A new approach for calculating internal dose estimates was developed through the use of a more realistic computational model of the human body. The present technique shows the capability to build a patient-specific phantom with tomography data (a voxel-based phantom) for the simulation of radiation transport and energy deposition using Monte Carlo methods such as in the MCNP-4B code. MCNP-4B absorbed fractions for photons in the mathematical phantom of Snyder et al. agreed well with reference values. Results obtained through radiation transport simulation in the voxel-based phantom, in general, agreed well with reference values. Considerable discrepancies, however, were found in some cases due to two major causes: differences in the organ masses between the phantoms and the occurrence of organ overlap in the voxel-based phantom, which is not considered in the mathematical phantom.

This paper describes the development of a tomographic model of a rat developed using CT images of an adult male Wistar rat for radiation transport studies. It also presents calculations of absorbed fractions (AFs) under internal photon and electron sources using this rat model and the Monte Carlo code MCNP. All data related to the developed phantom were made available for the scientific community as well as the MCNP inputs prepared for AF calculations in that phantom and also all estimated AF values, which could be used to obtain absorbed dose estimates--following the MIRD methodology--in rats similar in size to the presently developed model. Comparison between the rat model developed in this study and that published by Stabin et al (2006 J. Nucl. Med. 47 655) for a 248 g Sprague-Dawley rat, as well as between the estimated AF values for both models, has been presented.

A new approach for calculating internal dose estimates was developed through the use of a more realistic computational model of the human body. The study demonstrates the capability of building a patient-specific phantom with voxel-based data for the simulation of radiation transport and energy deposition using Monte Carlo methods such as the MCNP-4B code. MCNP-4B was used to calculate absorbed fractions for photons in a voxel-based phantom, and values were compared to reference values from traditional phantoms used for many years. Results obtained in general agreed well with previous values, but considerable differences were found in some cases due to two major causes; differences in the organ masses between the phantoms and the occurrence of organ overlap in the voxel-based phantom (which is not well modeled in the mathematical phantoms). These new techniques offer promise of developing a new generation of more realistic phantoms for internal, as well as external, dose assessment. The principal area of implementation in internal dose assessment should be the development of patient-specific dose estimates in nuclear medicine therapy, such as radioimmunotherapy (RIT). However, as new voxel-based phantoms for different individuals can be developed, they may also be used with the techniques developed here to derive new absorbed fractions and replace the traditional values usedfor other applications in internal and external dose assessment, which have been based on mathematical constructs that are not always very representative of real human organs.

Brachytherapy treatment planning systems that use model-based dose calculation algorithms employ a more accurate approach that replaces the TG43-U1 water dose formalism and adopt the TG-186 recommendations regarding composition and geometry of patients and other relevant effects. However, no recommendations were provided on the transit dose due to the source traveling inside the patient. This study describes a methodology to calculate the transit dose using information from the treatment planning system (TPS) and considering the source's instantaneous and average speed for two prostate and two gynecological cases. The trajectory of the (192)Ir HDR source was defined by importing applicator contour points and dwell positions from the TPS. The transit dose distribution was calculated using the maximum speed, the average speed and uniform accelerations obtained from the literature to obtain an approximate continuous source distribution simulated with a Monte Carlo code. The transit component can be negligible or significant depending on the speed profile adopted, which is not clearly reported in the literature. The significance of the transit dose can also be due to the treatment modality; in our study interstitial treatments exhibited the largest effects. Considering the worst case scenario the transit dose can reach 3% of the prescribed dose in a gynecological case with four catheters and up to 11.1% when comparing the average prostate dose for a case with 16 catheters. The transit dose component increases by increasing the number of catheters used for HDR brachytherapy, reducing the total dwell time per catheter or increasing the number of dwell positions with low dwell times. This contribution may become significant (>5%) if it is not corrected appropriately. The transit dose cannot be completely compensated using simple dwell time corrections since it may have a non-uniform distribution. An accurate measurement of the source acceleration and maximum speed should be incorporated in clinical practice or provided by the manufacturer to determine the transit dose component with high accuracy.

The source maintains its speed even for the short interdwell distances. Dose variations due to the transit dose component are much lower than the prescribed treatment doses for brachytherapy, although transit dose component should be evaluated individually for clinical cases.

Dose calculation in high dose rate brachytherapy with (192)Ir is usually based on the TG-43U1 protocol where all media are considered to be water. Several dose calculation algorithms have been developed that are capable of handling heterogeneities with two possibilities to report dose: dose-to-medium-in-medium (Dm,m) and dose-to-water-in-medium (Dw,m). The relation between Dm,m and Dw,m for (192)Ir is the main goal of this study, in particular the dependence of Dw,m on the dose calculation approach using either large cavity theory (LCT) or small cavity theory (SCT). A head and neck case was selected due to the presence of media with a large range of atomic numbers relevant to tissues and mass densities such as air, soft tissues and bone interfaces. This case was simulated using a Monte Carlo (MC) code to score: Dm,m, Dw,m (LCT), mean photon energy and photon fluence. Dw,m (SCT) was derived from MC simulations using the ratio between the unrestricted collisional stopping power of the actual medium and water. Differences between Dm,m and Dw,m (SCT or LCT) can be negligible (<1%) for some tissues e.g. muscle and significant for other tissues with differences of up to 14% for bone. Using SCT or LCT approaches leads to differences between Dw,m (SCT) and Dw,m (LCT) up to 29% for bone and 36% for teeth. The mean photon energy distribution ranges from 222 keV up to 356 keV. However, results obtained using mean photon energies are not equivalent to the ones obtained using the full, local photon spectrum. This work concludes that it is essential that brachytherapy studies clearly report the dose quantity. It further shows that while differences between Dm,m and Dw,m (SCT) mainly depend on tissue type, differences between Dm,m and Dw,m (LCT) are, in addition, significantly dependent on the local photon energy fluence spectrum which varies with distance to implanted sources.

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