Ionization chamber dosimetry of small photon fields: a Monte Carlo study on stopping-power ratios for radiosurgery and IMRT beams.

Absolute dosimetry with ionization chambers of the narrow photon fields used in stereotactic techniques and IMRT beamlets is constrained by lack of electron equilibrium in the radiation field. It is questionable that stopping-power ratio in dosimetry protocols, obtained for broad photon beams and quasi-electron equilibrium conditions, can be used in the dosimetry of narrow fields while keeping the uncertainty at the same level as for the broad beams used in accelerator calibrations. Monte Carlo simulations have been performed for two 6 MV clinical accelerators (Elekta SL-18 and Siemens Mevatron Primus), equipped with radiosurgery applicators and MLC. Narrow circular and Z-shaped on-axis and off-axis fields, as well as broad IMRT configured beams, have been simulated together with reference 10 x 10 cm2 beams. Phase-space data have been used to generate 3D dose distributions which have been compared satisfactorily with experimental profiles (ion chamber, diodes and film). Photon and electron spectra at various depths in water have been calculated, followed by Spencer-Attix (delta = 10 keV) stopping-power ratio calculations which have been compared to those used in the IAEA TRS-398 code of practice. For water/air and PMMA/air stopping-power ratios, agreements within 0.1% have been obtained for the 10 x 10 cm2 fields. For radiosurgery applicators and narrow MLC beams, the calculated s(w,air) values agree with the reference within +/-0.3%, well within the estimated standard uncertainty of the reference stopping-power ratios (0.5%). Ionization chamber dosimetry of narrow beams at the photon qualities used in this work (6 MV) can therefore be based on stopping-power ratios data in dosimetry protocols. For a modulated 6 MV broad beam used in clinical IMRT, s(w,air) agrees within 0.1% with the value for 10 x 10 cm2, confirming that at low energies IMRT absolute dosimetry can also be based on data for open reference fields. At higher energies (24 MV) the difference in s(w,air) was up to 1.1%, indicating that the use of protocol data for narrow beams in such cases is less accurate than at low energies, and detailed calculations of the dosimetry parameters involved should be performed if similar accuracy to that of 6 MV is sought.

Anisotropy function for 192Ir low-dose-rate brachytherapy sources: an EGS4 Monte Carlo study.

The anisotropy function of low-dose-rate 192Ir interstitial brachytherapy sources was studied. Absolute dose rates around 192Ir seeds with stainless steel or platinum cladding and a platinum covered wire have been estimated using the EGS4 Monte Carlo simulation system with a very well tested user code. Our results were compared with available experimental data. Excellent agreement between calculated and measured anisotropy function was found for stainless steel clad 192Ir sources, except along the longitudinal axis of the sources. Dosimetry data for the platinum covered seed and 3 mm long wire with platinum cladding as well as for the stainless steel clad 192Ir source are presented in TG43 format. The influence of phantom dimensions on the anisotropy function was found to be non-negligible over 7 cm, enhancing the anisotropy function by 1-2%. Our results have estimated statistical uncertainties below 1% at 1 sigma level up to 10 cm excluding the longitudinal axis where statistical uncertainties below 3% up to 10 cm are observed.

Anisotropy functions for low energy interstitial brachytherapy sources: an EGS4 Monte Carlo study.

Anisotropy functions for low energy interstitial brachytherapy sources are examined. Absolute dose rates around 103Pd seed model 200 and 125I seed models 6702 and 6711 have been estimated by means of the EGS4 Monte Carlo simulation system. The DLC-136/PHOTX cross section library, water molecular form factors, bound Compton scattering and Doppler broadening of the Compton-scattered photon energy were considered in the calculations. Following the formalism developed by the Interstitial Brachytherapy Collaborative Working Group, anisotropy functions, F(r, theta), have been calculated. Our Monte Carlo results were compared against a limited set of measured data selected from the literature and other Monte Carlo results. Binding corrections and phantom material selection have been found to have no influence on the anisotropy function. The accuracy of the geometrical source models used for the Monte Carlo calculations was validated against experimental measurements of in-air relative fluence at 100 cm from the source. More detailed knowledge about the geometrical design of 103Pd seed model 200 is needed in order to improve the agreement with experimentally measured in-air fluence. Values for in-air fluence of 125I model 6702 are sensitive to source position within the inner seed cylinder. Excellent agreement between calculated and measured in-air fluence is found for 125I model 6711. It was observed that using in-air relative fluence at 100 cm from the source to calculate the anisotropy function yields a less anisotropic dose distribution at distances close to the source than full Monte Carlo simulation, in contradiction with experimental data. Our results have estimated statistical uncertainties of 1%-3% at the 1sigma level within clinically relevant regions, but contain systematic uncertainties related to the assumed geometrical details.

Anisotropy functions for 169Yb brachytherapy seed models 5, 8 and X1267. An EGS4 Monte Carlo study.

Anisotropy functions for 169Yb sources used in interstitial brachytherapy are investigated. A comprehensive study of several factors affecting the angular dose distribution around four 169Yb seed models (Amersham International) has been undertaken. Absolute dose rates around 169Yb seed models 5, 8a, 8b and X1267 have been estimated by means of the EGS4 Monte Carlo Simulation System. An updated cross section library (DLC-136/PHOTX), binding corrections for Compton scattering and water molecular form factors were included in the calculations. Following the formalism developed by the Interstitial Brachytherapy Collaborative Working Group, anisotropy functions, F(r, theta), have been calculated and compared with other Monte Carlo results and whenever possible with experimental data. Excellent agreement is found with other Monte Carlo calculations. Considering the large experimental errors reported, a fairly good coincidence has been achieved between experimental and Monte Carlo data for models 8a and 8b. For model X1267 large discrepancies with experiment are obtained. Monte Carlo calculations for all seed models showed model 5 to be the least anisotropic and models 8b and X1267 to be almost identical. Statistical fluctuations can be drastically reduced computationally, offering an efficient alternative to measured data. Our results have estimated uncertainties of 0.5%-1.0% within one standard deviation everywhere excluding the longitudinal source axis, where uncertainties are below 3% up to 5 cm, this accuracy being excellent for clinical calculations.

Radial dose functions for 103Pd, 125I, 169Yb and 192Ir brachytherapy sources: an EGS4 Monte Carlo study.

Radial dose functions g(r) in water around 103Pd, 125I, 169Yb and 192Ir brachytherapy sources were estimated by means of the EGS4 simulation system and extensively compared with experimental as well as with theoretical results. The DLC-136/PHOTX cross section library, water molecular form factors, bound Compton scattering and Doppler broadening of the Compton-scattered photon energy were considered in the calculations. Use of the point source approach produces reasonably accurate values of the radial dose function only at distances beyond 0.5 cm for 103Pd sources. It is shown that binding corrections for Compton scattering have a negligible effect on radial dose function for 169Yb and 192Ir seeds and for 103Pd seeds under 5.0 cm from the source centre and for the 125I seed model 6702 under 8.0 cm. Beyond those limits there is an increasing influence of binding corrections on radial dose function for 103Pd and 125I sources. Results in solid water medium underestimate radial dose function for low-energy sources by as much as 6% for 103Pd and 2.5% for 125I already at 2 cm from source centre resulting in a direct underestimation of absolute dose rate values. It was found necessary to consider medium boundaries when comparing results for the radial dose function of 169Yb and 192Ir sources to avoid discrepancies due to the backscattering contribution in the phantom medium. Values of g(r) for all source types studied are presented. Uncertainties lie under 1% within one standard deviation.

Dose rate constants for 125I, 103Pd, 192Ir and 169Yb brachytherapy sources: an EGS4 Monte Carlo study.

An exhaustive revision of dosimetry data for 192Ir, 125I, 103Pd and 169Yb brachytherapy sources has been performed by means of the EGS4 simulation system. The DLC-136/PHOTX cross section library, water molecular form factors, bound Compton scattering and Doppler broadening of the Compton-scattered photon energy were considered in the calculations. The absorbed dose rate per unit contained activity in a medium at 1 cm in water and air-kerma strength per unit contained activity for each seed model were calculated, allowing the dose rate constant (DRC) A to be estimated. The influence of the calibration procedure on source strength for low-energy brachytherapy seeds is discussed. Conversion factors for 125I and 103Pd seeds to obtain the dose rate in liquid water from the dose rate measured in a solid water phantom with a detector calibrated for dose to water were calculated. A theoretical estimate of the DRC for a 103Pd model 200 seed equal to 0.669 +/- 0.002 cGy h(-1) U(-1) is obtained. Comparison of obtained DRCs with measured and calculated published results shows agreement within 1.5% for 192Ir, 169Yb and 125I sources.