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.

Small field dosimetry is complicated due to the lack of electronic equilibrium and to the high steep dose gradients. This works compares PDD curves, profiles and output factors measured with conventional detectors (film, diode, TLD and ionisation chamber) and calculated with Monte Carlo. The 6 MV nominal energy from a Philips SL-18 linac has been simulated by using the OMEGA code. MC calculation reveals itself as a convenient method to validate OF and profiles in special conditions, such as small fields.

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.

The precision of Radiosurgery Treatment planning systems is limited by the approximations of their algorithms and by their dosimetrical input data. This fact is especially important in small fields. However, the Monte Carlo methods is an accurate alternative as it considers every aspect of particle transport. In this work an acoustic neurinoma is studied by comparing the dose distribution of both a planning system and Monte Carlo. Relative shifts have been measured and furthermore, Dose-Volume Histograms have been calculated for target and adjacent organs at risk.

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.

The dramatic advance in the modem linacs has led to the implementation of new treatment techniques involving very small fields and non-homogeneous fluence beams. This has revealed some deficiencies in the commercial treatment planning systems as some effects, which are negligible in standard Radiotherapy and hence not properly accounted for, become now critical. Monte Carlo is the only method that considers accurately every single boundary crossing and non-equilibrium situation. However, there is still a main drawback for the routinary use of this technique, namely, the long calculation time needed. The aim of this work is then the reduction.

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].

Differences between the scatter conditions of dosimetry and treatment situation are more important in the case of large-field photon beams than in standard ones. In the former, the scattering volume is defined by the phantom cross section; in the latter, the radiation field size. Two factors should be considered: the thickness and the cross section of the phantom. Both of them have an effect on the Percentage Depth Dose (PDD) distribution. In a previous study we addressed the influence of backscatter thickness on dose delivered. The aim of this work is to measure the effect of cross section phantom on the PDD curves under our TBI treatment conditions. Results showed a strong dependence of the PDDs on this parameter. A semi-empirical expression has also been derived to calculate (within 0.5% uncertainty) the Lateral scatter Correction Factor (LCF). The model of LCF states a linear dependence on depth whilst slope of these curves depends exponentially on distance to the lateral surface. The algorithm is being applied to our practical Total Body Irradiation (TBI) procedure.

For total body irradiation (TBI) dose calculation requirements, anatomical information about the whole body is needed. Despite the fact that video image grabbing techniques are used by some treatment planning systems for standard radiotherapy, there are no such systems designed to generate anatomical parameters for TBI planning. The paper describes an anthropometrical computerised system based on video image grabbing which was purpose-built to provide anatomical data for a PC-based TBI planning system. Using software, the system controls the acquisition and digitalisation of the images (external images of the patient in treatment position) and the measurement procedure itself (on the external images or the digital CT information). An ASCII file, readable by the TBI planning system, is generated to store the required parameters of the dose calculation points, i.e. depth, backscatter tissue thickness, thickness of inhomogeneity, off-axis distance (OAD) and source to skin distance (SSD).

This work presents the verification of an on line in vivo dosimetry system based on semiconductors. Software and hardware has been designed to convert the diode signal into absorbed dose. Final verification was made in the form of an intercomparison with two independent thermoluminiscent (TLD) dosimetry systems, under TBI conditions.

The high level of accuracy required in radiotherapy treatment dosimetry makes necessary good treatment quality control. The common way is the use of in vivo dosimetry equipment that allows the direct measurement of dose delivered to the patient. Control of homogeneity and constancy of the incident beam on the patient can be achieved directly by means of entrance dose measurement; however, control of dose delivered to tumours and internal organs is difficult because of the impossibility of a direct measurement. In this case calculations are made using external measurements (entrance and exit sides of the patient) to obtain the dose delivered. In this work, an algorithm that allows the real-time knowledge of midline dose as a function of thickness and entrance and exit doses coming from semiconductor detectors is presented. By having the electrometer connected to the computer, these three values (entrance, midline, and exit dose) are displayed instantaneously when the algorithm is included in the acquisition program. The model has been developed both for standard (source to surface distance = 100 cm) and special treatment techniques such as total body irradiation (SSD = 314 cm). There is a good agreement of experimental and calculated values with differences below 0.04%.

The accuracy requirements in target dose delivery is, according to ICRU, +/- 5%. This is so not only in standard radiotherapy but also in total body irradiation (TBI). Physical dosimetry plays an important role in achieving this recommended level. The semi-infinite phantoms, customarily used for dosimetry purposes, give scatter conditions different to those of the finite thickness of the patient. So dose calculated in patient's points close to beam exit surface may be overestimated. It is then necessary to quantify the backscatter factor in order to decrease the uncertainty in this dose calculation. The backward scatter has been well studied at standard distances. The present work intends to evaluate the backscatter phenomenon under our particular TBI treatment conditions. As a consequence of this study, a semi-empirical expression has been derived to calculate (within 0.3% uncertainty) the backscatter factor. This factor depends lineally on the depth and exponentially on the underlying tissue. Differences found in the qualitative behavior with respect to standard distances are due to scatter in the bunker wall close to the measurement point.

We report on a series on 81 patients affected with acute leukemia undergoing TBI prior to BMT.