Dosimetric characterization of radioactive sources employed in prostate cancer therapy

Determination of task group 43 dosimetric parameters for CSM40 137Cs source for use in brachytherapy

The CSM40 ¹³⁷Cs source model is currently being used in clinical brachytherapy. According to the recommendations of task group No. 43 (TG-43) of the American Association of Physicists in Medicine, dosimetry parameters of brachytherapy sources should be determined by two independent investigators before their clinical use. The aim of this study was to determine the TG-43 dosimetry parameters for a medium-dose-rate CSM40 ¹³⁷Cs source. The determined dosimetric parameters included the air kerma strength, dose rate constant, radial dose function, and anisotropy function. To determine the source's dosimetric parameters, the CSM40 source was stimulated by the Monte Carlo N-Particle MCNP code. The TG-43 parameters were compared with the data of Vijande et al. on this source. The results showed that the dosimetry parameters for this source had good agreement with the results of Vijande et al. The dosimetric parameters of the CSM40 source can be used in treatment-planning systems incorporating this source model. The data can also be used for the quality assurance of treatment-planning systems.

Impact of interseed attenuation and tissue composition for permanent prostate implants

The purpose is to evaluate the impact of interseed attenuation and prostate composition for prostate treatment plans with 125I permanent seed implants using the Monte Carlo (MC) method. The effect of seed density (number of seeds per prostate unit volume) is specifically investigated. The study focuses on treatment plans that were generated for clinical cases. For each plan, four different dose calculation techniques are compared: TG-43 based calculation, superposition MC, full MC with water prostate, and full MC with realistic prostate tissue. The prostate tissue description is from the ICRP report 23 (W. S. Snyer, M. J. Cook, E. S. Nasset, L. R. Karkhausen, G. P. Howells, and I. H. Tipton, "Report of the task group on reference man," Technical Report 23, International Commission on Radiological Protection, 1974). According to the comparisons, the seed density has an influence on interseed attenuation. A plan with a typical low seed density (42 0.6 mCi seeds in a 26 cm3 prostate) suffers a 1.2% drop in the CTV D90 value due to interseed attenuation. A drop of 3.0% is calculated for a higher seed density (75 0.3 mCi seeds, same prostate). The influence of the prostate composition is similar for all seed densities and prostate sizes. The difference between MC simulations in water and MC simulations in prostate tissue is between 4.4% and 4.8% for the D90 parameter. Overall, the effect on D90 is ranging from 5.8% to 12.8% when comparing clinically approved TG-43 and MC simulations in prostate tissue. The impact varies from one patient to the other and depends on the prostate size and the number of seeds. This effect can reach a significant level when reporting correlations between clinical effect and deposited dose.

Absolute depth-dose-rate measurements for an Ir192 HDR brachytherapy source in water using MOSFET detectors

Reported MOSFET measurements concern mostly external radiotherapy and in vivo dosimetry. In this paper, we apply the technique for absolute dosimetry in the context of HDR brachytherapy using an 192Ir source. Measured radial dose rate distributions in water for different planes perpendicular to the source axis are presented and special attention is paid to the calibration of the R and K type detectors, and to the determination of appropriate correction factors for the sensitivity variation with the increase of the threshold voltage and the energy dependence. The experimental results are compared with Monte Carlo simulated dose rate distributions. The experimental results show a good agreement with the Monte Carlo simulations: the discrepancy between experimental and Monte Carlo results being within 5% for 82% of the points and within 10% for 95% of the points. Moreover, all points except two are found to lie within the experimental uncertainties, confirming thereby the quality of the results obtained.

144Ce as a potential candidate for interstitial and intravascular brachytherapy

PURPOSE

To investigate the suitability of (144)Ce for both interstitial and intravascular brachytherapy applications.

METHODS AND MATERIALS

Monte Carlo calculations of radial dose rate distributions in water were performed for (144)Ce in a spring-shaped source and compared with two commonly used interstitial and intravascular sources, (192)Ir and (32)P. The numeric simulations were checked experimentally with a calibrated ionization chamber in a water phantom. Other source characteristics, such as half-life and specific activity, were also compared.

RESULTS

For interstitial brachytherapy, (144)Ce presents dosimetric advantages over (192)Ir in terms of higher dose rate at shorter distances and lower irradiation of organs outside the tumor. The source size and shape reduce the anisotropy and the number of dwell positions necessary. The longer half-life of (144)Ce might also be advantageous over (192)Ir. For intravascular brachytherapy, (144)Ce permits the treatment of larger arteries as compared with (32)P, compensates centering errors more effectively, and has a more suitable half-life. The experimental validation showed good agreement (within 10%) with the Monte Carlo simulated dose rate distributions.

CONCLUSIONS

There are certain potential advantages of using (144)Ce as a source for both interstitial and intravascular brachytherapy. The basis for this finding is provided by the Monte Carlo radial dose rate comparisons with (192)Ir and (32)P, as well as by such characteristics as half-life and specific activity.