The effects of variations in the density and composition of eye materials on ophthalmic brachytherapy dosimetry.

In ophthalmic brachytherapy dosimetry, it is common to consider the water phantom as human eye anatomy. However, for better clinical analysis, there is a need for the dose determination in different parts of the eye. In this work, a full human eye is simulated with MCNP-4C code by considering all parts of the eye, i.e., the lens, cornea, retina, choroid, sclera, anterior chamber, optic nerve, and bulk of the eye comprising vitreous body and tumor. The average dose in different parts of this full model of the human eye is determined and the results are compared with the dose calculated in water phantom. The central axes depth dose and the dose in whole of the tumor for these 2 simulated eye models are calculated as well, and the results are compared.

Dose calculation and in-phantom measurement in BNCT using response matrix method.

In-phantom measurement of physical dose distribution is very important for Boron Neutron Capture Therapy (BNCT) planning validation. If any changes take place in therapeutic neutron beam due to the beam shaping assembly (BSA) change, the dose will be changed so another group of simulations should be carried out for dose calculation. To avoid this time consuming procedure and speed up the dose calculation to help patients not wait for a long time, response matrix method was used. This procedure was performed for neutron beam of the optimized BSA as a reference beam. These calculations were carried out using the MCNPX, Monte Carlo code. The calculated beam parameters were measured for a SNYDER head phantom placed 10 cm away from beam the exit of the BSA. The head phantom can be assumed as a linear system and neutron beam and dose distribution can be assumed as an input and a response of this system (head phantom), respectively. Neutron spectrum energy was digitized into 27 groups. Dose response of each group was calculated. Summation of these dose responses is equal to a total dose of the whole neutron/gamma spectrum. Response matrix is the double dimension matrix (energy/dose) in which each parameter represents a depth-dose resulted from specific energy. If the spectrum is changed, response of each energy group may be differed. By considering response matrix and energy vector, dose response can be calculated. This method was tested for some BSA, and calculations show statistical errors less than 10%.

Suggesting a new design for multileaf collimator leaves based on Monte Carlo simulation of two commercial systems.

Due to intensive use of multileaf collimators (MLCs) in clinics, finding an optimum design for the leaves becomes essential. There are several studies which deal with comparison of MLC systems, but there is no article with a focus on offering an optimum design using accurate methods like Monte Carlo. In this study, we describe some characteristics of MLC systems including the leaf tip transmission, beam hardening, leakage radiation and penumbra width for Varian and Elekta 80-leaf MLCs using MCNP4C code. The complex geometry of leaves in these two common MLC systems was simulated. It was assumed that all of the MLC systems were mounted on a Varian accelerator and with a similar thickness as Varian's and the same distance from the source. Considering the obtained results from Varian and Elekta leaf designs, an optimum design was suggested combining the advantages of three common MLC systems and the simulation results of this proposed one were compared with the Varian and the Elekta. The leakage from suggested design is 29.7% and 31.5% of the Varian and Elekta MLCs. In addition, other calculated parameters of the proposed MLC leaf design were better than those two commercial ones. Although it shows a wider penumbra in comparison with Varian and Elekta MLCs, taking into account the curved motion path of the leaves, providing a double focusing design will solve the problem. The suggested leaf design is a combination of advantages from three common vendors (Varian, Elekta and Siemens) which can show better results than each one. Using the results of this theoretical study may bring about superior practical outcomes.

Monte Carlo estimation of photoneutrons contamination from high-energy X-ray medical accelerators in treatment room and maze: a simplified model.

Despite all advantages associated with high-energy radiotherapy to improve therapeutic gain, the production of photoneutron via interaction of high-energy photons with high atomic number (Z) materials increases undesired dose to the patient and staff. Owing to the limitation and complication of experimental neutron dosimetry in mixed beam environment, including photon and neutron, the Monte Carlo (MC) simulation is a gold standard method for calculation of photoneutron contaminations. On the other hand, the complexity of treatment head makes the MC simulation more difficult and time-consuming. In this study, the possibility of using a simplified MC model for the simulation of treatment head has been investigated using MCNP4C general purpose MC code. As a part of comparative assessment strategy, the fluence, average energy and dose equivalent of photoneutrons were estimated and compared with other studies for several fields and energies at different points in treatment room and maze. The mean energy of photoneutrons was 0.17, 0.19 and 0.2 MeV at the patient plan for 10, 15 and 18 MeV, respectively. The calculated values differed, respectively, by a factor of 1.4, 0.7 and 0.61 compared with the reported measured data for 10, 15 and 18 MeV. Our simulation results in the maze showed that the neutron dose equivalent is attenuated by a factor of 10 for every 4.6 m of maze length while the related factor from Kersey analytical method is 5 m. The neutron dose equivalent was 4.1 mSv Gy(-1) at the isocentre and decreased to 0.79 mSv Gy(-1) at a distance of 100 cm away from the isocentre for 40 x 40 cm(2). There is good agreement between the data calculated using simplified model in this study and measurements. Considering the reported high uncertainties (up to 50%) in experimental neutron dosimetry, it can be concluded that the simplified model can be used as a useful tool for estimation of photoneutron contamination associated with high-energy photon radiotherapy.

Measurement of arm blood pressure using different oscillometry manometers compared to auscultatory readings.

Five different semiautomatic manometers were tested, where oscillometry is the measuring principle. Three of the manometers (Omron R4, A&D UB 322 and Braun) were wrist manometers, where the occluding cuff is placed around the volar surface of the wrist. Two of the manometers (A&D UA 777 and Omron M4) measure on the upper arm. The investigation included 72 patients with systolic blood pressure (SBP) ranging between 110 and 200, and diastolic blood pressure (DBP) between 62 and 114 mmHg. Forty-five of the subjects were on antihypertensive medication when the manometer tests were carried out. Each of the manometers was tested with double measurements of blood pressure against 2 x 2 auscultatory measurements done before and after the semiautomatic readings. The auscultatory measurements are all performed by the same observer, who was blinded for the measurements with semiautomatic manometers. The mean difference between the oscillometric recordings compared to auscultatory measurements varied from +1.2 to -8.5 mmHg for SBP and from -0.5 to -8.3 mmHg for DBP. However, the interindividual differences varied considerable with standard deviation of the difference varying from 8 to 18 mmHg for SBP with the highest values for wrist manometers. Concerning DBP, the standard deviation of difference for all five manometers was between 6 and 8 mmHg, with the highest values for wrist manometers. None of the tested manometers fulfilled the criteria for grading A or B in the previously introduced grading by the British Hypertension Society. To conclude, the upper-arm manometers have a measuring accuracy for SBP a little higher than that of the wrist manometers, while there is no bigger difference in the measuring accuracy of DBP. The most important point is that the measuring accuracy in a single patient is unpredictable. If home readings are prepared, a test of the accuracy against auscultatory recordings should be done in every single patient. In the clinical wards, it is important to be aware of the measuring accuracy if oscillometric measurements are introduced replacing auscultatory measurements.