Dose Rate Variation Research Articles

33. Comparative study of modulation for VMAT treatments planned on Eclipse and Raystation: Complexity index and experimental correlation

Introduction Volumetric modulated arc therapy technique allows to shape the isodose distribution around irregular or complex target and spare organ at risks. Those various dose distribution complexity will directly be translated in the level of modulation to be applied to the fluence and thus to the delivery system in terms of mechanical variations such as gantry speed, leaf movements, number of segments or dose rate variation. These last years, several studies have been carried out for the definition of fitted indexes that could be correlated to a plan complexity with regards to patient QA results [1] , [2] . In this context, two points have motivated this study: – Using existing indexes and defining new ones to compare VMAT plan issued from two different TPS. – Correlate these indexes to the experimental patient specific QA results. Methods In this study, two types of indexes have been used: the classical one described in the literature and simple derived indexes. The first derived index is based on the distance between two opposite leaves for each beam (not under a jaw) calculated for every control point of every arc of the plan. The first classical index is the MCSV [3] which the value is decreasing when the complexity is increasing. Among all VMAT plans for various indications, 450 VMAT plans have been analyzed (nearly 2/3 from Eclipse and 1/3 from Raystation) all of them treated on the same accelerator (Novalis TX). Patient specific QA have been performed with 2D diodes measurements using the ArcCheck phantom and a Pinpoint ionization chamber measurement located in the center of the ArcCheck. The analysis have been performed with local and global gamma indexes of 3%/3 mm with threshold set at 10% of the maximal dose. Results As a first step, we focused on the derived indexes and applied them to the Eclipse/Raystation plans. The median of the tip to tip distance of opposite leaves for all plans is about 10 mm and 18 mm, respectively, for Eclipse and Raystation. The mean proportion of opposite leaves having a tip to tip distance less than 10 mm is about 32% and 20%, respectively, for Eclipse and Raystation. As a second step we analyzed the exact same plans with the MCSv index. Results show a mean MCSV value of 0.25 and 0.39 and standard deviation of 0.08 (32%) and 0.13 (33%), respectively, for Eclipse and Raystation. These results show that the derived index we developed is in a good agreement with the standard index MCSV. Secondly, the results tend to show that plans, issued from Eclipse show or higher degree of modulation than those from Raystation. These tendency need to be correlated with the patient specific QA results. Conclusions The work already achieved in the framework of this study using plans performed according to our clinical standards, has showed clear differences in the complexity with regards to the TPS used. These results are confirmed by standard literature metric, such as the so called MCSv, but will be deeply investigated using other metrics. These results seems to corroborate a higher modulated solution on the Eclipse side that may lead to a more difficult plan to deliver, this has to be correlated with experimental QA.

Commissioning dosimetry and in situ dose mapping of a semi-industrial Cobalt-60 gamma-irradiation facility using Fricke and Ceric-cerous dosimetry system and comparison with Monte Carlo simulation data

Characterization of a 90 kCi (3330 TBq), semi-industrial, cobalt-60gamma irradiator was performed by commissioning dosimetry and in-situ dose mapping experiments with Ceric-cerous and Fricke dosimetry systems. Commissioning dosimetry was carried out to determine dose distribution pattern of absorbed dose in the irradiation cell and products. To determine maximum and minimum absorbed dose, overdose ratio and dwell time of the tote boxes, homogeneous dummy product (rice husk) with a bulk density of 0.13g/cm3 were used in the box positions of irradiation chamber. The regions of minimum absorbed dose of the tote boxes were observed in the lower zones of middle plane and maximum absorbed doses were found in the middle position of front plane. Moreover, as a part of dose mapping, dose rates in the wall positions and some selective strategic positions were also measured to carry out multiple irradiation program simultaneously, especially for low dose research irradiation program. In most of the cases, Monte Carlo simulation data, using Monte Carlo N-Particle eXtended code version MCNPX 2.7., were found to be in congruence with experimental values obtained from Ceric-cerous and Fricke dosimetry; however, in close proximity positions from the source, the dose rate variation between chemical dosimetry and MCNP was higher than distant positions.

Variance reduction technique in a beta radiation beam using an extrapolation chamber

This paper aims to show how the variance reduction technique “Geometry splitting/Russian roulette” improves the statistical error and reduces uncertainties in the determination of the absorbed dose rate in tissue using an extrapolation chamber for beta radiation. The results show that the use of this technique can increase the number of events in the chamber cavity leading to a closer approximation of simulation result with the physical problem. There was a good agreement among the experimental measurements, the certificate of manufacture and the simulation results of the absorbed dose rate values and uncertainties. The absorbed dose rate variation coefficient using the variance reduction technique “Geometry splitting/Russian roulette” was 2.85%.

Comparison of Homogeneous and Particulate Lung Dose Rates For Small Mammals.

Small, highly radioactive fragments of material incorporated into metallic matrices are commonly found at nuclear weapons test and accident sites and can be inhaled by wildlife. Inhaled particles often partition heterogeneously in the lungs, with aggregation occurring in the periphery of the lung, and are tenaciously retained. However, dose rates are typically calculated as if the material were homogeneously distributed throughout the entire organ. Here the authors quantify the variation in dose rates for alpha-, beta-, and gamma-emitting radionuclides with particle sizes from 0.01-150 μm (alpha) and 1-150 μm (beta, gamma) and considering three averaging volumes-the entire lung (64 cm), a 10-cm volume of tissue, and a 1-cm volume of tissue. Dose rates from beta-emitting particles (e.g., Sr) were approximately one order of magnitude higher than those from gamma-emitting radionuclides (e.g., Cs). Self-shielding within the particle, which reduces the dose rate to the surrounding tissue, was negligible for gammas and minor for betas. For alpha-emitting particles (e.g., Pu), self-shielding in larger particles is substantial, with >90% of emissions captured within particles of +20 μm diameter; but for smaller sizes of the respirable range of 0.01 to 5 μm, an average of 85% of the energy escapes the particle and is deposited in the surrounding tissues. These data provide more detail on respirable particles, which may remain lodged deep in the lung where they represent a considerable contribution to long-term lung dose rates. For practical dose rate calculation purposes, a graph of particle size vs. dose rates for plutonium-containing hot particles is provided. This study demonstrates one possible approach to dose assessments for biota in environments contaminated by radioactive particles, which may prove useful for those engaged in environmental radioprotection.

Evaluation of dose rate variation during very long term storage of RBMK-1500 reactor used fuel

The duration of dry storage of RBMK-1500 used fuel in casks is foreseen to be 50years. It is expected that a deep geological repository will be implemented in Lithuania after this storage period. Research activities on used nuclear fuel disposal in a deep geological repository have lasted for several decades and are still ongoing in different countries. However, until now there is no such repository in operation worldwide. Therefore, there is a possibility that due to the various reasons the deep geological repository cannot be constructed in the planned time. In such a case, an option of used fuel storage beyond 50years needs to be considered. Various nuclear research centres in different countries are already performing investigations on subjects related to extension of the used fuel storage period. This paper presents modelling results of the total dose rates for CASTOR®RBMK-1500 and CONSTOR®RBMK-1500 casks during long-term storage (up to 300years). Also, contributions of neutron and gamma doses to the total dose rate, variations of the neutron and gamma doses and influence of different gamma and neutron energy groups on dose rates during long-term storage are analysed.

Variations of radon concentration and gamma dose rate on tectonics in sedimentary rocks (Bohemian Massif)

Variations of radon concentration and gamma dose rate on tectonics in sedimentary rocks (Bohemian Massif)

Radioprotection de l’entourage des patients traités à l’iode 131 pour des cancers différenciés de la thyroïde : comparaison de deux méthodes d’estimation des temps de restriction de contact

In France, it is required to give patients some written radiation safety advice following an iodine-131 therapy of thyroid. However, there is no regulatory methodology proposed to assess the restriction times required to limit radiation dose received by their family and by third persons. In this study, we compared two methods to perform the calculation of restriction times for differentiated thyroid cancer: the first one, called Global method, follows general recommendations and the second one, called PaDe method, provides customized recommendations based on the effective half-life measured for each patient during the 48h of hospitalization. For these simulations, the input parameters for both methods were dose constraints, contact patterns, final dose rate measurement at discharge of the patient, effective half-life and dose rate variation with distance. For the Global method, these parameters were set and were not changed. For the PaDe method, the effective half-life and the final dose rate were adjusted for each patient. Comparison between the two methods was carried out for 103 patients. For 68.0 % of the patients, no restrictions were necessary according to the PaDe method whereas the Global method required restrictions to be applied during up to 2 days. For almost 16.5 % of them, restriction times estimated with the PaDe method were systematically lower. For the remaining 15.5 %, at least one contact pattern restriction had to be increased according to the PaDe method. Furthermore, with the Global method, the same instructions were given to 97.0 % of the patients. These results showed the limits of the Global method which does not consider individually tailored biological data. Eventually, the use of the personalized PaDe method optimizes the radiation protection by improving the well being of the patient or reinforcing the family and third person's protection.

NEW MODEL FOR MINES AND TRANSPORTATION TUNNELS EXTERNAL DOSE CALCULATION USING MONTE CARLO SIMULATION.

In this work, a new methodology is developed based on Monte Carlo simulation for tunnels and mines external dose calculation. Tunnels external dose evaluation model of a cylindrical shape of finite thickness with an entrance and with or without exit. A photon transportation model was applied for exposure dose calculations. A new software based on Monte Carlo solution was designed and programmed using Delphi programming language. The variation of external dose due to radioactive nuclei in a mine tunnel and the corresponding experimental data lies in the range 7.3-19.9%. The variation of specific external dose rate with position in, tunnel building material density and composition were studied. The given new model has more flexible for real external dose in any cylindrical tunnel structure calculations.

Assessment of monitor unit limiting strategy using volumetric modulated arc therapy for cancer of hypopharynx

PurposeTo quantify relative merit of MU deprived plans against freely optimized plans in terms of plan quality and report changes induced by progressive resolution optimizer algorithm (PRO3) to the dynamic parameters of RapidArc. Materials and methodsTen cases of carcinoma hypopharynx were retrospectively planned in three phases without using MU tool. Replicas of these baseline plans were reoptimized using “Intermediate dose” feature and “MU tool” to reduce MUs by 20%, 35%, and 50%. Overall quality indices for target and OAR, integral dose, dose-volume spread were assessed. All plans were appraised for changes induced in RapidArc dynamic parameters and pre-treatment quality assurance (QA). ResultsWith increasing MU reduction strength (MURS), MU/Gy values reduced, for all phases with an overall range of 8.6–34.7%; mean dose rate decreased among plans of each phase, phase3 plans recorded greater reductions. MURS20% showed good trade-off between MUs and plan quality. Dose-volume spread below 5Gy was higher for baseline plans while lower between 20 and 35Gy. Integral dose was lower for MURS0%, not exceeding 1.0%, compared against restrained plans. Mean leaf aperture and control point areas increased systematically, correlated negatively with increasing MURS. Absolute delta dose rate variations were least for MURS0%. MU deprived plans exhibited GAI (>93%), better than MURS0% plans. ConclusionBaseline plans are superior to MU restrained plans. However, MURS20% offers equivalent and acceptable plan quality with mileage of MUs, improved GAI for complex cases. MU tool may be adopted to tailor treatment plans using PRO3.

Dosimetric characterization of a novel dual-energy medical linear accelerator without a flattening filter

Objective To study the dosimetric characteristics of flattened and flattening filter free (FFF) beams with 6 MV and 10 MV photon energy using a novel dual-energy medical linear accelerator (Elekta Versa HDTM), to identify the dosimetric characteristics and advantages of FFF beams, and to provide a basis for their clinical application. Methods The percentage depth dose (PDD), profiles/dose rate of off-axis ratio (OAR), field size, penumbral width, dose out of the fields, collimator scatter factor (Sc), and total scatter factor (Sc, p) were compared between flattened and FFF beams. Results (1) After beam energy matching, the FFF beams had the same beam energy with the flattened beams. The matching error of PDD at a depth of 10 cm was less than 1% between fields.(2) The FFF beams had a smaller variation in dose rate of OAR with the depth than the flattened beams.(3) The FFF beams had smaller variations in field size and penumbral width than the flattened beams. Moreover, the penumbral width of the FFF beams increased with the increasing field size or depth. The FFF beams had a lower dose out of the fields than the flattened beams.(4) The FFF beams had smaller variations in Sc and Sc, p with the field size and depth than the flattened beams. Conclusions Removal of the flattening filter can substantially improve the dose rate, shorten radiotherapy time, and reduce leakage and scattering of the head. The dosimetric advantages make the FFF beams appropriate for clinical treatment. Key words: Linear accelerator; Flattening filter; Flattening filter free

Dedicated high dose rate 192Ir brachytherapy radiation fields for in vitro cell exposures at variable source-target cell distances: killing of mammalian cells depends on temporal dose rate fluctuation

Afterloading brachytherapy is conducted by the stepwise movement of a radioactive source through surgically implanted applicator tubes where at predefined dwell positions calculated dwell times optimize spatial dose delivery with respect to a planned dose level. The temporal exposure pattern exhibits drastic fluctuations in dose rate at a given coordinate and within a single treatment session because of the discontinuous and repeated source movement into the target volume. This could potentially affect biological response. Therefore, mammalian cells were exposed as monolayers to a high dose rate 192Ir source by utilizing a dedicated irradiation device where the distance between a planar array of radioactive source positions and the plane of the cell monolayer could be varied from 2.5 mm to 40 mm, thus varying dose rate pattern for any chosen total dose.The Gammamed IIi afterloading system equipped with a nominal 370 GBq (10 Ci) 192-Ir source was used to irradiate V79 Chinese hamster lung fibroblasts from both confluent and from exponential growth phase with dose up to 12 Gy (at room temperature, total exposure not exceeding 1 h). For comparison, V79 cells were also exposed to 6 MV x-rays from a clinical linear accelerator (dose rate of 2.5 Gy min−1). As biological endpoint, cell survival was determined by standard colony forming assay. Dose measurements were conducted with a diamond detector (sensitive area 7.3 mm2), calibrated by means of 60Co radiation. Additionally, dose delivery was simulated by Monte Carlo calculations using the EGSnrc code system. The calculated secondary electron fluence spectra at the cell location did not indicate a significant change of radiation quality (i.e. higher linear energy transfer) at the lower distances.Clonogenic cell survival curves obtained after brachytherapy exhibited an altered biological response compared to x-rays which was characterized by a significant reduction of the survival curve shoulder when dose rate fluctuations were high. Therefore, also for the time scale of the present investigation, cellular effects of radiation are not invariant to the temporal pattern in dose rate. We propose that with high dose rate variation the cells activate less efficiently their DNA damage response than after continuous irradiation.

Temporal Variations of Air Dose Rates in East Fukushima During Japanese Fiscal Years 2012 and 2013.

Temporal variations of ambient air dose rates in eastern Fukushima prefecture during Japanese fiscal years 2012 and 2013 are analyzed. The average overall variation rate of air dose rates in east Fukushima during the examined period is found to be 0.49 (51% down) compared to the theoretically predicted value 0.65 (35% down) based on physical decay of radioactive cesium nuclides. On average, local dose rates declined almost linearly for the relatively short period. Temporal characteristics of air dose rates may be classified into variation rates, peaks, spikes, and oscillations. During the examined period, a typical dose-rate curve formed a long-term peak in summer that lasted one through a few months as well as a long-term spike in winter that lasted likewise. Otherwise, occasional short-term peaks and short-term spikes, in addition to long-term oscillations, were observed. Air dose rates may be effectively modulated at short timescales mainly by precipitation. Moreover, it is likely that winds may oscillate air dose rates due to resuspension of radio-dusts.

Variation of ambient gamma dose rate and indoor radon/thoron concentration in different villages of Udhampur district, Jammu and Kashmir State, India

The exposure of natural background radiation imparts a major contribution to inhalation doses received by the public, and its amount depends on the lithology, altitude, and building construction materials. The preliminary results of indoor and outdoor gamma-ray dose rate of Udhampur district, Jammu and Kashmir, India are presented. Indoor radon and thoron gas concentration have also been measured by using the LR-115 based pin-hole detectors in the same villages and the estimated concentrations are within the recommended level of the International Commission of Radiological Protection. For the outdoor environment, the minimum and maximum gamma dose rate were 0.06 and 0.4 μSvh−1, whereas for indoor environment, the minimum and maximum gamma dose rates were 0.09 and 0.21 μSvh−1, respectively. Effects of lithology on indoor radon/thoron concentration and also on gamma dose rates have also been investigated. The average values of annual effective dose from background gamma radiations were well within the safe limit and does not cause health hazard to the inhabitants.

P34. Configuration, validation and evaluation of the dose calculation algorithm Acuros XB

Purpose The purpose of this study is to implement dosimetric data libraries in a dose calculation algorithm Acuros XB (developed by Varian) in a treatment planning system, to validate the libraries and evaluate the performances of the algorithm with several tests in order to be used in the radiotherapy department. Materials and Methods Acuros XB (AXB) is implanted in the Eclipse TPS Version 10. The methodology used is that of the ESTRO booklet n°7 for the calculation of dose and monitor units (MU) with several levels of tests (level 1, 2 and 3 for static techniques, non-IMRT photon beams and one test with RapidArc technique). Level 1 tests assess AXB in simple configurations (square, rectangular fields) in absolute and relative dose, level 2 tests verify the accuracy consideration of beams modifiers (wedge filters, asymmetric fields), Level 3 tests, using the quality index methodology assess the modeling of the dose distribution with heterogeneity and scatter dose (small complex fields, thin central block) for 6 and 15 MV. Finally a test assesses AXB performances in a complex configuration (small fields, variation of dose rate, MLC movement) with the RapidArc technique. Dose calculations made by AXB on the Eclipse TPS are comparing to measurements which are the references in the same conditions. The tolerances fixed are those presented in the reports of the ESTRO 7 (static techniques, non-IMRT) and ESTRO 9 (IMRT and RapidArc). Main Results and Discussion The calculations made by AXB are in agreement with the measurements for all tests level. Some differences are observed, their number is nevertheless low. Differences between the calculations and measurements are observed at the interface water–air interface; however, they are less important than those observed in AAA and PBC. Conclusion and Discussion Acuros XB models and calculates accurately the dose in simple configurations, with modifiers beams, and with the presence of heterogeneities for static irradiation techniques, and also with a more complex configuration with the RapidArc technique. Acuros XB can be used as a dose calculation algorithm in the Eclipse TPS at the radiotherapy department of the Institut Curie Paris.

20. Radiation protection following iodine-131 therapy for thyroid cancers: Comparison of two methods to calculate restriction times

Introduction In France, it is required to give patients some radiation safety advice following an iodine-131 therapy of thyroid. However, there is no regulatory methodology proposed to assess the restriction times required to limit radiation dose received by their family and by third persons. In this study, we compared two methods to perform the calculation of restriction times for differentiated thyroid cancer: the first one followed the recommendations of the Conseil Superieur d’Hygiene Publique de France (CSHPF) [1] and the second one, called the “patient-dependent” method (PaDe), used the effective half-life measured for each patient during the 48h-of hospitalization. Material and methods For these simulations, the input parameters were dose constraints, contact patterns, dose rate measurements and dose rate variation with distance. For the CSHPF method, these parameters were fixed and were not changed. For the PaDe method, the input parameters were mostly defined according to the literature [2] . The dose constraint for members of the public was set to 1 mSv. The effective half-life was calculated for each individual patient from a mono-exponential fit of dose rate measurements performed during the hospitalization. Using this effective half-life and the dose rate measured at the time of discharge, restriction times were computed for each contact pattern. Comparison between the two methods was carried out for 59 patients. Results For 63% of the patients (37/59), no restrictions were necessary according to the PaDe method whereas the CSHPF method required the restrictions to be applied during up to 2 days. For almost 20% of them (12/59), restriction times estimated with the PaDe method were systematically lower. For the remaining 17% (10/59), the CSHPF method underestimated the restrictions for at least one contact pattern. Furthermore, with the CSHPF method, the same instructions were given to 97% of the patients (57/59). These results showed the limits of the CSHPF method which does not consider individually tailored biological data and applies a theoretical inverse square law to derive dose rates at different distances. Conclusion In order to improve the well-being of the patient and to optimize the radiation protection, this study made us change our method to follow the PaDe computed recommendations.

Seasonal Variation of Terrestrial Gamma Radiation Dose and Evaluation of Annual Effective Dose in AECD Campus, Dhaka, Bangladesh

Background: Seasonal variations of environmental terrestrial gamma dose rates were measured at the Atomic Energy Centre Dhaka (AECD) campus in Shahbag Thana under Dhaka City from January to December 2015. Aim of the study: This kind of study is required to detect the natural and artificial radionuclides (if any) releasing from nuclear facilities in the country or from neighbouring countries. Materials and Methods: The measurement was performed using a portable High Purity Germanium (HPGe) detector (Model No. GEM25P4-83). The portable HPGe detector was placed at 1 meter above the ground facing downward and data acquisition time was 10,000 sec. Four locations were selected in the AECD campus to obtain the spectrum and those locations were marked out using global positioning system. Results: The average dose rate of the four locations from location-1 to location-4 were 0.428 ± 0.041, 0.344 ± 0.073, 0.355 ± 0.049 and 0.405 ± 0.054 µGy.h-1 respectively. The average dose rate of six seasons such as winter, spring, summer, rainy, autumn and late autumn were 0.377 ± 0.088, 0.421 ± 0.071, 0.385 ± 0.066, 0.358 ± 0.031, 0.281 ± 0.106 and 0.421 ± 0.023 µGy.h-1 respectively. The annual effective dose of the population due to the terrestrial gamma radiation were also calculated and it was varied from 0.208 - 0.627 mSv. The mean annual effective dose was found to be 0.472 ± 0.081 mSv which is comparable to the worldwide average value of 0.48 mSv.y-1. Conclusion: It was observed that winter, spring, summer and late autumn season’s dose rates are higher than those of rainy & autumn seasons.

Variations of radon concentration and gamma dose rate on tectonic structures in metamorphic rocks and granitoids (Bohemian Massif)

Variations of radon concentration and gamma dose rate on tectonic structures in metamorphic rocks and granitoids (Bohemian Massif)

Variations of radon concentration and gamma dose rate on tectonic structures in metamorphic rocks and granitoids (Bohemian Massif)

Variations of radon concentration and gamma dose rate on tectonic structures in metamorphic rocks and granitoids (Bohemian Massif)

SU‐F‐T‐68: Characterizes of Microdetectors in Electron Beam Dosimetry

Purpose:Electron beam dosimetry requires high resolution data due to finite range that can be accomplished with small volume detectors. The small‐field used in advance technologies in photon beam has created a market for microdetectors, however characteristics are significantly variable in photon beams and relatively unknown in electron beam that is investigated in this study.Methods:Among nearly 2 dozen microdetectors that have been investigated in small fields of photon beam, two popular detectors (microDiamond 60019 (PTW)) and W1 plastic scintillator detector (Standard Imaging)) that are tissue equivalent and have very small sensitive volume are selected. Electron beams from Varian linear accelerators were used to investigate dose linearity dose rate dependence, energy dependence, depth dose and profiles in a reference condition in a water phantom. For W1 that has its own Supermax electrometer point by point measurements were performed. For microDiamond, a PTW‐scanning tank was used for both scanning and point dose measurements.Results:W1 detector showed excellent dose linearity (r2 =1.0) from 5–500 MU either with variation of dose rate or beam energy. Similar findings were also observed for microdiamond with r2=1.0. Percent variations in dose/MU for W1 and microDiamond were 0.2–1.1% and 0.4–1.2%, respectively among dose rate and beam energy. This variation was random for microDiamond, whereas it decreased with beam energy and dose rate for W1. The depth dose and profiles were within ±1 mm for both detectors. Both detectors did not show any energy dependence in electron beams.Conclusion:Both microDiamond and W1 detectors provided superior characteristics of beam parameters in electron beam including dose, dose rate linearity and energy independence. Both can be used in electron beam except W1 require point by point measurements and microdiamond requires 1500 MU for initial quenching.

SU‐F‐T‐412: Spatiotemporal Distribution of Dose Rates in VMAT Plans

Purpose:To assess dose rates in volumetric modulated arc therapy (VMAT) plans.Methods:For dose rate analysis, using Monte Carlo methods, dose distributions are calculated in sectors of consecutive DICOM control points and each multiplied with the MU‐rate obtaining dose rate distributions for every voxel at every time‐point during a treatment fraction. Dose rate distributions were calculated for a clinical head and neck, cranial (2 Gy fraction dose) and a SBRT liver treatment case (5 Gy per fraction). To assess optimizer influence on dose rate distributions, five conventionally fractionated head and neck cases were optimized using the progressive resolution optimizer (PRO) versions 2 and 3 (Varian Medical Systems, Palo Alto, CA).Results:Firstly, histograms of dose rates over the course of treatment delivery in volumes of interest were generated for the PTV and spine or brainstem of the clinical treatment plans. Secondly, dose rates were assessed as color wash overlay on top of CT slices, allowing the user to explore the dose rate variation over time. For the cases considered, dose rates were spread out over a continuous spectrum with a mean value in the lower half. PRO3 generated plans showed higher maximum dose rates compared to PRO2 (699.80 cGy/min vs. 543.46 cGy/min) and modulation of MU rates in blocks as opposed to gradual modulation.Conclusion:Calculation and visualization of dose rate distributions in VMAT have been performed. Dose rates in clinical relevant VMAT cases are spread out over a continuous spectrum with a mean in the lower part for both conventional and SBRT test cases. Different optimization algorithms may lead to substantially different dose rate distributions in the same cases. The development of the Swiss Monte Carlo Plan framework has been partially supported by Varian Medical Systems