This article presents an investigation of the influence of the ion chamber response, including buildup caps, on the measurement of in-air off-axis ratio (OAR) profiles in megavoltage photon beams using Monte Carlo simulations with the EGSnrc system. Two new techniques for the calculation of OAR profiles are presented. Results of the Monte Carlo simulations are compared to measurements performed in 6, 10 and 25 MV photon beams produced by an Elekta Precise linac and shown to agree within the experimental and simulation uncertainties. Comparisons with calculated in-air kerma profiles demonstrate that using a plastic mini phantom gives more accurate air-kerma measurements than using high-Z material buildup caps and that the variation of chamber response with distance from the central axis must be taken into account.

OBJECTIVE. The objective of this study was to optimize CT protocols for whole-body PET/CT by reducing radiation dose while minimizing effects on image quality. MATERIALS AND METHODS. Before protocol optimization, a survey of 140 consecutive patients was conducted to establish the baseline dose from a whole-body PET/CT examination. Another sample of 100 patients was surveyed to evaluate the reduction of radiation dose after implementation of the new protocol. Effective dose from the CT component of the examination was estimated using dose-length product (DLP) values from reports generated by the scanner and anatomy-specific conversion factors. Twenty-six patients who underwent studies before and after the optimization were included in an analysis of image quality. All 26 patients had maintained the same weight between the examinations and were scanned in the same position using a similar technique except for the changes made for CT dose optimization. The studies were randomized and blinded for an experienced PET and CT reader who graded the imaging quality of anatomic structures. RESULTS. CT protocol optimization resulted in a 32% reduction of the mean CT radiation dose: The mean effective dose was reduced from 8.1 to 5.5 mSv. The blinded analysis of image quality showed no clinically significant degradation of the lower-dose studies. The only structures visualized statistically better on the higher-dose CT scans were the carotid arteries and the region of the posterior triangle. CONCLUSION. The results of this study showed that optimization of CT acquisition can effectively reduce radiation dose in a whole-body PET/CT examination without significantly sacrificing image quality.

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