[Palliation by radiation is also our business !]

Radiotherapy (RT), both with a curative and a palliative intent, is one of the cornerstones of oncological treatments. A variety of symptoms linked to cancer can be relieved with RT (such as pain, bleeding, compression exerted by a tumour lesion…). Very often, palliative RT is proposed when other medical treatments (painkillers, morphine…) are no longer efficient, or the patient does not tolerate them anymore. Palliative RT is an integral part of the global supportive oncological care. Indeed, patients' wishes and prognosis are taken into account in each and every step of the treatment pathway. Every treatment deserves an individualized approach and benefits from the best available techniques.

Dynamic MLC Tracking Using 4D Lung Tumor Motion Modelling and EPID Feedback

BACKGROUND

Respiratory motion causes thoracic movement and reduces targeting accuracy in radiotherapy.

OBJECTIVE

This study proposes an approach to generate a model to track lung tumor motion by controlling dynamic multi-leaf collimators.

MATERIAL AND METHODS

All slices which contained tumor were contoured in the 4D-CT images for 10 patients. For modelling of respiratory motion, the end-exhale phase of these images has been considered as the reference and they were analyzed using neuro-fuzzy method to predict the magnitude of displacement of the lung tumor. Then, the predicted data were used to determine the leaf motion in MLC. Finally, the trained algorithm was figured out using Shaper software to show how MLCs could track the moving tumor and then imported on the Varian Linac equipped with EPID.

RESULTS

The root mean square error (RMSE) was used as a statistical criterion in order to investigate the accuracy of neuro-fuzzy performance in lung tumor prediction. The results showed that RMSE did not have a considerable variation. Also, there was a good agreement between the images obtained by EPID and Shaper for a respiratory cycle.

CONCLUSION

The approach used in this study can track the moving tumor with MLC based on the 4D modelling, so it can improve treatment accuracy, dose conformity and sparing of healthy tissues because of low error in margins that can be ignored. Therefore, this method can work more accurately as compared with the gating and invasive approaches using markers.

Enhancement of the Dose on 12 MV Linac with Free Flattening Filter Mode

PURPOSE

In the last years, some studies investigated dosimetric benefits of a free flattening filter for the photon mode in the radiotherapy field. This study aims to provide a theoretical study published and analysis of basic dosimetric properties for a Saturne 43 Linac head to implement free flattening filter beams clinically.

MATERIAL AND METHODS

This is the first Monte Carlo study for the head of Saturne 43 with replacement flattening filter mode investigating beam dosimetric characteristics, including central axis absorbed doses, beam profiles and photon energy spectra. The later ones were analyzed for flattening filter and replacement flattening filter beams using BEAMnrc and DOSXYZnrc Monte Carlo codes for 10 × 10 cm², 5 × 5 cm² and 2 × 2 cm² square field sizes.

RESULTS

A 3.94-fold increase of dose rate and electron contaminating increased by 246.4 % with the replacement flattening filter mode for field size of 10 × 10 cm². Reduction was made by replacement flattening filter beam in the peripheral dose up to 30%, and the time was reduced more than 50 %.

CONCLUSION

Results obtained from our study revealed that some characteristic dosimetries such as the maximum increase in depth dose rate, decrease in out-of-depth dose, and reducing time can be beneficial for the unflattened beam to be used in the radiotherapy for the advanced techniques.

An Efficiency Studying of an Ion Chamber Simulation Using Vriance Reduction Techniques with EGSnrc

BACKGROUND

Radiotherapy is an important technique of cancer treatment using ionizing radiation. The determination of total dose in reference conditions is an important contribution to uncertainty that could achieve 2%. The source of this uncertainty comes from cavity theory that relates the in-air cavity dose and the dose to water. These correction factors are determined from Monte Carlo calculations of ionization chambers. The main problem of this type of calculation is the extremely long computation time to achieve reasonable statistics.

OBJECTIVE

The main purpose of this work is to present a combination with variance reduction techniques for the case of an ionization chamber in water.

METHODS

The egs_chamber code allows for very efficient computation of ionization chamber doses and dose ratios by using various variance reduction techniques, and also permits realistic simulations of the experimental setup due to the use of EGSnrc C++ library. Russian roulette and Photon Cross Section Enhancement were used with egs_chamber code. Tests were performed to obtain the parameters of variance reduction techniques resulting in a maximum efficiency.

RESULTS

It can be seen that the parameters which result in improved Monte Carlo calculation of the efficiency values are XCSE 64 and Russian Roulette (RR) 128.

CONCLUSION

This study determines the parameters of variance reduction techniques that result in an optimal computational efficiency.

A Feasibility Study of IMRT of Lung Cancer Using Gafchromic EBT3 Film

BACKGROUND

Intensity modulated radiation therapy (IMRT) is an advanced method for delivery of three dimensional therapies, which provides optimal dose distribution with giving multiple nonuniform fluency to the patient. The complex dose distribution of IMRT should be checked to ensure that the accurate dose is delivered. Today, film dosimetry is a powerful tool for radiotherapy treatment Quality Assurance (QA) and a good method to verify dose distribution in phantoms.

OBJECTIVE

This study aimed to evaluate the accuracy of IMRT treatment planning system, Prowess Panther^® software, with Gafchromic EBT3 films in a inhomogeneity phantom.

MATERIAL AND METHODS

The IMRT plan was generated by Prowess Panther^® treatment planning system (TPS) version 5.2 on a inhomogeneity phantom, then it was irradiated by ONCOR linear accelerator (Linac) with 6 (MV) photon beam energy. The Gafchromic EBT3 film located between the phantom has measured the dose distribution. To compare between TPS calculated doses and film measured doses, Gamma criteria 3%/3 mm, 4%/4 mm, 5%/5 mm, 6%/6 mm and 7%/7 mm Dose Difference (DD) and Distance to Agreement (DTA), respectively were used.

RESULTS

Gammas passing rates for PTV are obtained 67.5% for 3%/3mm, 78.8% for 4%/4mm, 86.3% for 5%/5mm, 91.2% for 6%/6mm and 94.3% for 7%/7mm and for organs at risk is 72.4% for 3%/3mm, 82.8% for 4%/4mm, 89.8% for 5%/5mm, 93.3% for 6%/6mm and 95.4% for 7%/7mm (respectively DD/DTA). By increasing the range of criteria the capability increased.

CONCLUSION

The results show that the use of EBT3 film in a inhomogeneity phantoms allows us to evaluate the dose differences between the EBT3 measured dose distribution and TPS calculated dose distribution .Hence, a result Prowess Panther^® TPS can be used for IMRT technique treatment.

Benchmarking of Siemens Linac in Electron Modes: 8-14 MeV Electron Beams

BACKGROUND

Radiation therapy using electron beams is a promising method due to its physical dose distribution. Monte Carlo (MC) code is the best and most accurate technique for forespeaking the distribution of dose in radiation treatment of patients.

MATERIAL AND METHODS

We report an MC simulation of a linac head and depth dose on central axis, along with profile calculations. The purpose of the present research is to carefully analyze the application of MC methods for the calculation of dosimetric parameters for electron beams with energies of 8-14 MeV at a Siemens Primus linac. The principal components of the linac head were simulated using MCNPX code for different applicators.

RESULTS

The consequences of measurements and simulations revealed a good agreement. Gamma index values were below 1 for most points, for all energy values and all applicators in percent depth dose and dose profile computations. A number of states exhibited rather large gamma indices; these points were located at the tail of the percent depth dose graph; these points were less used in in radiotherapy. In the dose profile graph, gamma indices of most parts were below 1. The discrepancies between the simulation results and measurements in terms of Z_max, R₉₀, R₈₀ and R₅₀ were insignificant. The results of Monte Carlo simulations showed a good agreement with the measurements.

CONCLUSION

The software can be used for simulating electron modes of a Siemens Primus linac when direct experimental measurements are not feasible.

[Could we consider that radiotherapy is effective outside the irradiation area ? The abscopal effect]

Radiotherapy is known for its action on local tumoral control. However, it is also able to induce immunomodulatory effects at a systemic level. The abscopal effect (from latin ab scopus which means «away from the target») is an illustration of this phenomenon. It is defined as a tumor regression observed outside and at a distance of the irradiation fields. The potential application of this effect of treatment in disseminated cancers is a fast-growing field of research. The optimal therapeutic strategy to achieve this effect remains unknown.

Monte Carlo Simulation of Siemens Primus plus Linac for 6 and 18 MV Photon Beams

OBJECTIVE

The aim of the present study is to simulate 6 MV and 18 MV photon beam energies of a Siemens Primus Plus medical linear accelerator (Linac) and to verify the simulation by comparing the results with the measured data.

METHODS

The main components of the head of Siemens Primus Plus linac were simulated using MCNPX Monte Carlo (MC) code. To verify the results, experimental data of percentage depth dose (PDD) and beam dose profile for 5 × 5 cm², 10 × 10 cm² and 20 × 20 cm2 field sizes were measured and compared with simulation results. Moreover, gamma function was used to compare the measurement and simulation data.

RESULTS

The results show a good agreement, within 1%, was observed between the data calculated by the simulations and those obtained by measurement for 6 MV photon beam, while it was within 2% for 18 MV photon beam, except in the build-up region for both beams. Gamma index values were less than unity in most data points for all the mentioned energies and fields. To calculate the dose in the phantom, cells were selected in different modes, one of the modes due to the lack of dose gradient and overlapping, produced better results than others produce.

CONCLUSION

There was good settlement between measured and MC simulation values in this research. The simulation programs can be used for photon modes of Siemens Primus Plus linac in conditions in which it is not possible to perform experimental measurements.