈部肿瘤患者的IMRT计划分别移植到经过CT扫描的调强体模,生成验证计划,将0.13cc电离室放置到调强体模中在加速器下执行验证计划,在治疗计划系统中算出电离室所在区域的吸收剂量为计划剂量,按验证计划照射测量到的电离室吸收剂量为实测剂量,将二者进行比较得出误差.相对误差=(计划剂量-实测剂量)/实测剂量.百分误差超过±5％,说明计划在执行中剂量误差过大,计划需要修正.结果:20例患者中有17例患者验证的误差在±5％以内,表明计划通过;有3例患者误差超过±5％以内,计划需重新修改,计划通过率为85％.结论:剂量学验证确定IMRT治疗剂量的置信度,保证治疗计划的准确实施,提供了临床评价治疗计划的依据.     

Advanced kernel methods vs. Monte Carlo-based dose calculation for high energy photon beams

Purpose

The aim of this study was to compare the dose calculation accuracy of advanced kernel-based methods and Monte Carlo algorithms in commercially available treatment planning systems.

Materials and methods

Following dose calculation algorithms and treatment planning (TPS) systems were compared: the collapsed cone (CC) convolution algorithm available in Oncentra Masterplan, the XVMC Monte Carlo algorithm implemented in iPlan and Monaco, and the analytical anisotropic algorithm (AAA) implemented in Eclipse. Measurements were performed with a calibrated ionization chamber and radiochromic EBT type films in a homogenous polystyrene phantom and in heterogeneous lung phantoms. Single beam tests, conformal treatment plans and IMRT plans were validated. Dosimetric evaluations included absolute dose measurements, 1D γ-evaluation of depth-dose curves and profiles using 2 mm and 2% dose difference criteria for single beam tests, and γ-evaluation of axial planes for composite treatment plans applying 3 mm and 3% dose difference criteria.

Results

Absolute dosimetry revealed no large differences between MC and advanced kernel dose calculations. 1D γ-evaluation showed significant discrepancies between depth-dose curves in different phantom geometries. For the CC algorithm γ_mean values were 0.90 ± 0.74 vs. 0.43 ± 0.41 in heterogeneous vs. homogeneous conditions and for the AAA γ_mean values were 1.13 ± 0.91 vs. 0.41 ± 0.28, respectively. In general, 1D γ results obtained with both MC TPS were similar in both phantoms and on average equal to 0.5 both for profiles and depth-dose curves. The results obtained with the CC algorithm in heterogeneous phantoms were slightly better in comparison to the AAA algorithm. The 2D γ-evaluation results of IMRT plans and four-field plans showed smaller mean γ-values for MC dose calculations compared to the advanced kernel algorithms (γ_mean for four-field plan and IMRT obtained with Monaco MC were 0.28 and 0.5, respectively, vs. 0.40 and 0.54 for the AAA).

Conclusion

All TPS investigated in this study demonstrated accurate dose calculation in homogenous and heterogeneous phantoms. Commercially available TPS with Monte Carlo option performed best in heterogeneous phantoms. However, the difference between the CC and the MC algorithms was found to be small.     

Clinical introduction of a linac head-mounted 2D detector array based quality assurance system in head and neck IMRT

Background and purpose

IMRT QA is commonly performed in a phantom geometry but the clinical interpretation of the results in a 2D phantom plane is difficult. The main objective of our work is to move from film measurement based QA to 3D dose reconstruction in a patient CT scan. In principle, this could be achieved using a dose reconstruction method from 2D detector array measurements as available in the COMPASS system (IBA Dosimetry). The first step in the clinical introduction of this system instead of the currently used film QA procedures is to test the reliability of the dose reconstruction. In this paper we investigated the validation of the method in a homogeneous phantom with the film QA procedure as a reference. We tested whether COMPASS QA results correctly identified treatment plans that did or did not fulfil QA requirements in head and neck (H&N) IMRT.

Materials and methods

A total number of 24 treatments were selected from an existing database with more than 100 film based H&N IMRT QA results. The QA results were classified as either good, just acceptable or clinically rejected (mean gamma index <0.4, 0.4-0.5 or >0.5, respectively with 3%/3 mm criteria). Film QA was repeated and compared to COMPASS QA with a MatriXX detector measurement performed on the same day.

Results

Good agreement was found between COMPASS reconstructed dose and film measured dose in a phantom (mean gamma 0.83 ± 0.09, 1SD with 1%/1 mm criteria, 0.33 ± 0.04 with 3%/3 mm criteria). COMPASS QA results correlated well with film QA, identifying the same patients with less good QA results. Repeated measurements with film and COMPASS showed changes in delivery after a modified MLC calibration, also visible in a standard MLC check in COMPASS. The time required for QA reduced by half by using COMPASS instead of film.

Conclusions

Agreement of COMPASS QA results with film based QA supports its clinical introduction for a phantom geometry. A standard MLC calibration check is sensitive to <1 mm changes that could be significant in H&N IMRT. These findings offer opportunities to further investigate the method based on a 2D detector array to 3D dose reconstruction in a patient anatomy.     

Dose verification in clinical IMRT prostate incidents

PURPOSE: In view of the need for dose-validation procedures on each individual intensity-modulated radiation therapy (IMRT) plan, dose-verification measurements by film, by ionization chamber, and by polymer gel-MRI dosimetry were performed for a prostate-treatment plan configuration. Treatment planning system (TPS) calculations were evaluated against dose measurements. METHODS AND MATERIALS: Intensity-modulated radiation therapy (IMRT) treatments were planned on a commercial TPS. Kodak EDR-2 films were used for the verification of two-dimensional (2D) dose distributions at 1 coronal and 5 axial planes in a water-equivalent phantom. Full three-dimensional (3D) dose distributions were measured by use of a novel polymer gel formulation and a 3D magnetic resonance imaging (MRI) readout technique. Calculations were compared against measurements by means of isocontour maps, gamma-index maps (3% dose difference, 3-mm distance to agreement) and dose-volume histograms. RESULTS: A good agreement was found between film measurements and TPS predictions for points within the 60% isocontour, for all the examined plans (gamma-index <1 for 96% of pixels). Three-dimensional dose distributions obtained with the polymer gel-MRI method were adequately matched with corresponding TPS calculations, for measurements in a gel phantom covering the planning-target volume (PTV). CONCLUSIONS: Measured 2D and 3D dose distributions suggest that, for the investigated prostate IMRT plan configuration, TPS calculations provide clinically acceptable accuracy.     

A multi-institutional initiative on patient-related quality assurance: Independent computational dose verification of fluence-modulated treatment techniques

PurposeThis multi-institutional study investigates whether computational verification of fluence-modulated treatment plans using independent software with its own Strahlerkopfmodel is an appropriate method for patient-related quality assurance (PRQA) in the context of various combinations of linear accelerators (linacs), treatment techniques and treatment planning systems (TPS).Materials and methodsThe PRQA-software's (Mobius3D) recalculations of 9 institutions’ treatment plans were analyzed for a horseshoe-shaped planning target volume (PTV) inside a phantom. The recomputed dose distributions were compared to a) the dose distributions as calculated by all TPS's and b) the measured dose distributions, which were acquired using the same independent measuring system for all institutions. Furthermore, dose volume histograms were examined. The penumbra deviations and mean gamma values were quantified using Verisoft (PTW). Additionally, workflow requirements for computational verification were discussed.ResultsMobius3D is compatible with all examined TPSs, treatment techniques and linacs. The mean PTV dose differences (Mobius3D-TPS, <3.0%) and 3D gamma passing rates (>95.0%) led to a positive plan acceptance result in all cases. These results are similar to the outcome of the dosimetric measurements with one exception. The mean gamma values (<0.5) show a good agreement between Mobius3D and the TPS dose distributions.ConclusionUsing Mobius3D was proven to be an appropriate computational PRQA method for the tested combinations of linacs, treatment techniques and TPS's. The clinical use of Mobius3D has to be complemented with regular dosimetric measurements and thorough linac and TPS QA. Mobius3D's computational verification reduced measurement effort and personnel needs in comparison to dosimetric verifications.     

Clinical evaluation of monitor unit software and the application of action levels.

PURPOSE: The aim of this study was the clinical evaluation of an independent dose and monitor unit verification (MUV) software which is based on sophisticated semi-analytical modelling. The software was developed within the framework of an ESTRO project. Finally, consistent handling of dose calculation deviations applying individual action levels is discussed. MATERIALS AND METHODS: A Matlab-based software ("MUV") was distributed to five well-established treatment centres in Europe (Vienna, Graz, Basel, Copenhagen, and Ume?) and evaluated as a quality assurance (QA) tool in clinical routine. Results were acquired for 226 individual treatment plans including a total of 815 radiation fields. About 150 beam verification measurements were performed for a portion of the individual treatment plans, mainly with time variable fluence patterns. The deviations between dose calculations performed with a treatment planning system (TPS) and the MUV software were scored with respect to treatment area, treatment technique, geometrical depth, radiological depth, etc. RESULTS: In general good agreement was found between calculations performed with the different TPSs and MUV, with a mean deviation per field of 0.2+/-3.5% (1 SD) and mean deviations of 0.2+/-2.2% for composite treatment plans. For pelvic treatments less than 10% of all fields showed deviations larger than 3%. In general, when using the radiological depth for verification calculations the results and the spread in the results improved significantly, especially for head-and-neck and for thorax treatments. For IMRT head-and-neck beams, mean deviations between MUV and the local TPS were -1.0+/-7.3% for dynamic, and -1.3+/-3.2% for step-and-shoot IMRT delivery. For dynamic IMRT beams in the pelvis good agreement was obtained between MUV and the local TPS (mean: -1.6+/-1.5%). Treatment site and treatment technique dependent action levels between +/-3% and +/-5% seem to be clinically realistic if a radiological depth correction is performed, even for dynamic wedges and IMRT. CONCLUSION: The software MUV is well suited for patient specific treatment plan QA applications and can handle all currently available treatment techniques that can be applied with standard linear accelerators. The highly sophisticated dose calculation model implemented in MUV allows investigation of systematic TPS deviations by performing calculations in homogeneous conditions.     

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 (γ)-index, distance-to-agreement (DTA), and superimposed dose distributions. The treatment plans were also compared based on dose-volume indices and 3-D γ index for targets and critical structures.

Results

Flat-phantom comparisons demonstrated that the MC algorithm predicted measurements better than the SC algorithm. The average PTV_prostate 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 PTV_prostate and PTV_nodes, 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.     

A comparison of the quality assurance of four dosimetric tools for intensity modulated radiation therapy

Background

This study was designed to compare the quality assurance (QA) results of four dosimetric tools used for intensity modulated radiation therapy (IMRT) and to suggest universal criteria for the passing rate in QA, irrespective of the dosimetric tool used.

Materials and methods.

Thirty fields of IMRT plans from five patients were selected, followed by irradiation onto radiochromic film, a diode array (Mapcheck), an ion chamber array (MatriXX) and an electronic portal imaging device (EPID) for patient-specific QA. The measured doses from the four dosimetric tools were compared with the dose calculated by the treatment planning system. The passing rates of the four dosimetric tools were calculated using the gamma index method, using as criteria a dose difference of 3% and a distance-to-agreement of 3 mm.

Results

The QA results based on Mapcheck, MatriXX and EPID showed good agreement, with average passing rates of 99.61%, 99.04% and 99.29%, respectively. However, the average passing rate based on film measurement was significantly lower, 95.88%. The average uncertainty (1 standard deviation) of passing rates for 6 intensity modulated fields was around 0.31 for film measurement, larger than those of the other three dosimetric tools.

Conclusions

QA results and consistencies depend on the choice of dosimetric tool. Universal passing rates should depend on the normalization or inter-comparisons of dosimetric tools if more than one dosimetric tool is used for patient specific QA.     

A national dosimetric audit of IMRT

Background and purpose

A dosimetric audit of IMRT has been carried out within the UK between June 2009 and March 2010 in order to provide an independent check of safe implementation and to identify problems in the modelling and delivery of IMRT.

Methods and materials

A mail based audit involving film and alanine dosimeters was utilized. Measurements were made for each individual field in an IMRT plan isocentrically in a flat water-equivalent phantom at a depth of 5 cm. The films and alanine dosimeters were processed and analysed centrally; additional ion chamber measurements were made by each participating centre.

Results

57 of 62 centres participated, with a total of 78 plans submitted. For the film measurements, all 176 fields from the less complex IMRT plans (including prostate and breast plans) achieved over 95% pixels passing a gamma criterion of 3%/3 mm within the 20% isodose. For the more complex IMRT plans (mainly head and neck) 8/245 fields (3.3%) achieved less than 95% pixels passing a 4%/4 mm gamma criterion. Of the alanine measurements, 4/78 (5.1%) of the measurements differed by >5% from the dose predicted by the treatment planning system. Three of these were large deviations of −77.1%, −29.1% and 14.1% respectively. Excluding the three measurements outside 10%, the mean difference was 0.05% with a standard deviation of 1.5%. The out of tolerance results have been subjected to further investigations.

Conclusions

A dosimetric audit has been successfully carried out of IMRT implementation by over 90% of UK radiotherapy departments. The audit shows that modelling and delivery of IMRT is accurate, suggesting that the implementation of IMRT has been carried out safely.     

Dose discrepancy between planning system estimation and measurement in spine stereotactic body radiation therapy: A case report

Stereotactic body radiation therapy (SBRT) to treat spinal metastases has shown excellent clinical outcomes for local control. High dose gradients wrapping around spinal cord make this treatment technically challenging. In this work, we present a spine SBRT case where a dosimetric error was identified during pre‐treatment dosimetric quality assurance (QA). A patient with metastasis in T7 vertebral body consented to undergo SBRT. A dual arc volumetric modulated arc therapy plan was generated on the Pinnacle treatment planning system (TPS) with a 6 MV Elekta machine using gantry control point spacing of 4°. Standard pre‐treatment QA measurements were performed, including ArcCHECK, ion chamber in CTV and spinal cord (SC) region and film measurements in multiple planes. While the dose measured at CTV region showed good agreement with TPS, the dose measured to the SC was significantly higher than reported by TPS in the original and repeat plans. Acceptable agreement was only achieved when the gantry control point spacing was reduced to 3°. A potentially harmful dose error was identified by pre‐treatment QA. TPS parameter settings used safely in conventional treatments should be re‐assessed for complex treatments.     

Analysis of the dose calculation accuracy for IMRT in lung: a 2D approach

The purpose of this study was to compare the dosimetric accuracy of IMRT plans for targets in lung with the accuracy of standard uniform-intensity conformal radiotherapy for different dose calculation algorithms. Tests were performed utilizing a special phantom manufactured from cork and polystyrene in order to quantify the uncertainty of two commercial TPS for IMRT in the lung. Ionization and film measurements were performed at various measuring points/planes. Additionally, single-beam and uniform-intensity multiple-beam tests were performed, in order to investigate deviations due to other characteristics of IMRT. Helax-TMS V6.1(A) was tested for 6, 10 and 25 MV and BrainSCAN 5.2 for 6 MV photon beams, respectively. Pencil beam (PB) with simple inhomogeneity correction and 'collapsed cone' (CC) algorithms were applied for dose calculations. However, the latter was not incorporated during optimization hence only post-optimization recalculation was tested. Two-dimensional dose distributions were evaluated applying the gamma index concept. Conformal plans showed the same accuracy as IMRT plans. Ionization chamber measurements detected deviations of up to 5% when a PB algorithm was used for IMRT dose calculations. Significant improvement (deviations approximately 2%) was observed when IMRT plans were recalculated with the CC algorithm, especially for the highest nominal energy. All gamma evaluations confirmed substantial improvement with the CC algorithm in 2D. While PB dose distributions showed most discrepancies in lower (<50%) and high (>90%) dose regions, the CC dose distributions deviated mainly in the high dose gradient (20-80%) region. The advantages of IMRT (conformity, intra-target dose control) should be counterbalanced with possible calculation inaccuracies for targets in the lung. Until no superior dose calculation algorithms are involved in the iterative optimization process it should be used with great care. When only PB algorithm with simple inhomogeneity correction is used, lower energy photon beams should be utilized.     

A comparison of several modulated radiotherapy techniques for head and neck cancer and dosimetric validation of VMAT

Purpose

Volumetric modulated arc therapy (VMAT) has the potential to shorten treatment times for fluence modulated radiotherapy. We compared dose distributions of VMAT, step-and-shoot IMRT and serial tomotherapy for typical head and neck (H&N) planning target volumes (PTV) with sparing of one parotid, a complex paradigm and a situation often encountered in H&N radiotherapy. Finally, we validated the dosimetric accuracy of VMAT delivery.

Material and methods

Based on CT datasets of 10 patients treated for H&N cancer (PTV1:60 Gy/PTV2:56 Gy) with IMRT (7/9 fields), serial tomotherapy (MIMiC) and VMAT were compared with regard to plan quality and treatment efficiency. Plan quality was assessed by calculating homogeneity/conformity index (HI/CI), mean dose to parotid and brain stem and the maximum dose to the spinal cord. For plan efficiency evaluation, total treatment time (TTT) and number of monitor units (MU) were considered. A dosimetric evaluation of VMAT was performed using radiosensitive film, ion chamber and 2D-array.

Results

For MIMiC/IMRT_7F/IMRT_9F/VMAT, mean CI was 1.98/2.23/2.23/1.82, HI_PTV1 was 1.12/1.20/1.20/1.11 and HI_PTV2 was 1.11/1.15/1.13/1.12. Mean doses to the shielded parotid were 19.5 Gy/14.1 Gy/13.9 Gy/14.9 Gy and the spinal cord received maximum doses of 43.6 Gy/40.8 Gy/41.6 Gy/42.6 Gy. The mean MU’s were 2551/945/925/521 and the mean TTT was 12.8 min/7.6 min/8.5 min/4.32 min. The ion chamber measurements showed an absolute deviation of 0.08 ± 1.10% and 98.45 ± 3.25% pixels passed γ-analyses for 3%/3 mm and 99.95 ± 0.09% for 5%/5 mm for films. 2D-array measurements reported an agreement for 3%/3 mm of 95.65 ± 2.47%-98.33 ± 0.65% and for 5%/5 mm 99.79 ± 0.24%-99.92 ± 0.09% depending on the measurement protocol.

Conclusion

All treatment paradigms produced plans of excellent quality and dosimetric accuracy with IMRT providing best OAR sparing and VMAT being the most efficient treatment option in our comparison of treatment plans with high complexity.     

Intensity modulated radiosurgery of brain metastases with flattening filter-free beams

Purpose

Flattening filter free (FFF) irradiation potentially reduces treatment delivery time in radiosurgery thus eliminating intrafraction motion and increasing patient comfort. We compared plan quality and efficiency of VMAT and IMRT plans for FFF- and standard delivery for brain metastases with single fraction doses of 20 Gy and validated the dosimetric accuracy of the FFF delivery.

Material and Methods

CT data of 15 patients with brain metastases were included in this study. For every patient, 2 IMRT- and 2 VMAT-plans were created using a high-resolution MLC with two different delivery modes (6 MV standard vs. 6 MV FFF). Plan quality and efficiency was assessed by analysis of conformity, homogeneity, dose gradients, treatment delivery time and number of monitor units (MU). Dosimetric evaluation was performed for 10 FFF plans with radiochromic film and ion chamber.

Results

Plan quality was similar for both approaches. FFF provided a mean treatment time reduction of 51.5% with similar MU for VMAT and IMRT for this low-modulation paradigm. The dosimetric validations showed an absolute dose deviation of +0.93 ± 0.99% and γ-index analysis (3%/3 mm and 3%/1 mm) resulted in agreement of 99.08 ± 1.58% respectively 93.46 ± 2.41%.

Conclusion

FFF radiosurgery is an efficient technique for intensity modulated hypofractionated or single fraction treatments with similar plan quality when compared to flattened beams at reduced treatment time.     

Micro ionization chamber dosimetry in IMRT verification: clinical implications of dosimetric errors in the PTV.

BACKGROUND AND PURPOSE: Absolute dose measurements for Intensity Modulated Radiotherapy (IMRT) beamlets is difficult due to the lack of lateral electron equilibrium. Recently we found that the absolute dosimetry in the penumbra region of the IMRT beamlet, can suffer from significant errors (Capote et al., Med Phys 31 (2004) 2416-2422). This work has the goal to estimate the error made when measuring the Planning Target Volume's (PTV) absolute dose by a micro ion chamber (microIC) in typical IMRT treatment. The dose error comes from the assumption that the dosimetric parameters determining the absolute dose are the same as for the reference conditions. MATERIALS AND METHODS: Two IMRT treatment plans for common prostate carcinoma case, derived by forward and inverse optimisation, were considered. Detailed geometrical simulation of the microIC and the dose verification set-up was performed. The Monte Carlo (MC) simulation allows us to calculate the delivered dose to water and the dose delivered to the active volume of the ion chamber. However, the measured dose in water is usually derived from chamber readings assuming reference conditions. The MC simulation provides needed correction factors for ion chamber dosimetry in non reference conditions. RESULTS: Dose calculations were carried out for some representative beamlets, a combination of segments and for the delivered IMRT treatments. We observe that the largest dose errors (i.e. the largest correction factors) correspond to the smaller contribution of the corresponding IMRT beamlets to the total dose delivered in the ionization chamber within PTV. CONCLUSION: The clinical impact of the calculated dose error in PTV measured dose was found to be negligible for studied IMRT treatments.     

Simulation of realistic linac motion improves the accuracy of a Monte Carlo based VMAT plan QA system

Purpose

To investigate the use of a software-based pre-treatment QA system for VMAT, which incorporates realistic linac motion during delivery.

Methods

A beam model was produced using the GATE platform for GEANT4 Monte Carlo dose calculations. Initially validated against static measurements, the model was then integrated with a VMAT delivery emulator, which reads plan files and generates a set of dynamic delivery instructions analogous to the linac control system. Monte Carlo simulations were compared to measurements on dosimetric phantoms for prostate and head and neck VMAT plans. Comparisons were made between calculations using fixed control points, and simulations of continuous motion utilising the emulator. For routine use, the model was incorporated into an automated pre-treatment QA system.

Results

The model showed better agreement with measurements when incorporating linac motion: mean gamma pass (Γ < 1) over 5 prostate plans was 100.0% at 3%/3 mm and 97.4% at 2%/2 mm when compared to measurement. For the head and neck plans, delivered to the anatomical phantom, gamma passes were 99.4% at 4%/4 mm and 94.94% at 3%/3 mm. For example simulations within patient CT data, gamma passes were observed which are within our centre’s tolerance for pre-treatment QA.

Conclusions

Through comparison to phantom measurements, it was found that the incorporation of a realistic linac motion improves the accuracy of the model compared to the simulation of fixed control points. The ability to accurately calculate dose as a second check of the planning system, and determine realistic delivery characteristics, may allow for the reduction of machine-based pre-treatment plan QA for VMAT.     

基于解剖在线测量的调强放疗三维剂量验证系统测试与应用

目的 测试基于患者解剖图像和矩阵探测器在线测量进行重建(RDBMOM)的调强放疗(IMRT)三维剂量验证系统的准确性,评估其临床应用可行性。方法 分别在体模设计规则野和非规则野测试计划,同时以指形电离室和二维电离室阵列测量各测试例的点剂量和平面剂量分布,评估RDBMOM系统剂量重建精度。选择 2例鼻咽癌IMRT计划做RDBMOM验证,分析验证结果的临床应用意义。结果 与指形电离室点测量结果比较,RDBMOM系统对各测试例的重建剂量偏差均<1%(3 cm×3 cm小野除外),IMRT测试例的重建剂量偏差最大为2.12%。与电离室阵列测量比较,RDBMOM重建的测量平面内离轴剂量分布曲线符合良好,两者比较的γ通过率(3%/3 mm)为 94.56%~100%。2例IMRT计划的RDBMOM验证结果整体γ通过率>99%;计划靶体积γ通过率>98%且 D₉₅误差<0.4%,腮腺和晶体平均剂量的最大误差分别达2.97%和59.58%。结论 测试系统剂量重建精度可满IMRT验证要求,并能给出与患者解剖结构相关的体积剂量误差与误差位置等信息,有利于评估其对临床的影响。     

Quality assurance of patient dosimetry in boron neutron capture therapy

The verification of the correctness of planned and executed treatments is imperative for safety in radiotherapy. The purpose of the present work is to describe and evaluate the quality assurance (QA) procedures for patient dosimetry implemented at the boron neutron capture therapy (BNCT) facility at Studsvik, Sweden. The dosimetric complexity of the mixed neutron-photon field during BNCT suggests a careful verification of routine procedures, specifically the treatment planning calculations. In the present study, two methods for QA of patient dosimetry are presented. The first is executed prior to radiotherapy and involves an independent check of the planned absorbed dose to be delivered to a point in the patient for each treatment field. The second QA procedure involves in vivo dosimetry measurements using post-treatment activation analysis. Absorbed dose conversion factors taking the difference in material composition and geometry of the patient and the PMMA phantom used for reference dosimetry were determined using the Monte Carlo method. The agreement of the QA procedure prior to radiotherapy reveals an acceptably small deviation for 60 treatment fields of ±4.2% (1 SD), while the in vivo dosimetry method presented may benefit from improvements, as the deviations observed were quite substantial (±12%, 1 SD), and were unlikely to be due to actual errors in the clinical dosimetry.