| 1. |
- Källman, Hans-Erik, PhD, 1960-, et al.
(författare)
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Toward automated and personalized organ dose determination in CT examinations : A comparison of two tissue characterization models for Monte Carlo organ dose calculation with a Therapy Planning System
- 2019
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Ingår i: Medical physics (Lancaster). - : WILEY. - 0094-2405 .- 2473-4209. ; 46:2, s. 1012-1023
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Tidskriftsartikel (refereegranskat)abstract
- Purpose: Computed tomography (CT) is a versatile tool in diagnostic radiology with rapidly increasing number of examinations per year globally. Routine adaption of the exposure level for patient anatomy and examination protocol cause the patients' exposures to become diversified and harder to predict by simple methods. To facilitate individualized organ dose estimates, we explore the possibility to automate organ dose calculations using a radiotherapy treatment planning system (TPS). In particular, the mapping of CT number to elemental composition for Monte Carlo (MC) dose calculations is investigated.Methods: Organ dose calculations were done for a female thorax examination test case with a TPS (Raystation, Raysearch Laboratories AB, Stockholm, Sweden) utilizing a MC dose engine with a CT source model presented in a previous study. The TPS's inherent tissue characterization model for mapping of CT number to elemental composition of the tissues was calibrated using a phantom with known elemental compositions and validated through comparison of MC calculated dose with dose measured with Thermo Luminescence Dosimeters (TLD) in an anthropomorphic phantom. Given the segmentation tools of the TPS, organ segmentation strategies suitable for automation were analyzed for high contrast organs, utilizing CT number thresholding and model-based segmentation, and for low contrast organs utilizing water replacements in larger tissue volumes. Organ doses calculated with a selection of organ segmentation methods in combination with mapping of CT numbers to elemental composition (RT model), normally used in radiotherapy, were compared to a tissue characterization model with organ segmentation and elemental compositions defined by replacement materials [International Commission on Radiological Protection (ICRP) model], frequently favored in imaging dosimetry.Results: The results of the validation with the anthropomorphic phantom yielded mean deviations from the dose to water calculated with the RT and ICRP model as measured with TLD of 1.1% and 1.5% with maximum deviations of 6.1% and 8.7% respectively over all locations in the phantom. A strategy for automated organ segmentation was evaluated for two different risk organ groups, that is, low contrast soft organs and high contrast organs. The relative deviation between organ doses calculated with the RT model and with the ICRP model varied between 0% and 20% for the thorax/upper abdomen risk organs.Conclusions: After calibration, the RT model in the TPS provides accurate MC dose results as compared to measurements with TLD and the ICRP model. Dosimetric feasible segmentation of the risk organs for a female thorax demonstrates a possibility for automation using the segmentation tool available in a TPS for high contrast organs. Low contrast soft organs can be represented by water volumes, but organ dose to the esophagus and thyroid must be determined using standardized organ shapes. The uncertainties of the organ doses are small compared to the overall uncertainty, at least an order of magnitude larger, in the estimates of lifetime attributable risk (LAR) based on organ doses. Large-scale and automated individual organ dose calculations could provide an improvement in cancer incidence estimates from epidemiological studies.
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| 2. |
- Ödén, Jakob, et al.
(författare)
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Spatial correlation of linear energy transfer and relative biological effectiveness with treatment related toxicities following proton therapy for intracranial tumors
- 2020
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Ingår i: Medical physics (Lancaster). - : Wiley. - 0094-2405 .- 2473-4209. ; 47:2, s. 342-351
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Tidskriftsartikel (refereegranskat)abstract
- Purpose: The enhanced relative biological effectiveness (RBE) at the end of the proton range might increase the risk of radiation-induced toxicities. This is of special concern for intracranial treatments where several critical organs at risk (OARs) surround the tumor. In the light of this, a retrospective analysis of dose-averaged linear energy transfer (LETd) and RBE-weighted dose (DRBE) distributions was conducted for three clinical cases with suspected treatment related toxicities following intracranial proton therapy. Alternative treatment strategies aiming to reduce toxicity risks are also presented.Methods: The clinical single-field optimized (SFO) plans were recalculated for 81 error scenarios with a Monte Carlo dose engine. The fractionation DRBE was 1.8 Gy (RBE) in 28 or 30 fractions assuming a constant RBE of 1.1. Two LETd- and α/β-dependent variable RBE models were used for evaluation, including a sensitivity analysis of the α/β parameter. Resulting distributions of DRBE and LETd were analyzed together with normal tissue complication probabilities (NTCPs). Subsequently, four multi-field optimized (MFO) plans, with an additional beam and/or objectives penalizing protons stopping in OARs, were created to investigate the potential reduction of LETd, DRBE and NTCP.Results: The two variable RBE models agreed well and predicted average RBE values around 1.3 in the toxicity volumes, resulting in increased near-maximum DRBE of 7-11 Gy (RBE) compared to RBE=1.1 in the nominal scenario. The corresponding NTCP estimates increased from 0.8%, 0.0% and 3.7% (RBE=1.1) to 15.5%, 1.8% and 45.7% (Wedenberg RBE model) for the three patients, respectively. The MFO plans generally allowed for LETd, DRBE and NTCP reductions in OARs, without compromising the target dose. Compared to the clinical SFO plans, the maximum reduction of the near-maximum LETd was 56%, 63% and 72% in the OAR exhibiting the toxicity for the three patients, respectively.Conclusions: Although a direct causality between RBE and toxicity cannot be established here, high LETd and DRBE correlated spatially with the observed toxicities, whereas setup and range uncertainties had a minor impact. Individual factors, which might affect the patient-specific radiosensitivity, were however not included in these calculations. The MFO plans using both an additional beam and proton track-end objectives allowed the largest reductions in LETd, DRBE and NTCP, and might be future tools for similar cases.
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| 3. |
- Ahnesjö, Anders, et al.
(författare)
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Beam modeling and verification of a photon beam multisource model.
- 2005
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Ingår i: Medical Physics. - : Wiley. - 0094-2405. ; 32:6, s. 1722-37
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Tidskriftsartikel (refereegranskat)abstract
- Dose calculations for treatment planning of photon beam radiotherapy require a model of the beam to drive the dose calculation models. The beam shaping process involves scattering and filtering that yield radiation components which vary with collimator settings. The necessity to model these components has motivated the development of multisource beam models. We describe and evaluate clinical photon beam modeling based on multisource models, including lateral beam quality variations. The evaluation is based on user data for a pencil kernel algorithm and a point kernel algorithm (collapsed cone) used in the clinical treatment planning systems Helax-TMS and Nucletron-Oncentra. The pencil kernel implementations treat the beam spectrum as lateral invariant while the collapsed cone involves off axis softening of the spectrum. Both algorithms include modeling of head scatter components. The parameters of the beam model are derived from measured beam data in a semiautomatic process called RDH (radiation data handling) that, in sequential steps, minimizes the deviations in calculated dose versus the measured data. The RDH procedure is reviewed and the results of processing data from a large number of treatment units are analyzed for the two dose calculation algorithms. The results for both algorithms are similar, with slightly better results for the collapsed cone implementations. For open beams, 87% of the machines have maximum errors less than 2.5%. For wedged beams the errors were found to increase with increasing wedge angle. Internal, motorized wedges did yield slightly larger errors than external wedges. These results reflect the increased complexity, both experimentally and computationally, when wedges are used compared to open beams.
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| 4. |
- Källman, Hans-Erik, et al.
(författare)
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Source modeling for Monte Carlo dose calculation of CT examinations with a radiotherapy treatment planning system
- 2016
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Ingår i: Medical physics (Lancaster). - : Wiley. - 0094-2405. ; 43:11, s. 6118-6128
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Tidskriftsartikel (refereegranskat)abstract
- Purpose:Radiation dose to patients undergoing examinations with Multislice Computed Tomography (MSCT) as well as Cone Beam Computed Tomography (CBCT) is a matter of concern. Risk management could benefit from efficient replace rational dose calculation tools. The paper aims to verify MSCT dose calculations using a Treatment Planning System (TPS) for radiotherapy and to evaluate four different variations of bow-tie filter characterizations for the beam model used in the dose calculations.Methods:A TPS (RayStation™, RaySearch Laboratories, Stockholm, Sweden) was configured to calculate dose from a MSCT (GE Healthcare, Wauwatosa, WI, USA). The x-ray beam was characterized in a stationary position the by measurements of the Half-Value Layer (HVL) in aluminum and kerma along the principal axes of the isocenter plane perpendicular to the beam. A Monte Carlo source model for the dose calculation was applied with four different variations on the beam-shaping bow-tie filter, taking into account the different degrees of HVL information but reconstructing the measured kerma distribution after the bow-tie filter by adjusting the photon sampling function. The resulting dose calculations were verified by comparison with measurements in solid water as well as in an anthropomorphic phantom.Results:The calculated depth dose in solid water as well as the relative dose profiles was in agreement with the corresponding measured values. Doses calculated in the anthropomorphic phantom in the range 26–55 mGy agreed with the corresponding thermo luminescence dosimeter (TLD) measurements. Deviations between measurements and calculations were of the order of the measurement uncertainties. There was no significant difference between the different variations on the bow-tie filter modeling.Conclusions:Under the assumption that the calculated kerma after the bow-tie filter replicates the measured kerma, the central specification of the HVL of the x-ray beam together with the kerma distribution can be used to characterize the beam. Thus, within the limits of the study, a flat bow-tie filter with an HVL specified by the vendor suffices to calculate the dose distribution. The TPS could be successfully configured to replicate the beam movement and intensity modulation of a spiral scan with dose modulation, on the basis of the specifications available in the metadata of the digital images and the log file of the CT.
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