Currently, most clinical range-modulated proton beams are assumed to have a fixed overall relative biological effectiveness (RBE) of 1.1. However, it is well known that the RBE increases with depth in the spread-out Bragg peak (SOBP) and becomes about 10% higher than mid-SOBP RBE at 2 mm from the distal edge (Paganetti 2003 Technol. Cancer Res. Treat. 2 413-26) and can reach values of 1.3-1.4 in vitro at the distal edge (Robertson et al 1975 Cancer 35 1664-77, Courdi et al 1994 Br. J. Radiol. 67 800-4). We present a fast method for applying a variable RBE correction with linear energy transfer (LET) dependent tissue-specific parameters based on the alpharef/betaref ratios suitable for implementation in a treatment planning system. The influence of applying this variable RBE correction on a clinical multiple beam proton dose plan is presented here. The treatment plan is evaluated by RBE weighted dose volume histograms (DVHs) and the calculation of tumour control probability (TCP) and normal tissue complication probability (NTCP) values. The variable RBE correction yields DVHs for the clinical target volumes (CTVs), a primary advanced hypopharynx cancer and subclinical disease in the lymph nodes, that are slightly higher than those achieved by multiplying the absorbed dose with RBE=1.1. Although, more importantly, the RBE weighted DVH for an organ at risk, the spinal cord is considerably increased for the variable RBE. As the spinal cord in this particular case is located 8 mm behind the planning target volume (PTV) and hence receives only low total doses, the NTCP values are zero in spite of the significant increase in the RBE weighted DVHs for the variable RBE. However, high NTCP values for the non-target normal tissue were obtained when applying the variable RBE correction. As RBE variations tend to be smaller for in vivo systems, this study-based on in vitro data since human tissue RBE values are scarce and have large uncertainties-can be interpreted as showing the upper limits of the possible effects of utilizing a variable RBE correction. In conclusion, the results obtained here still indicate a significant difference in introducing a variable RBE compared to applying a generic RBE of 1.1, suggesting it is worth considering such a correction in clinical proton therapy planning, especially when risk organs are located immediately behind the target volume.
Purpose: To explore the possibility of separately quantifying the leakage and scatter components of the peripheral radiation dose resulting from Leksell GammaKnife R Perfexion™ radiotherapy. Methods and Materials: In this study, peripheral dose was defined as the dose arising at points outside the paths of the primary beams. Consider a spherical skull phantom centered at Leksell coordinates (100,100,100). Peripheral dose may arise from: (1) Inphantom scattered radiation originating from primary beams; (2) Radiation first scattered within the collimator structure (3) Photons undergoing multiple scattering and exiting through adjacent collimator openings and (4) Leakage radiation that passes directly through the collimator body. Contributions (1) and (2) were separately quantified via Monte‐Carlo simulations. Types (3) and (4) were measured using EBTII Gafchromic film inserts and placing the sources in“beam‐off” position (i.e. between collimator openings). Additionally, the total peripheral dose arising from a 16‐mm shot was measured using film. The combination of simulated type_1 and type_2 doses and measured type_3 and type_4 were compared to the measured total dose at 40 mm superior and inferior to the isocentre. Results: At the isocentre of the 16‐mm shot, the dose‐rate determined by Monte Carlo simulations agreed with measured dose‐rates to 0.8%, thereby establishing consistency between measurement and simulation. The simulated sum of Type_1 and Type_2 contributions at 40 mm inferior and superior to the isocentre was 0.40% and 0.35% of the isocentre dose‐rate, respectively. Of those totals, 87% was Type_1 a 13% came from Type 2. The measured total peripheral dose‐rate was 0.40% and 0.38% at 40 mm inferiorly and superiorly, respectively. The measured Type_3 and Type_4 dose‐rates were less than 0.01% of the isocentre dose‐rate in either direction. Conclusion: The results of this study indicate that peripheral dose arising from a single 16‐mm shot arises primarily from Type_1, with <0.01% contribution from leakage radiation.
The American Association of Physicists in Medicine (AAPM) formed Task Group 178 (TG‐178) to perform the following tasks: review in‐phantom and in‐air calibration protocols for gamma stereotactic radiosurgery (GSR), suggest a dose rate calibration protocol that can be successfully utilized with all gamma stereotactic radiosurgery (GSR) devices, and update quality assurance (QA) protocols in TG‐42 (AAPM Report 54, 1995) for static GSR devices. The TG‐178 report recommends a GSR dose rate calibration formalism and provides tabulated data to implement it for ionization chambers commonly used in GSR dosimetry. The report also describes routine mechanical, dosimetric, and safety checks for GSR devices, and provides treatment process quality assurance recommendations. Sample worksheets, checklists, and practical suggestions regarding some QA procedures are given in appendices. The overall goal of the report is to make recommendations that help standardize GSR physics practices and promote the safe implementation of GSR technologies.
Fler och er anvander idag kohesiva modeller vid simulering av limfogar mednita element analys (FEA). Ett stort problem ar emellertid bristen pa bra ochdetaljerad materialdata. Denna rapport beskriv ...
The aim of this study is to improve the characterization and modeling of the radiation field surrounding the Leksell Gamma Knife-Perfexion. The improved characterization of the radiation field enables more accurate shielding calculations to be performed for the areas adjacent to the treatment room. With the aid of a high-purity germanium detector and a satellite dose rate meter, gamma-ray spectra and ambient dose equivalent data were acquired at various locations in the field of a Leksell Gamma Knife unit in a treatment room at Karolinska University Hospital, Sweden. These measurements were utilized to validate the results of the PEGASOS Monte Carlo simulation system with a PENELOPE kernel. The levels of the radiation that passes through the shielding of the machine (leakage radiation) are shown to be much lower than what is suggested by various bodies, e.g. the National Council on Radiation Protection and Measurements, to be used when calculating radiation shielding barriers.The results clearly indicate that Monte Carlo simulations may be used in structural shielding design calculations for gamma rays from the Leksell Gamma Knife.
<i>Background:</i> Gamma knife surgery (GKS) is used at subnecrotic doses for temporal lobe epilepsy (TLE) treatment. Rat models of TLE have been used to probe the mechanisms underlying GKS. Previous GKS studies on rats have used the Leksell GammaPlan® (LGP) treatment planning system to determine the irradiation time to achieve the dose to deliver. Since LGP is not designed for such small structures, it is important to calibrate the system for the rat brain. <i>Methods:</i> We have used a Monte Carlo simulation (MCS) radiation transport scheme, with CT data as anatomical and tissue-specific information, to simulate the dose distribution in a rat brain when using a Leksell Gamma Knife®. <i>Results:</i> We show how dose distributions obtained by MCS quantitatively compare to those predicted by LGP, and discuss whether LGP should be used for studies involving rats. The energy deposited when using the 4-mm collimators was calculated for targets on both sides of the rat brain in the dorsal hippocampus, which allowed us to determine the exact time to irradiate rats with a given dose. <i>Conclusion:</i> The MCS method used in this study can easily be used for future GKS studies on small animals when accurate dose distributions are required.
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