MO‐E‐T‐617‐06: Influence of Ion Chamber Response On In‐Air Profile Measurements in Megavoltage Photon Beams

Purpose: To investigate the influence of the ion chamber response including build‐up cap materials, on the measurements of in‐air off‐axis ratio (OAR) profiles in megavoltage photon beams using Monte Carlo (MC) stimulations with the EGSnrc system. Method and Materials: Two new techniques were developed for the calculations of OARs when the ion chamber is oriented horizontally or vertically. For a horizontally oriented chamber pre‐calculated tables of the response of an ion chamber inserted in a build‐up cap for different photon energies was used to compute the dose deposited in the air cavity on‐the‐fly within the BEAMnrc simulation. For a vertically oriented chamber the BEAMnrc code was modified so that it can be compiled into a shared library that serves as a particle source for the CAVRZnrc user code. With these BEAMnrc and CAVRZnrc changes the OAR could be calculated on the fly without intermediate phase‐space file generation. Results of the simulations were compared with experimental profiles from the 6, 10 25 MV photon beams from an Elekta Precise linac. Results: The calculated and measured in‐air profiles for all investigated beams and build‐up caps (brass, hevimet and two PMMA miniphantoms) are in a good agreement within the statistical and experimental uncertainties. The comparison between the calculated air‐kerma and OAR profiles shows 3–6% differences between air‐kerma and in‐air profiles measured with hevimet and brass caps and 0.5–1% differences for measurements with PMMA mini‐phantoms. Conclusion: The change of chamber response with distance from the central axis must be taken into account. For in‐air profiles measurements PMMA mini‐phantoms should be recommended over high‐Z material build‐up caps

Influence of ion chamber response on in-air profile measurements in megavoltage photon beams

NRC publication: Ye

A multi institutional comparison of imaging dose and technique protocols for neonatal chest radiography

Introduction: The focus on pediatric radiation dose reduction supports the re-evaluation of pediatric imaging protocols. This is particularly important in the neonates where chest radiographs are frequently requested to assess respiratory illness and line placement. This study aims to assess the impact of neonatal chest radiographic protocols on patient dose in four hospitals in different countries. Methods: Exposure parameters, collimation, focus to skin distance (FSD) and radiation dose from 200 neonatal chest radiographs were registered prospectively. Inclusion criteria consisted of both premature and full-term neonates weighing between 1000 and 5000 g. Only data from the examinations meeting diagnostic criteria and approved for the clinical use were included. The radiation dose was assessed using a dose area product (DAP). Results: The lowest DAP value (4.58 mGy cm2) was recorded in the Norwegian hospital, employing a high kVp, low mAs protocol using a DR system. The Canadian hospital recorded the highest DAP (9.48), using lower kVp and higher mAs with a CR system, including the addition of a lateral projection. The difference in the mean DAP, weight, the field of view (FOV) and kVp between the hospitals is statistically significant (p<0.001). Conclusion: The use of non-standardized imaging protocols in neonatal chest radiography results in differences in patient dose across hospitals included in the study. Using higher kVp, lower mAs and reducing the number of lateral projections to clinically relevant indications result in a lower DAP measured in the infant sample studied. Further studies to examine image quality based on exposure factors and added filtration are recommended. Implications for practice: Reevaluation of pediatric imaging protocols presents an opportunity to reduce patient dose in a population with increased sensitivity to ionizing radiation.info:eu-repo/semantics/publishedVersio