fast model for prediction of respiratory lung motion for image-guided radiotherapy: a feasibility study

The aim of this work was to study the feasibility of constructing a fast thorax model suitable for simulating lung motion due to respiration using only one CT dataset. For each of six patients with different thorax sizes, two sets of CT images were obtained in single-breath-hold inhale and exhale stages in the supine position. The CT images were then analyzed by measurements of the displacements due to respiration in the thorax region. Lung and thorax were 3D reconstructed and then transferred to the ABAQUS software for biomechanical fast finite element [FFE] modeling. The FFE model parameters were tuned based on three of the patients, and then was tested in a predictive mode for the remaining patients to predict lung and thorax motion and deformation following respiration. Starting from end-exhale stage, the model, tuned for a patient created lung wall motion at end-inhale stage that matched the measurements for that patient within 1 mm [its limit of accuracy]. In the predictive mode, the mean discrepancy between the imaged landmarks and those predicted by the model [formed from averaged data of two patients] was 4.2 mm. The average computation time in the fast predictive mode was 89 sec. Fast prediction of approximate, lung and thorax shapes in the respiratory cycle has been feasible due to the linear elastic material approximation, used in the FFE model

Comparison of dosimetry parameters of two commercially available Iodine brachytherapy seeds using Monte Carlo calculations

Iodine brachytherapy sources with low photon energies have been widely used in treating cancerous tumors. Dosimetric parameters of brachytherapy sources should be determined according to AAPM TG-43U1 recommendations before clinical use. Monte Carlo codes are reliable tools in calculation of these parameters for brachytherapy sources. Dosimetric parameters [dose rate constant, radial dose function, and anisotropy function] of two I-125 brachytherapy sources [models LS-1 and Intersource] were calculated with MCNP4C Monte Carlo code following task group number 43 [TG-43U1] recommendations of American Assossiation of Physicists in Medicine. The simulations were done inside a spherical water phantom because of its tissue equivalent properties. The Monte Carlo simulations for radial dose function were performed at distances ranging from 0.25 to 10 cm from the source center. The anisotropy functions F[r, theta], for both sources, were calculated at distances of 1, 2, 3, 5 and 7 cm from the source center for angles ranging from 15 to 90 degree. The results of the Monte Carlo simulation indicated a dose rate constant of 0.952 cGyh -1U-1 and 0.986 cGyh -1U-1 for models LS-1 and Intersource, respectively. The tabulated data and fifth order polynomial coefficients for radial dose functions along the source are described in this paper .The results indicated that the anisotropy in dose distribution increased along the source axis. The obtained results were in good agreement with measurements and calculations of other investigators, using other Monte Carlo codes

Simulation of the shielding effects of an applicator on the AAPM TG-43 parameters of CS-137 Selectron LDR brachytherapy sources

The dose rate distribution delivered by a low dose rate [137]Cs pellet source, a spherical source used within the source trains of the Selectron gynecological brachytherapy system, was investigated using the MCNP4C Monte Carlo code. The calculations were performed in both water and Plexiglas and the absolute dose rate distribution for a single pellet source and the AAPM TG-43 parameters were computed. A spherical phantom with dimensions large enough [60 cm] was used to provide full scattering conditions. In order to score dose at different distances from the source centre, this sphere was divided into a set of 600 concentric spherical shells of 0.05 cm thickness. The calculations were performed up to a distance of 10 cm from the source centre. To calculate the effect of the applicator and dummy pellets on dose rate constant and radial dose function, a single pellet source was simulated inside the vaginal applicator, and spherical tally cells with radius of 0.05 cm were used in the simulations. The F6 tally was used to score the absolute dose rate at a given point in the phantom. The dose rate constant for a single active pellet was found to be 1.102 +/- 0.007 cGyh[-1]U[-1], and the dose rate constant for an active pellet inside the applicator was 1.095 +/- 0.009 cGyh[-1]U[-1]. The tabulated data and 5th order polynomial fit coefficients for the radial dose function along with the dose rate constant are provided for both cases. The effect of applicator and dummy pellets on anisotropy function of the source was also investigated. The error resulting from ignoring the applicator was reduced using the data of a single pellet. The results indicate that F[r, theta] decreases towards the applicator