M. Perucha

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.

Monte Carlo simulation of complex radiotherapy treatments

Monte Carlo simulation is an accurate way of assessing radiotherapy dose distribution in nonhomogeneous volumes, but it requires long processing times. A new distribution model simulates radiotherapy treatments and runs on a PC network, which reduces the processing time and makes for a powerful treatment-verification tool.

Total skin electron therapy treatment verification: Monte Carlo simulation and beam characteristics of large non-standard electron fields

Total skin electron therapy (TSET) is a complex technique which requires non-standard measurements and dosimetric procedures. This paper investigates an essential first step towards TSET Monte Carlo (MC) verification. The non-standard 6 MeV 40 x 40 cm2 electron beam at a source to surface distance (SSD) of 100 cm as well as its horizontal projection behind a polymethylmethacrylate (PMMA) screen to SSD = 380 cm were evaluated. The EGS4 OMEGA-BEAM code package running on a Linux home made 47 PCs cluster was used for the MC simulations. Percentage depth-dose curves and profiles were calculated and measured experimentally for the 40 x 40 cm2 field at both SSD = 100 cm and patient surface SSD = 380 cm. The output factor (OF) between the reference 40 x 40 cm2 open field and its horizontal projection as TSET beam at SSD = 380 cm was also measured for comparison with MC results. The accuracy of the simulated beam was validated by the good agreement to within 2% between measured relative dose distributions, including the beam characteristic parameters (R50, R80, R100, Rp, E0) and the MC calculated results. The energy spectrum, fluence and angular distribution at different stages of the beam (at SSD = 100 cm, at SSD = 364.2 cm, behind the PMMA beam spoiler screen and at treatment surface SSD = 380 cm) were derived from MC simulations. Results showed a final decrease in mean energy of almost 56% from the exit window to the treatment surface. A broader angular distribution (FWHM of the angular distribution increased from 13 degrees at SSD = 100 cm to more than 30 degrees at the treatment surface) was fully attributable to the PMMA beam spoiler screen. OF calculations and measurements agreed to less than 1%. The effect of changing the electron energy cut-off from 0.7 MeV to 0.521 MeV and air density fluctuations in the bunker which could affect the MC results were shown to have a negligible impact on the beam fluence distributions. Results proved the applicability of using MC as a treatment verification tool for complex radiotherapy techniques.

Computational methods for treatment verification: the Full Monte Carlo contribution

The aim of Radiophysics is, within the frame of Radiotherapy, the precise knowledge of the dose distribution inside the human body due to the energy delivery coming from a radiation beam, either from a particle accelerator or from a radioactive source. The planning systems are mainly devoted to this purpose. These pieces of software, which run usually on powerful computers, simulate the process by means of experimental data and specially suited algorithms. Although the precision of these systems, some uncertainties still remain mainly regarding scattering effects and inhomogeneities. This is so because the calculations are not made generally in true 3D and on the other hand, inhomogeneities are not accounted for in detail. In standard Radiotherapy these uncertainties are may be small, but in the case of special techniques or situations, the accurate knowledge of the dose becomes more critical[1,2,3,4,5,6].

A conformal technique for a ring shaped conjunctive lymphoma treatment

Radiotherapy is commonly utilised as standard treatment in the so called mucosa-associated lymphoid tissues (MALT), due to the low probability of distant relapse. The particularities of the lesion, make necessary both energy degradation and beam conformation. To keep homogeneity within acceptable limits, a lengthener attached to the electron applicator has been devised to closely fit the anatomy of the patient. Considering the small area of the outcoming field, film dosimetry is preferred, since the dimensions of an ionisation chamber and even of a semiconductor probe might be comparable to the field size.

A Monte Carlo approach for small electron beam dosimetry.

BACKGROUND AND PURPOSE In treatments where it is necessary to conform the field shape yielding a very small effective beam area, dosimetry and conventional treatment planning may be inaccurate. The Monte Carlo (MC) method can be an alternative to verify dose calculations. A conjunctival mucosa-associated lymphoid tissues lymphoma is presented, to show the importance of an independent assessment in critical situations. MATERIALS AND METHODS In this work, the MC technique has been employed using the program BEAM (based on EGS4 code). Electron beam simulation has been performed and the results have been compared with those obtained with films. The patient dose distribution has been obtained by two methods: the full Monte Carlo (FMC) simulation and a conventional planning system (PLATO). RESULTS Concerning dosimetry, some differences have been observed in the comparison of profiles obtained with film and those obtained with the MC method. Moreover, significant differences were found in the patient isodose distribution between both calculation methods. CONCLUSIONS The results highlight that, in treatments where small beams are needed, conventional dosimetry and planning systems have some limitations. Therefore, an independent and more accurate assessment, such as MC, would be desirable.

Monte carlo clinical dosimetry

Abstract The choice of the most appropriate strategy for radiotherapy treatment is mainly based on the use of a planning system. With the introduction of new techniques (conformal and/or small fields, asymmetrical and non coplanar beams, true 3D calculation, IMRT) the trustworthiness of the algorithms used is questioned. An alternative verification procedure has become increasingly more necessary to warranty treatment delivery. The reliability of the Monte Carlo method is generally acknowledged. However, its clinical use has not been practical due to the high CPU time required. During the last few years our objective has decreased CPU time by means of a new process distribution technique. This reduction has made it feasible, not only to apply physical dosimetry under special conditions, but also to use it in numerous clinical cases employing photon and electron conformal fields, in radiosurgery, and IMRT. The procedure carried out is presented. Furthermore, conventional Treatment Planning System calculations are compared with the Monte Carlo simulations.

PC-Based Process Distribution to Solve Iterative Monte Carlo Simulations in Physical Dosimetry

A distribution model to simulate physical dosimetry measurements with Monte Carlo (MC) techniques has been developed. This approach is indicated to solve the simulations where there are continuous changes of measurement conditions (and hence of the input parameters) such as a TPR curve or the estimation of the resolution limit of an optical densitometer in the case of small field profiles. As a comparison, a high resolution scan for narrow beams with no iterative process is presented. The model has been installed on a network PCs without any resident software. The only requirement for these PCs has been a small and temporal Linux partition in the hard disks and to be connecting by the net with our server PC.

Monte Carlo Conformal Treatment Planning as an Independent Assessment

The wide range of possibilities available in Radiotherapy with conformai fields cannot be covered experimentally. For this reason, dosimetrical and planning procedures are based on approximate algorithms or systematic measurements. Dose distribution calculations based on Monte Carlo (MC) simulations can be used to check results. In this work, two examples of conformai field treatments are shown: A prostate carcinoma and an ocular lymphoma. The dose distributions obtained with a conventional Planning System and with MC have been compared. Some significant differences have been found.

65. Physical and clinical dosimetry by means of Monte Carlo using a process distribution tool

The choice of the most appropriate strategy in a Radiotherapy treatment is mainly based on the use of a planning system. With the introduction of new techniques (conformal and/or small fields, asymmetrical and non coplanar beams, true 3D calculations, IMRT) the trustworthiness of the algorithms is being questioned. An alternative verification procedure is every time more necessary to warranty a treatment delivery. The reliability of Monte Carlo is generally accepted. However, its clinical use has not been operative due to the high CPU times needed. During the last few years our objective has been focussed to reduce this time by means of new process distribution techniques. Tnis drop has made it feasible, not only the physical dosimetry under special conditions, but also a numerous variety of clinical cases: photon and electron conformal fields, Radiosurgery and IMRT. The carried out procedure is presented. Furthermore, experimental dosimetry data as well as conventional TPS calculations are compared with Monte Carlo simulations.