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A calculation of the correct dose in radiation therapy requires an accurate description of the radiation source because uncertainties in characterization of the linac photon spectrum are propagated through the dose calculations. Unfortunately, detailed knowledge of the initial electron beam parameters is not readily available, and many researchers adjust the initial electron fluence values by trial-and-error methods. The main goal of this work was to develop a methodology to characterize the fluence of initial electrons before they hit the tungsten target of an Elekta Precise medical linear accelerator. To this end, we used a Monte Carlo technique to analyze the influence of the characteristics of the initial electron beam on the distribution of absorbed dose from a 6 MV linac photon beam in a water phantom. The technique is based on calculations with Software for Uncertainty and Sensitivity Analysis (SUSA) and Monte Carlo simulations with the MCNP5 transport code. The free parameters used in the SUSA calculations were the mean energy and full-width-at-half-maximum (FWHM) of the initial electron distribution. A total of 93 combinations of these parameters gave initial electron fluence configurations. The electron spectra thus obtained were used in a simulation of the electron transport through the target of the linear accelerator, which produced different photon (Bremsstrahlung) spectra. The simulated photon spectra were compared with the 6-MV photon spectrum provided by the linac manufacturer (Elekta). This comparison revealed how the mean energy and FWHM of the initial electron fluence affect the spectrum of the generated photons. This study has made it possible to fine-tune the examined electron beam parameters to obtain the resulted absorbed doses with acceptable accuracy (error<1%).
RESUMO
INTRODUCTION: Colonoscopy and bowel preparation cause a number of serum electrolytes changes. AIMS: To determine the prevalence of these changes in patients who underwent colonoscopy and to identify risk factors for these effects. PATIENTS AND METHODS: Forty five patients undergoing colonoscopy were assessed prospectively. They have been previously randomized to receive either sodium phosphate (NaP) or polyethylene glycol (PEG) as preparation. Serum levels of sodium and potassium were analyzed before colonoscopy, immediately after the end of colonoscopy, and 1 hour thereafter. Serum calcium and phosphorus levels were measured just before colonoscopy. RESULTS: Twenty five patients (55.5%) received PEG and 17 (37.7%) NaP. Three patients (6.6%) did not follow the recommended bowel preparation instructions and were excluded from the study. Five patients (11%) developed hyponatremia, of whom, in 4 cases (8.8%), it occurred after the procedure. Thirteen patients (28.8%) developed hypokalemia, of whom it occurred after the end of the procedure in seven (15.5%). There was a non-significant trend to decreased serum potassium levels 1 hour after colonoscopy in patients prepared with NaP (63.6 vs 36.4%). The multivariate analysis showed that low potassium levels were independently associated with age and NaP preparation. Hypocalcemia was observed in 2 patients (4%) and hypophosphatemia in 8 (18%). Hyperphosphatemia was found in 8 cases (18%). Non-significant increases in phosphorus levels were observed in the NaP group. CONCLUSIONS: Temporal sequences of the development of serum electrolyte disturbances suggest that colonoscopy itself might play a role in the pathogenesis of these changes. Preparation for colonoscopy with NaP and the age of patients are risk factors for the development of hypokalemia.
