Development of a particle source model for a synergy linear accelerator to be used in Monte Carlo radiation dose calculations for cancer therapy

[摘要] English: In oncology patients are treated for cancer with various methods such as surgery, chemotherapy and radiation therapy. Accurate radiation treatment planning and dose delivery to thetumour is necessary for the successful outcome of cancer treatment. In order to achieve thisgoal accurate radiation dose calculation codes must be utilized. EGSnrc based Monte Carlo(MC) codes such as BEAMnrc and DOSXYZnrc have been developed for just this purpose.The problem that arises in using these MC codes is that they lack suitable x-ray beam sourcemodels. These models must be accurate in order to replicate the true clinical x-ray beamemanating from the linear accelerator. One such machine for which radiation source datamust be derived is currently being used at the Oncology department in Universitas HospitalAnnex. It is desirable to model this linear accelerator in order to perform MC based dosecalculations for radiation treatment.The use of MC based dose calculations is certainly not new in the radiation physicsenvironment. Various authors have studied the replication of radiation beam characteristicsusing source models to simulate the phase-space parameters of particles produced by thelinear accelerator. These parameters include the charge, energy, direction, and position ofeach particle as it crosses a certain reference plane below the linear accelerator. An accuratesource model should be able to re-generate particles with the exact set of above-mentionedparameters as would be produced by the real linear accelerator. Sources can be very simplesuch as a single point from which the particles are radiating with a single invariant energyspectrum. Studies have shown that these beam models can yield accurate beam data over relatively small field sizes and is not general enough to use over a whole range of clinicallyuseful field sizes.A graphical user interface (GUI) was developed that can assist in the construction of thesource model. The source model can describe energy and fluence distributions for photonsand electrons as separate point sources each with their own SSD. The accuracy of the modelwas validated by comparing simulated profiles with measured data for an Elekta Synergylinear accelerator.The modified Schiff formula was used to derive the bremsstrahlung spectra emanatingfrom the target. The x-ray fluence Gaussian distribution consisted of the primary fluencefrom the target, which was modified by the primary collimator, secondary collimators as wellas the multileaf collimators. The truncation and beam scatter caused by the face of thecollimators were modelled with error functions. Exponential functions were used to modeloff-axis collimator transmission.Profiles and percentage depth dose curves were obtained with the source for square fieldsizes of 1 × 1 cm2up to a 40 × 40 cm2. Offset fields for 10 × 10 cm2, 15 × 15 cm2and 20 ×20 cm2, rectangular fields as well as wedged fields were included. Irregular field shapes weresimulated to evaluate the source model's capability of reproducing complex treatment fields.Film dose verification was done in an anthropomorphic Rando® phantom and compared withthe MC source model for 6 MV x-ray beams. A criterion of 2% / 2 mm was used to compareMC data and measured data.This study demonstrated that a diversity of field sizes and percentage depth dose curvescan be modelled within 2% / 2 mm. The model can replicate irregular field sizes used forcomplex treatments. Minor discrepancies were found for the relative dose comparisonsbetween the MC and film data for the anthropomorphic phantom.

[发布机构] University of the Free State